The SLL Lighting Handbook

The Society of Light and Lighting

The Society of Light and Lighting

The Society of Light and Lighting is part of the Chartered Institution of Building Services Engineers

The SLL Lighting Handbook

The Society of Light and Lighting

The SLL Lighting Handbook

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The SLL Lighting Handbook

222 Balham High Road, London SW12 9BS +44 (0)20 8675 5211 www.cibse.org

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This document is based on the best knowledge available at the time of publication. However, no responsibility of any kind for any injury, death, loss, damage or delay however caused resulting from the use of these recommendations can be accepted by the Chartered Institution of Building Services Engineers, The Society of Light and Lighting, the authors or others involved in its publication. In adopting these recommendations for use each adopter by doing so agrees to accept full responsibility for any personal injury, death, loss, damage or delay arising out of or in connection with their use by or on behalf of such adopter irrespective of the cause or reason therefore and agrees to defend, indemnify and hold harmless the Chartered Institution of Building Services Engineers, The Society of Light and Lighting, the authors and others involved in their publication from any and all liability arising out of or in connection with such use as aforesaid and irrespective of any negligence on the part of those indemnified. The rights of publication or translation are reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means without the prior permission of the publisher. Note from the publisher This publication is primarily intended to give guidance. It is not intended to be exhaustive or definitive, and it will be necessary for users of the guidance given to exercise their own professional judgement when deciding whether to abide by or depart from it. © February 2009 The Society of Light and Lighting

The Society of Light and Lighting

The Society of Light and Lighting is part of the Chartered Institution of Building Services Engineers

The Society is part of CIBSE, which is a registered charity, number 278104. ISBN 978-1-906846-02-2 Project and Print management by entiveon Ltd. www.entiveon.com Design, linework and typsetting by Squarefox Design Ltd. www.squarefox.co.uk Printed in England on FSC certified Mixed Sources paper by Stones the Printers Ltd. www.stonestheprinters.co.uk

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FOREWORD 2009 is the centenary of the formation of the Illuminating Engineering Society, the progenitor of the Society of Light and Lighting. This handbook has been written to celebrate this anniversary and to fill a gap in the Society’s publications. The Society of Light and Lighting’s major publications are: The SLL Code for lighting, which offers recommendations on lighting for a wide range of applications The SLL Lighting Guides, which provide detailed guidance on specific lighting applications The SLL Lighting Handbook has been written to forge a link between them. It is designed to be complementary to the SLL Code for lighting but to go beyond it in terms of applications and background information without getting into the fine detail of the Lighting Guides. The SLL Lighting Handbook is intended to be the first-stop for anyone seeking information on lighting. It is aimed not just at lighting practitioners but also at lighting specifiers and students of lighting. For all three groups, we have tried to make it comprehensive, up-to-date and easily understandable. The contents summarise the fundamentals of light and vision, the technology of lighting and guidance on a wide range of applications, both interior and exterior. Authors Peter Boyce PhD, FSLL, FIESNA Peter Raynham BSc MSc CEng FSLL MCIBSE MILE Acknowledgements John Fitzpatrick Lou Bedocs (Thorn Lighting) Ted Glenny (Philips Lighting) Jennifer Brons for Figure 20.2 Kit Cuttle for Figures 13.1 and 13.2 Lighting Research Center for Figures 9.1, 10.3, 18.8, 18.9 and 20.3 McGraw Hill Inc, for Figures 2.4 and 2.9 Mick Stevens for Figures 20.3 and 22.1 The Illuminating Engineering Society of North America for Figures 1.5, 1.6, 1.7, 1.8, 2.8 and 2.13 Philips Lighting, iGuzzini Illuminazione, Havells Sylvania & Luxo Charlotte Wood Photography for Figures 14.1, 14.2 and 14.3 Editors Stuart Boreham (entiveon Ltd.) Peter Hadley (Squarefox Design Ltd.) SLL Secretary Liz Peck CIBSE Editorial Manager Ken Butcher CIBSE Director of Information Jacqueline Balian

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CONTENTS PART 1: FUNDAMENTALS Chapter 1: Light 1.1 The nature of light 1.2 The CIE standard observers 1.3 The measurement of light — photometry 1.3.1 Luminous flux 1.3.2 Luminous intensity 1.3.3 Illuminance 1.3.4 Luminance 1.3.5 Reflectance 1.3.6 Obsolete units 1.3.7 Typical values 1.4 The measurement of light — colourimetry 1.4.1 The CIE chromaticity diagrams 1.4.2 The CIE colour spaces 1.4.3 Correlated colour temperature 1.4.4 CIE colour rendering index 1.4.5 Colour gamut 1.4.6 Scotopic/photopic ratio 1.4.7 Colour order systems

1 1 3 3 3 4 4 4 6 6 7 7 10 11 12 13 14 14

Chapter 2: Vision 2.1 The structure of the visual system 2.1.1 The visual field 2.1.2 Eye movements 2.1.3 Optics of the eye 2.1.4 The structure of the retina 2.1.5 The functioning of the retina 2.1.6 The central visual pathways 2.1.7 Colour vision 2.2 Continuous adjustments of the visual system 2.2.1 Adaptation 2.2.2 Photopic, scotopic and mesopic vision 2.2.3 Accommodation 2.3 Capabilities of the visual system 2.3.1 Threshold measures 2.3.2 Factors determining visual threshold 2.3.3 Spatial thresholds 2.3.4 Temporal thresholds 2.3.5 Colour thresholds 2.3.6 Light spectrum and movement 2.4 Suprathreshold performance 2.5 Visual search 2.6 Visual discomfort 2.6.1 Insufficient light 2.6.2 Illuminance uniformity 2.6.3 Glare 2.6.4 Veiling reflections 2.6.5 Shadows 2.6.6 Flicker

16 16 16 17 19 22 23 23 24 24 25 26 26 26 28 28 30 31 32 32 34 37 37 37 38 39 40 41

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2.7 Perception through the visual system 2.7.1 The constancies 2.7.2 Attributes and modes of appearance 2.8 Anomolies of vision 2.8.1 Defective colour vision 2.8.2 Low vision

41 41 42 44 44 45

PART 2: TECHNOLOGY Chapter 3: Light sources 3.1 Production of radiation 3.1.1 Incandescence 3.1.2 Electric discharges 3.1.3 Electroluminescence 3.1.4 Luminescence 3.1.5 Radioluminescence 3.1.6 Cathodoluminescence 3.1.7 Chemiluminescence 3.1.8 Thermoluminescence 3.2 Daylight 3.2.1 Sunlight 3.2.2 Skylight 3.3 Electric light 3.3.1 Incandescent 3.3.2 Tungsten halogen 3.3.3 Fluorescent 3.3.4 High pressure mercury 3.3.5 Metal halide 3.3.6 Low pressure sodium 3.3.7 High pressure sodium 3.3.8 Induction 3.3.9 Light emitting diodes 3.3.10 Electroluminescent 3.4 Electric light source characteristics 3.4.1 Luminous flux 3.4.2 Power demand 3.4.3 Luminous efficacy 3.4.4 Lumen maintenance 3.4.5 Life 3.4.6 Colour properties 3.4.7 Run-up time 3.4.8 Restrike time 3.4.9 Other factors 3.4.10 Summary of lamp characteristics 3.5 Flames 3.5.1 Candle 3.5.2 Oil 3.5.3 Gas

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48 48 49 51 51 51 52 52 52 52 52 54 57 57 59 60 64 66 69 70 74 75 76 77 77 77 78 78 78 78 78 79 79 79 82 82 82 83

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Chapter 4: Luminaires 4.1 Basic requirements 4.1.1 Electrical 4.1.2 Mechanical 4.1.3 Optical control 4.1.4 Efficiency 4.1.5 Thermal 4.1.6 Acoustics 4.1.7 Environmental 4.2 Luminaire types 4.2.1 Interior lighting 4.2.2 Exterior lighting 4.3 Certification and classification 4.3.1 Certification 4.3.2 Classification

84 84 85 86 91 91 93 94 94 94 98 100 100 105

Chapter 5: Electrics 5.1 Control gear 5.1.1 Ballasts for discharge light sources 5.1.2 Transformers for low voltage light sources 5.1.3 Drivers for LEDs 5.2 Lighting controls 5.2.1 Options for control 5.2.2 Input devices 5.2.3 Control processes and systems

109 109 114 114 115 115 115 116

PART 3: APPLICATIONS Chapter 6: Lighting design 6.1 Objectives and constraints 6.2 A holistic strategy for lighting 6.2.1 Legal requirements 6.2.2 Visual function 6.2.3 Visual amenity 6.2.4 Lighting and architectural integration 6.2.5 Energy efficiency and sustainability 6.2.6 Maintenance 6.2.7 Lighting costs 6.2.8 Photopic or mesopic vision 6.2.9 Light trespass and skyglow 6.3 Basic design decisions 6.3.1 Use of daylight 6.3.2 Choice of electric lighting system 6.3.3 Integration 6.3.4 Equal and approved

117 117 118 118 119 120 120 121 121 121 122 124 124 124 125 128

Chapter 7: Daylighting 7.1 7.2 7.3 7.4

Benefits of daylight Daylight availability Daylight as a contribution to room brightness Daylight for task illumination

129 131 133 133 vii

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7.5 Types of daylighting 7.5.1 Windows 7.5.2 Clerestories 7.5.3 Rooflights 7.5.4 Atria 7.5.5 Remote distribution 7.5.6 Borrowed light 7.6 Problems of daylighting 7.6.1 Visual problems 7.6.2 Thermal problems 7.6.3 Privacy problems 7.7 Maintenance

133 133 135 135 136 136 137 137 137 139 139 139

Chapter 8: Emergency lighting 8.1 Legislation and standards 8.2 Forms of emergency lighting 8.2.1 Escape route lighting 8.2.2 Signage 8.2.3 Open area lighting 8.2.4 High risk area 8.2.5 Standby lighting 8.3 Design approaches 8.4 Emergency lighting equipment 8.4.1 Power sources 8.4.2 Circuits 8.4.3 Luminaires 8.4.4 Luminaire classification 8.4.5 Light sources 8.4.6 Others 8.5 Scheme planning 8.5.1 Risk assessment 8.5.2 Recommended systems for specific places 8.5.3 Planning sequence 8.6 Installation, testing and maintenance 8.6.1 Installation 8.6.2 Maintenance and inspection 8.6.3 Documentation 8.6.4 Commissioning and certification 8.6.5 Completion certificate

140 141 141 142 142 144 144 144 145 145 146 147 148 148 149 149 149 150 153 153 153 153 154 154 155

Chapter 9: Office lighting 9.1 Functions of lighting in offices 9.2 Factors to be considered 9.2.1 Legislation and guidance 9.2.2 Type of work done 9.2.3 Screen type 9.2.4 Daylight availability 9.2.5 Ceiling height 9.2.6 Obstruction 9.2.7 Surface finishes

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156 156 156 157 157 158 158 159 159

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9.3 Lighting recommendations 9.3.1 Illuminances 9.3.2 Light distribution 9.3.3 Maximum luminances 9.3.4 Discomfort glare control 9.3.5 Light source colour properties 9.4 Approaches to office lighting 9.4.1 Direct lighting 9.4.2 Indirect lighting 9.4.3 Direct/indirect lighting 9.4.4 Localised lighting 9.4.5 Supplementary task lighting 9.4.6 Cove lighting 9.4.7 Luminous ceilings 9.4.8 Daylight

162 162 164 165 166 167 168 168 169 170 170 171 171 172 172

Chapter 10: Industrial lighting 10.1 Functions of lighting in industrial premises 10.2 Factors to be considered 10.2.1 Legislation and guidance 10.2.2 The environment 10.2.3 Daylight availability 10.2.4 Need for good colour vision 10.2.5 Obstruction 10.2.6 Directions of view 10.2.7 Access 10.2.8 Rotating machinery 10.2.9 Safety and emergency egress 10.3 Lighting recommendations 10.3.1 Control rooms 10.3.2 Storage 10.3.3 Ancillary areas 10.3.4 Speculative factory units 10.4 Approaches to industrial lighting 10.4.1 General lighting 10.4.2 Localised lighting 10.4.3 Local lighting 10.4.4 Visual inspection 10.4.5 Visual aids

173 173 173 174 174 175 175 176 177 177 177 177 178 180 181 182 182 182 183 183 183 184

Chapter 11: Lighting for educational premises 11.1 Functions of lighting for educational premises 11.2 Factors to be considered 11.2.1 Students’ capabilities 11.2.2 Daylight or electric light 11.2.3 Common lines of sight 11.2.4 Flat or raked floor 11.2.5 Presence of visual aids 11.2.6 Surface finishes

185 185 185 186 186 186 186 186

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11.3 Lighting recommendations 11.3.1 Illuminances 11.3.2 Illuminance uniformity 11.3.3 Glare control 11.3.4 Light source colour properties 11.3.5 Control systems 11.4 Approaches to lighting educational premises 11.4.1 Classrooms and lecture halls 11.4.2 IT room 11.4.3 Arts studio 11.4.4 Science laboratories 11.4.5 Seminar room 11.4.6 Library 11.4.7 Assembly hall 11.4.8 Music room 11.4.9 Drama studio

187 187 187 187 188 188 189 189 189 189 189 190 190 190 190 190

Chapter 12: Retail lighting 12.1 Functions of retail lighting 12.2 Factors to be considered 12.2.1 Shop profile 12.2.2 Daylight or electric light 12.2.3 Nature of merchandise 12.2.4 Obstruction 12.3 Lighting recommendations 12.3.1 Illuminances 12.3.2 Illuminance uniformity 12.3.3 Luminances 12.3.4 Light source colour properties 12.4 Approaches to retail lighting 12.4.1 General lighting 12.4.2 Accent lighting 12.4.3 Display lighting

191 191 191 192 192 192 192 192 193 193 193 194 194 194 195

Chapter 13: Lighting for museums and art galleries

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13.1 Functions of lighting in museums and art galleries 13.2 Factors to be considered 13.2.1 Daylight or electric light 13.2.2 Conservation of exhibits 13.2.3 Light source colour rendering properties 13.2.4 Adaptation 13.2.5 Balance 13.2.6 Shadows and modelling 13.2.7 Glare 13.2.8 Veiling reflections and highlights 13.2.9 Out-of-hours activities 13.2.10 Security and emergency 13.2.11 Maintenance 13.2.12 Flexibility 13.3 Lighting approaches for museums and art galleries 13.3.1 Wall mounted displays 13.3.2 Three-dimensional displays 13.3.3 Showcase lighting

198 198 198 198 199 199 199 200 200 200 200 201 201 201 201 201 201 202

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Chapter 14: Lighting for hospitals 14.1 Functions of lighting in hospitals 14.2 Factors to be considered 14.2.1 Daylight 14.2.2 Lines of sight 14.2.3 Colour rendering requirements 14.2.4 Observation without disturbance to sleep 14.2.5 Emergency lighting 14.2.6 Luminaire safety 14.2.7 Cleanliness 14.2.8 Electro-magnetic compatibility (EMC) 14.3 Approaches for the lighting of different areas in hospitals 14.3.1 Entrance halls, waiting areas and lift halls 14.3.2 Reception and enquiry desks 14.3.3 Hospital streets and general corridors 14.3.4 Changing rooms, cubicles, toilets, bath, wash and shower rooms 14.3.5 Wards 14.3.6 Reading lighting 14.3.7 Night lighting 14.3.8 Night observation lighting (watch lighting) 14.3.9 Clinical areas and operating departments 14.3.10 Operating theatres

203 203 203 203 203 204 204 204 205 205 205 206 206 207 207 207 211 211 211 212 212

Chapter 15: Quasi-domestic lighting 15.1 Functions of quasi-domestic lighting 15.2 Factors to be considered 15.2.1 Occupants’ capabilities 15.2.2 Daylight 15.2.3 Light source colour properties 15.2.4 Energy efficiency 15.2.5 Safety 15.2.6 Security 15.3 Lighting recommendations 15.4 Approaches to lighting quasi-domestic buildings 15.4.1 Entrances 15.4.2 Corridors and stairs 15.4.3 Study bedrooms 15.4.4 Kitchens and utility rooms 15.4.5 Lounges 15.4.6 Dining halls 15.4.7 Games room

214 214 214 214 214 215 215 216 216 217 217 217 218 218 219 219 219

Chapter 16: Road lighting 16.1 Road classification 16.2 Lighting for traffic routes 16.2.1 Lighting recommendations for traffic routes 16.2.2 Lighting recommendations for areas adjacent to the carriageway 16.2.3 Lighting recommendations for conflict areas 16.2.4 Coordination 16.2.5 Traffic route lighting design

220 220 220 223 224 225 225 xi

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16.3 Lighting for subsidiary roads 16.3.1 Lighting recommendations for subsidiary roads 16.3.2 Lighting design for subsidiary roads 16.4 Lighting for urban centres and public amenity areas 16.5 Tunnel lighting

230 230 232 232 233

Chapter 17: Exterior workplace lighting 17.1 Functions of lighting in exterior workplaces 17.2 Factors to be considered 17.2.1 Scale 17.2.2 Nature of work 17.2.3 Need for good colour vision 17.2.4 Obstruction 17.2.5 Interference with complementary activities 17.2.6 Hours of operation 17.2.7 Impact on the surrounding area 17.2.8 Atmospheric conditions 17.3 Lighting recommendations 17.3.1 Illuminance and illuminance uniformity 17.3.2 Glare control 17.3.3 Light source colour properties 17.3.4 Loading areas 17.3.5 Chemical and fuel industries 17.3.6 Sidings, marshalling yards and goods yards 17.4 Approaches to exterior workplace lighting 17.4.1 High mast floodlighting 17.4.2 Integrated lighting 17.4.3 Localised lighting

236 236 236 236 237 237 237 237 238 238 238 238 239 239 239 240 241 243 243 243 244

Chapter 18: Security lighting

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18.1 Functions of security lighting 18.2 Factors to be considered 18.2.1 Type of site 18.2.2 Site features 18.2.3 Ambient light levels 18.2.4 Crime risk 18.2.5 CCTV surveillance 18.2.6 Impact on the surrounding area 18.3 Lighting recommendations 18.3.1 Illuminance and illuminance uniformity 18.3.2 Glare control 18.3.3 Light source colour properties 18.4 Approaches to security lighting 18.4.1 Secure areas 18.4.2 Public spaces 18.4.3 Private areas 18.4.4 Multi-occupancy dwellings 18.5 Lighting Equipment 18.5.1 Light sources 18.5.2 Luminaires 18.5.3 Lighting columns 18.5.4 Lighting controls 18.5.5 Maintenance

245 245 245 246 247 247 247 247 247 247 249 249 249 249 252 253 254 254 254 255 255 256 256

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Chapter 19: Sports lighting 19.1 Functions of lighting for sports 19.2 Factors to be considered 19.2.1 Standard of play and viewing distance 19.2.2 Playing area 19.2.3 Luminaires 19.2.4 Television 19.2.5 Coping with power failures 19.2.6 Obtrusive light 19.3 Lighting recommendations 19.3.1 Athletics 19.3.2 Bowls 19.3.3 Cricket 19.3.4 Five-a-side football (indoor) 19.3.5 Fitness training 19.3.6 Football (Association, Gaelic and American) 19.3.7 Lawn tennis 19.3.8 Rugby (Union and League) 19.3.9 Swimming 19.4 Lighting in large facilities 19.4.1 Multi-use sports halls 19.4.2 Small sports stadia 19.4.3 Indoor arenas 19.4.4 Swimming pools

257 257 257 258 258 258 259 260 261 261 262 263 264 264 265 265 266 266 267 267 267 268 268

Chapter 20: Lighting performance verification 20.1 The need for performance verification 20.2 Relevant operating conditions 20.3. Instrumentation 20.3.1 Illuminance meters 20.3.2 Luminance meters 20.4 Methods of measurement 20.4.1 Average illuminance 20.4.2 Interior lighting 20.4.3 Exterior lighting 20.5 Measurement of illuminance variation 20.5.1 Illuminance diversity 20.5.2 Illuminance uniformity 20.6 Luminance measurements 20.7 Measurement of reflectance

270 270 271 271 271 272 272 272 274 275 275 276 276 276

Chapter 21: Lighting maintenance 21.1 The need for lighting maintenance 21.2 Lamp replacement 21.3 Cleaning luminaires 21.4 Room surface cleaning 21.5 Maintained illuminance 21.6 Designing for lighting maintenance 21.7 Determination of maintenance factor for interior lighting 21.7.1 Lamp lumen maintenance factor (LLMF) 21.7.2 Lamp survival factor (LSF) 21.7.3 Luminaire maintenance factor (LMF) 21.7.4 Room surface maintenance factor (RSMF)

278 278 278 280 280 280 280 281 281 282 284

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21.8 Determination of maintenance factor for exterior lighting 21.9 Disposal of lighting equipment

285 286

Chapter 22: On the horizon 22.1. Changes and challenges 22.2. The changes and challenges facing lighting practice 22.2.1 Costs 22.2.2 Technologies 22.2.3 New knowledge 22.2.4 External influences 22.3 The evolution of lighting practice

287 287 287 287 287 290 290

Chapter 23: Bibliography

xiv

23.1 Standards 23.2 Guidance 23.3 References

293 296 298

Index

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Chapter 1: Light 1.1 The nature of light Light is part of the electromagnetic spectrum that stretches from cosmic rays to radio waves (Figure 1.1). What distinguishes the wavelength region between 380–780 nanometers from the rest is the response of the human visual system. Photoreceptors in the human eye absorb energy in this wavelength range and thereby initiate the process of seeing.

Chapter One: Light

PART 1. FUNDAMENTALS

104

102

RADIO WAVES

100

10–2

780 nm MICRO WAVES 700

10–4

INFRA RED 600

10–6

10–8

VISIBLE

ULTRA VIOLET 500

10–10

10–12

X RAYS

400 380 nm

GAMMA RAYS

10–14

10–16

COSMIC RAYS

Wavelength (m)

Figure 1.1 A schematic diagram of the electromagnetic spectrum showing the location of the visible spectrum. The divisions between the different types of electromagnetic radiation are indicative only. 1.2 The CIE standard observers The sensitivity of the human visual system is not the same at all wavelengths in the range 380 nm to 780 nm. This makes it impossible to adopt the radiometric quantities conventionally used to measure the characteristics of the electromagnetic spectrum for quantifying light. Rather, a special set of quantities has to be derived from the radiometric quantities by weighting them by the spectral sensitivity of the human visual system. The result is the photometry system (see Section 1.3). The Commission Internationale de l’Eclairage (CIE) has established three standard observers to represent the sensitivity of the human visual system to light at different wavelengths, in different conditions. In 1924, the CIE adopted the Standard Photopic Observer to characterise the spectral sensitivity of the human visual system by day. 1

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Chapter One: Light

In 1990, in the interests of greater photometric accuracy, the CIE produced a Modified Photopic Observer, having greater sensitivity than the CIE Standard Photopic Observer at wavelengths below 460 nm. This CIE Modified Photopic Observer is considered to be a supplement to the CIE Standard Photopic Observer not a replacement for it. As a result, the CIE Standard Photopic Observer has continued to be widely used by the lighting industry. This is acceptable because the modified sensitivity at wavelengths below 460 nm has been shown to make little difference to the photometric properties of light sources that emit radiation over a wide range of wavelengths. It is only for light sources that emit significant amounts of radiation below 460 nm that changing from the CIE Standard Photopic Observer to the CIE Modified Photopic Observer makes a significant difference to photometric properties. Some narrow band light sources, such as blue light emitting diodes, fall into this category. In 1951, the CIE adopted the CIE Standard Scotopic Observer to characterise the spectral sensitivity of the human visual system by night. The Standard Scotopic Observer is used by the lighting industry to quantify the efficiency of a light source at stimulating the rod photoreceptors of the eye (see Section 2.1.4). The CIE Standard and Modified Photopic Observers and the CIE Standard Scotopic Observer are shown in Figure 1.2, the Standard and Modified Photopic Observers having maximum sensitivities at 555 nm and the Standard Scotopic Observer having a maximum sensitivity at 507 nm. These relative spectral sensitivity curves are formally known as the 1924 CIE Spectral Luminous Efficiency Function for Photopic Vision, the CIE 1988 Modified Two Degree Spectral Luminous Efficiency Function for Photopic Vision, and the 1951 CIE Spectral Luminous Efficiency Function for Scotopic Vision, respectively. More commonly, they are known as the CIE V (λ), CIE VM (λ), and the CIE V’ (λ) curves. These curves are the basis of the conversion from radiometric quantities to the photometric quantities used to characterise light. Relative luminous efficiency 1.0

0.8

= Standard photopic observer = Modified photopic observer = Standard scotopic observer = 10 degree field

0.6

0.4

0.2

0

300

400

500

600

700

800

Wavelength (nm)

Figure 1.2 The relative luminous efficiency functions for the CIE Standard Photopic Observer, the CIE Modified Photopic Observer, the CIE Standard Scotopic Observer, and the relative luminous efficiency function for a 10 degree field of view in photopic conditions 2

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1.3.1 Luminous flux The most fundamental measure of the electromagnetic radiation emitted by a source is its radiant flux. This is the rate of flow of energy emitted and is measured in watts. The most fundamental quantity used to measure light is luminous flux. Luminous flux is radiant flux multiplied, wavelength by wavelength, by the relative spectral sensitivity of the human visual system, over the wavelength range 380 nm to 780 nm (Figure 1.3). This process can be represented by the equation:

Chapter One: Light

1.3 The measurement of light — photometry

Φ = Km Σ Ψλ Vλ ∆λ where: Φ = luminous flux (lumens) Ψλ = radiant flux in a small wavelength interval ∆λ (watts) Vλ = the relative luminous efficiency function for the conditions Km= constant (lumens/watt) ∆λ = wavelength interval In System Internationale (SI) units, the radiant flux is measured in watts (W) and the luminous flux in lumens (lm). The values of Km are 683 lm/W for the CIE Standard and Modified Photopic Observers and 1699 lm/W for the CIE Standard Scotopic Observer. It is always important to identify which of the CIE Standard Observers is being used in any particular measurement or calculation. The CIE recommends that whenever the Standard Scotopic Observer is being used, the word scotopic should precede the measured quantity, i.e. scotopic luminous flux. Luminous flux is used to quantify the total light output of a light source in all directions. Energy output

V(λ)

Light output

350 400 450 500 550 600 650 700 750 800 350 400 450 500 550 600 650 700 750 800 350 400 450 500 550 600 650 700 750 800

Figure 1.3 The process for converting from radiometric to photometric quantities. The lefthand figure shows the spectral power distribution of a light source in radiometric quantities (watts/wavelength interval). The centre figure shows the CIE Standard Photopic Observer. Multiplying the spectral power at each wavelength by the luminous efficiency at the same wavelength given by the CIE Standard Photopic Observer, the right hand figure is produced. The right hand figure is the spectral luminous flux distribution in photometric quantities (lumens/wavelength interval). 1.3.2 Luminous intensity Luminous intensity is the luminous flux emitted/unit solid angle, in a specified direction. Solid angle is given by area divided by the square of the distance and is measured in steradians. An area of 1 square metre at a distance of 1 metre from the origin subtends one steradian. The unit of measurement of luminous intensity is the candela, which is equivalent to one lumen/steradian. Luminous intensity is used to quantify the distribution of light from a luminaire. 3

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Chapter One: Light

1.3.3 Illuminance Illuminance is the luminous flux falling on unit area of a surface. The unit of measurement of illuminance is the lumen/m2 or lux. The illuminance incident on a surface is the most widely used electric lighting design criterion. Figure 1.4 shows some typical illuminances on different surfaces under the noonday sun in temperate climates.

100 lux

2500 lux

5000 lux

10,000 lux

100,000 lux

Figure 1.4 Typical illuminances on different surfaces under the noonday sun in temperate climates 1.3.4 Luminance The luminance of a surface is the luminous intensity emitted per unit projected area of the surface in a given direction. The unit of measurement of luminance is the candela/m2. Luminance is widely used to define stimuli presented to the visual system. 1.3.5 Reflectance As might be expected, there is a relationship between the amount of light incident on a surface and the amount of light reflected from the same surface. The simplest form of the relationship is quantified by the luminance coefficient. The luminance coefficient is the ratio of the luminance of the surface to the illuminance incident on the surface and has units of candela/lumen. The luminance coefficient of a given surface is dependent on the nature of the surface and the geometry between the lighting, surface and observer. There are two other quantities commonly used to express the relationship between the luminance of a surface and the illuminance incident on it. For a perfectly diffusely-reflecting surface, the relationship is given by the equation: luminance =

(illuminance × reflectance)

π

where luminance is expressed in candela/m2 and illuminance is expressed in lumens/m2. 4

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Chapter One: Light

For a diffusely-reflecting surface, reflectance is defined as the ratio of reflected luminous flux to incident luminous flux. For a non-diffusely-reflecting surface, i.e. a surface with some specularity, the same equation between luminance and illuminance applies but reflectance is replaced with luminance factor. Luminance factor is defined as the ratio of the luminance of the surface viewed from a specific position and lit in a specified way to the luminance of a diffusely-reflecting white surface viewed from the same direction and lit in the same way. It should be clear from this definition, that a non-diffusely-reflecting surface can have many different values of the luminance factor. Table 1.1 summarises these definitions. Table 1.1 The photometric quantities. Measure

Definition

Units

Luminous flux

That quantity of radiant flux which expresses its capacity to produce visual sensation

lumens (lm)

Luminous intensity

The luminous flux emitted in a very narrow cone containing the given direction divided by the solid angle of the cone, i.e. luminous flux/unit solid angle

candela (cd)

Illuminance

The luminous flux/unit area at a point on a surface

lumen/m2

Luminance

The luminous flux emitted in a given direction divided by the product of the projected area of the source element perpendicular to the direction and the solid angle containing that direction, i.e. luminous intensity/unit area

candela/m2

Luminance coefficient

The ratio of the luminance of a surface to the illuminance incident on it

candela/lumen

Reflectance

The ratio of the luminous flux reflected from a surface to the luminous flux incident on it

For a diffuse surface:

Luminance factor

For a non-diffuse surface, for a specific direction and lighting geometry:

luminance = (illuminance × reflectance ) / π

The ratio of the luminance of a reflecting surface viewed from a given direction to that of a perfect white uniform diffusing surface identically illuminated

luminance = (illuminance × luminance factor) / π 5

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1.3.6 Obsolete units Photometry has a long history that has generated a number of different units of measurement for illuminance and luminance. Table 1.2 lists some of these obsolete units, together with the multiplying factors necessary to convert from the alternative unit to the SI units of lumens/m2 for illuminance and candela/m2 for luminance. Table 1.2 Some photometric units of measurement for illuminance and luminance and the multiplying factors necessary to change them to System Internationale (SI) units Quantity

Unit

Dimensions

Multiplying factor

Illuminance

lux metre candle phot footcandle

lumen/m2 lumen/m2 lumen/cm2 lumen/ft2

1.00 1.00 10,000 10.76

Luminance

nit stilb

candela/m2 candela/cm2 candela/in2 candela/ft2

1.00 10,000 1,550 10.76

Luminous exitance*

apostilb* blondel* lambert* footlambert*

lumen/m2 lumen/m2 lumen/cm2 lumen/ft2

0.32 0.32 3,183 3.43

* Luminous exitance is the product of the illuminance on the surface and the reflectance of the surface. It is only meaningful for completely diffusely reflecting surfaces. Luminous exitance has the dimensions of lumens/unit area. Luminous exitance is deprecated in the SI system. 1.3.7 Typical values Table 1.3 shows some illuminances and luminances typical of commonly occurring situations, all measured using the CIE Standard Photopic Observer. Table 1.3 Typical illuminance and luminance values.

6

Situation

Illuminance (lm/m2)

Typical surface

Luminance (cd/m2)

Clear sky in summer in temperate zones

100,000

Grass

1,910

Overcast sky in summer in temperate zones

16,000

Grass

300

Textile inspection

1,500

Light grey cloth

140

Office work

500

White paper

120

Heavy engineering

300

Steel

20

Good road lighting

20

Concrete road surface

2.0

Moonlight

0.5

Asphalt road surface

0.01

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Photometry does not take into account the wavelength combination of the light. Thus it is possible for two surfaces to have the same luminance but the reflected light to be made up of totally different combinations of wavelengths. In this situation, and provided there is enough light for colour vision to operate, the two surfaces will look different in colour. The CIE colourimetry system provides a means to quantify colour. 1.4.1 The CIE chromaticity diagrams The basis of the CIE colourimetry system is colour matching. The CIE Colour Matching Functions are the relative spectral sensitivity curves of the human observer with normal colour vision and can be considered as another form of standard observer. The CIE colour matching functions are mathematical constructs that reflect the relative spectral sensitivities required to ensure that all the wavelength combinations that are seen as the same colour have the same position in the CIE colourimetry system and that all wavelength combinations that are seen as different in colour occupy different positions. Figure 1.5 shows two sets of colour matching functions. The CIE 1931 Standard Observer is used for colours occupying visual fields up to 4° of angular subtense. The CIE 1964 Standard Observer is used for colours covering visual fields greater than 4° in angular subtense. The values of the colour matching functions at different wavelengths are known as the spectral tristimulus values.

Chapter One: Light

1.4 The measurement of light — colourimetry

2.5

2.0

z 1.5

1.0 y

x

0.5

0 400

450

500

550

600

650

700

Wavelength (nm)

Figure 1.5 Two sets of colour matching functions: The CIE 1931standard observer (2 degrees) (solid line) and the CIE 1964 standard observer (10 degrees) (dashed line). The colour of a light source can be represented mathematically by multiplying the spectral power distribution of the light source, wavelength by wavelength, by each of the three colour matching functions x(λ), y(λ) and z(λ), the outcome being the amounts of three imaginary primary colours X, Y, and Z required to match the light source colour. In the form of equations, X, Y and Z are given by: 7

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Chapter One: Light

X = h Σ S(λ) x(λ) λ Y = h Σ S(λ) y(λ) λ Z = h Σ S(λ) z(λ) λ where: S(λ) = spectral radiant flux of the light source (W/nm) x(λ), y(λ), z(λ) = spectral tristimulus values from the appropriate colour matching function λ = wavelength interval (nm) h = arbitrary constant If only relative values of the X, Y and Z are required, an appropriate value of h is one that makes Y = 100. If absolute values of the X, Y, and Z are required it is convenient to take h = 683 since then the value of Y is the luminous flux in lumens. If the colour being calculated is for light reflected from a surface or transmitted through a material, the spectral reflectance or spectral transmittance is included as a multiplier in the above equations. For a reflecting surface, an appropriate value of h is one that makes Y =100 for a perfect white reflecting surface because then the actual value of Y is the percentage reflectance of the surface. Having obtained the X, Y, and Z values, the next step is to express their individual values as proportions of their sum, i.e. x = X / (X+Y+Z) y = Y / (X+Y+Z) z = Z / (X+Y+Z) The values x, y and z are known as the CIE chromaticity coordinates. As x + y + z = 1, only two of the coordinates are required to define the chromaticity of a colour. By convention, the x and y coordinates are used. Given that a colour can be represented by two coordinates, then all colours can be represented on a two dimensional surface. Figure 1.6 shows the CIE 1931 chromaticity diagram. The outer curved boundary of the CIE 1931 chromaticity diagram is called the spectrum locus. All pure colours, i.e. those that consist of a single wavelength, lie on this curve. The straight line joining the ends of the spectrum locus is the purple boundary and is the locus of the most saturated purples obtainable. At the centre of the diagram is a point called the equal energy point, where a colourless surface will be located. Close to the equal energy point is a curve called the Planckian locus. This curve passes through the chromaticity coordinates of objects that operate as a black body, i.e. the spectral power distribution of the light source is determined solely by its temperature. The CIE 1931 chromaticity diagram can be considered as a map of the relative location of colours. The saturation of a colour increases as the chromaticity coordinates get closer to the spectrum locus and further from the equal energy point. The hue of the colour is determined by the direction in which the chromaticity coordinates move. The CIE 1931 chromaticity diagram is useful for indicating approximately how a colour will appear, a value recognised by the CIE in that it specifies chromaticity coordinate limits for signal lights and surfaces so that they will be recognised as red, green, yellow, and blue (CIE Publication 107:1994).

8

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Chapter One: Light

520 nanometers 530 0.8 540

510

0.7

550

560 0.6 570 500 0.5

580

Y 590 3500

0.4

2360

1900 1500

4800

510

6500 0.3

490

620 630 640

Equal energy

10,000

600

24,000

780

0.2

480 nanometers 0.1 470 460 450

0 0

0.1

380 0.2

0.3

0.4

0.5

0.6

0.7

0.8

X

Figure 1.6 The CIE 1931 Chromaticity Diagram showing the spectrum locus, the Planckian locus and the equal energy point) The CIE 1931 chromaticity diagram is perceptually non-uniform. Green colours cover a large area while red colours are compressed in the bottom right corner. This perceptual non-uniformity makes any attempt to quantify large colour differences using the CIE 1931 chromaticity diagram futile. In an attempt to improve this situation, the CIE first introduced the CIE 1960 Uniform Chromaticity Scale (UCS) diagram and then, in 1976, recommended the use of the CIE 1976 UCS diagram. Both diagrams are simply linear transformations of the CIE 1931 chromaticity diagram. The axes for the CIE 1976 UCS diagram are u' = 4x / (–2x +12y +3) v' = 9y / (–2x + 12y + 3) where x and y are the CIE 1931 chromaticity coordinates. Figure 1.7 shows the CIE 1976 UCS diagram.

9

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0.6

520

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560 580 600 620

500

640 770 nm

0.5

0.4

V' 0.3 480

0.2

470 0.1 460 450 440 420 0

0.1

0.2

400 nm 0.3

0.4

0.5

0.6

U'

Figure 1.7 The CIE 1976 Uniform Chromaticity Scale diagram (from the IESNA Lighting Handbook) 1.4.2 The CIE colour spaces All chromaticity diagrams are of limited value for quantifying colour differences because such diagrams are two-dimensional, considering only the hue and saturation of the colour. To completely describe a colour a third dimension is needed, that of brightness for a selfluminous object and lightness for a reflecting object. In 1964, the CIE introduced the U*, V*, W* colour space for use with surface colours, where U* = 13 W* (u – un) V* = 13 W* (v – vn) W* = 25 Y 0.33 – 17 (where Y has a range from 1 to 100)

10

W* is called a lightness index and approximates the Munsell value of a surface colour (see Section 1.4.7). The coordinates u, v, refer to the chromaticity coordinates of the surface colour in the CIE 1960 UCS diagram while the chromaticity coordinates un, vn refer to a spectrally neutral colour lit by the source, that is placed at the origin of the U*, V* system. This U*, V*, W* system is little used now, about the only purpose for which it is routinely used is the calculation of the CIE colour rendering indices (see Section 1.4.4). For other purposes, the U*, V*, W* colour space has been superseded by two other colour spaces known by the initialisms CIELUV and CIELAB.

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L* = (116 (Y/Yn) 0.33 – 16) for Y/Yn > 0.008856 L* = 903.29 (Y/Yn) for Y/Yn ≤ 0.008856 u* = 13 L* (u' – u'n) v* = 13 L* (v' – v'n)

Chapter One: Light

The three coordinates of the CIELUV colour space are given by the expressions:

where u' and v' are the chromaticity coordinates from the CIE 1976 UCS diagram and u'n, v'n, Yn are values for a nominally achromatic colour, usually the surface with 100% reflectance (Y = 100) lit by the light source. The three coordinates of the CIELAB colour space are given by the expressions: L* = 116 f (Y/Yn) – 16 a* = 500 (f(X/Xn) – f(Y/Yn)) b* = 200(f(Y/Yn) – f(Z/Zn))

where f(q) = q 0.33 for q > 0.008856 and f(q) = 7.787 q + 0.1379 for q ≤ 0.008856 q = X/Xn or Y/Yn or Z/Zn Again, Xn, Yn, Zn are the values of the X, Y and Z for a nominally achromatic surface, usually that of the light source with Yn = 100. Each of these colour spaces have a colour difference formula associated with them. For the CIELUV colour space, the colour difference is given by E*uv = ((L*)2 + (u*)2 + (v*)2)0.5 For the CIELAB colour space, the colour difference is given by E*ab = ((L*)2 + (a*)2 + (b*)2)0.5 These two colour spaces are now widely used to set colour tolerances for manufacture in many industries. 1.4.3 Correlated colour temperature While the CIE colourimetry system is the most exact means of quantifying colour, it is complex. Therefore, the lighting industry has used the CIE colourimetry system to derive two single-number metrics to characterise the colour properties of light sources. The metric used to characterise the colour appearance of the light emitted by a light source is the correlated colour temperature. The basis of this measure is the fact that the spectral power distribution of a black body is defined by Planck's Radiation Law and hence is a function of its temperature only (see Section 3.1.1). 11

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Chapter One: Light

Figure 1.8 shows a section of the CIE 1931 chromaticity diagram with the Planckian locus shown. The locus is the curved line joining the chromaticity coordinates of black bodies at different temperatures. The lines running across the Planckian locus are iso-temperature lines. When the CIE 1931 chromaticity coordinates of a light source lie directly on the Planckian locus, the colour appearance of that light source is expressed by the colour temperature, i.e. the temperature of the black body that has the same chromaticity coordinates. For light sources that have chromaticity coordinates close to the Planckian locus but not on it, their colour appearance is quantified as the correlated colour temperature, i.e. the temperature of the isotemperature line that is closest to the actual chromaticity coordinates of the light source. The temperatures are usually given in kelvins (K). As a rough guide, nominally-white light sources have correlated colour temperatures ranging from 2,700 K to 7,500 K. A 2,700 K light source, such as an incandescent lamp, will have a yellowish colour appearance and be described as ‘warm’, while a 7,500 K lamp, such as some types of fluorescent lamp, will have a bluish appearance and be described as ‘cold’. It is important to appreciate that light sources that have chromaticity coordinates that lie beyond the range of the iso-temperature lines shown in Figure 1.8 should not be given a correlated colour temperature. The light from such light sources will appear greenish when the chromaticity coordinates lie above the Planckian locus or purplish if they lie below it.

0 ,00 10

Y

1, 51 5

A

5,000

0.400

2, 00 0

3,3 33

2,5 00

0.500

D65 C

0.300

0

0.200

0.200

0.300

0.400

0.500

0.600

X

Figure 1.8 The Planckian locus and lines of constant correlated colour temperature plotted on the CIE 1931 (x,y) chromaticity diagram. Also shown are the chromaticity coordinates of CIE Standard Illuminants, A, C, and D65 (from the IESNA Lighting Handbook). 1.4.4 CIE colour rendering index The CIE colour rendering index measures how well a given light source renders a set of standard test colours relative to their rendering under a reference light source of the same correlated colour temperature as the light source of interest. 12

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Chapter One: Light

The reference light source used is an incandescent light source for light sources with a correlated colour temperature below 5000 K and some form of daylight for light sources with correlated colour temperature above 5000 K. The actual calculation involves obtaining the positions of a surface colour in the CIE 1964, U*, V*, W*, colour space under the reference light source and under the light source of interest, correcting for any difference in white point under the two light sources and expressing the difference between the two positions on a scale that gives perfect agreement between the two positions a value of 100. The CIE has fourteen standard test colours. The first eight form a set of pastel colours arranged around the hue circle. Test colours nine to fourteen represent colours of special significance, such as skin tones and vegetation. The result of the calculation for any single colour is called the CIE special colour rendering index, for that colour. The average of the special colour rendering indices for the first eight test colours is called the CIE general colour rendering index (Ra). It is the CIE general colour rendering index that is usually presented in light source manufacturers’ catalogues. The CIE general colour rendering index varies widely across light sources (see Section 3.4.10). 1.4.5 Colour gamut The colour gamut of a light source is obtained by calculating the position of the first eight CIE standard test colours under the light source of interest and plotting them on the CIE 1976 UCS diagram. When the plotted positions are joined together, the colour gamut is formed. The colour gamut can be reduced to a single number by calculating the gamut area. Figure 1.9 shows the colour gamuts for a number of different light sources. A great deal can be learnt from the colour gamut. From a consideration of its shape and the spacing between the positions of the individual test colours, the extent to which the different parts of the hue circle can be discriminated is apparent. From its location on the CIE 1976 UCS diagram, the appearance of colours can be appreciated to some degree. By plotting different light sources on the same diagram it is easy to make comparisons between light sources. Further, by including the colour gamut of an ideal light source, such as daylight, it is possible to evaluate how close to the ideal light source is the light source of interest, as far as colour rendering is concerned. 0.58

0.56

*

0.54

*

+

* 0.52

*

+ V'

+

0.50

* *+

0.48

*

* 0.46

+

0.44

*

+

0.42

+

+ 0.14

0.16

0.18

0.20

+ 0.22

0.24

0.26

0.28

= = = = = = =

spectrum locus Planckian locus metal halide high pressure sodium fluorescent incandescent daylight

0.30

0.32

0.34

U'

Figure 1.9 The colour gamuts for high pressure sodium, incandescent, fluorescent and metal halide light sources, and for the CIE Standard Illuminant D65, simulating daylight, all plotted on the CIE 1976 uniform chromaticity scale diagram. The dotted curve is the Planckian locus. 13

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Chapter One: Light

1.4.6 Scotopic/photopic ratio One other measure of light source colour characteristics that has been gaining interest in recent years is the scotopic/photopic ratio (Berman, 1992). This is calculated by taking the relative spectral power distribution, in radiometric units, of the light source and weighting it by the CIE Standard Scotopic and Photopic Observers and expressing the resulting scotopic lumens and photopic lumens as a ratio. The value of scotopic/photopic ratios is that they express the relative effectiveness of different light sources in stimulating the rod and cone photoreceptors in the human visual system. A light source with a higher scotopic/photopic ratio will stimulate the rods more than a light source with a lower scotopic/photopic ratio when both produce the same photopic luminous flux. This information is useful when considering light sources for applications where the operation of both rod and cone photoreceptors is likely. Table 1.4 gives scotopic/photopic ratios for a number of commonly used light sources. Table 1.4 Scotopic/photopic ratios for a number of widely used electric light sources (from He et al., 1997) Light source

Photopic efficacy (lm/W)

Scotopic efficacy (lm/W)

Scotopic/photopic ratio

Incandescent

14.7

20.3

1.38

Fluorescent

84.9

115.9

1.36

Mercury vapour

52.3

66.8

1.28

Metal halide

107.4

181.7

1.69

High pressure sodium

126.9

80.5

0.63

Low pressure sodium

180.0

40.8

0.23

1.4.7 Colour order systems A colour ordering system is a physical, three-dimensional representation of colour space. There are several different colour ordering systems but one of the most widely used is the Munsell system. Figure 1.10 shows the organisation of the Munsell system. The azimuthal hue dimension consists of 100 steps arranged around a circle, with five principal hues (red, yellow, green, blue and purple) and five intermediate hues (yellow-red, green-yellow, bluegreen, purple-blue and red-purple). The vertical value scale contains ten steps from black to white. The horizontal chroma scale contains up to 20 steps from gray to highly saturated. The position of any colour in the Munsell system is identified by an alphanumeric reference made up of three terms, hue, value and chroma, e.g. a strong red is given the alphanumeric 7.5R/4/12. Achromatic surfaces, i.e. colours that lie along the vertical value axis and hence have no hue or chroma, are coded as Neutral 1, Neutral 2 etc. depending on their reflectance. To a first approximation, the percentage reflectance of a surface is given by the product of V and (V–1) of the surface, where V is the Munsell value of the surface. 14

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Chapter One: Light

White 10

Value scale

9 Hue scale

5PB 10B

5B

5P

10P 5RP

10BG

7

10RP

6

5BG 6

8

5

4

3

2

5R 1

1

2

3

4

5

Chroma scale

4

10G

6

10R

5G

5YR

10GY 5GY

10YR

10Y 2

5Y

1

Black

Figure 1.10 The organisation of the Munsell colour order system. The hue letters are B = blue, PB = purple/blue, P = purple, RP = red/purple, R = red, YR = yellow/red, Y = yellow, GY = green/yellow, G = green, BG = blue/green. The existence of several other colour ordering systems, such as the Natural Colour System, the DIN system and BS 5252 system, would seem to be a recipe for confusion. This is avoided by the fact that conversions are available between many of the colour ordering systems. For more detail on the Munsell system, other colour ordering systems and the relationships between them, see the SLL Lighting Guide 11: Surface reflectance and colour.

15

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Chapter Two: Vision

Chapter 2: Vision 2.1 The structure of the visual system The visual system consists of the eye and brain working together. Functionally, the visual system is an image-processing system that extracts specific aspects of the retinal image for interpretation by the brain. 2.1.1 The visual field Humans have two eyes, mounted frontally. Figure 2.1 shows the approximate extent of the visual field of the two eyes in humans, measured in degrees from the point of fixation. The enclosed white area can be seen with both eyes. The shaded area to the left is visible to the left eye only. The shaded area to the right is visible to the right eye only.

100

80 60

40

20

20 40

60

80 100

Figure 2.1 The binocular visual field expressed in degrees deviation from the point of fixation. The shaded areas are visible to only one eye (after Boff and Lincoln, 1988). Given this limited field of view for a fixed position, it is necessary for the two eyes to be able to move. There are two ways this can be done; by moving the head and by moving the eyes in the head. Humans have a limited range of head movements but a wide range of eye movements. 2.1.2 Eye movements The movement of the eye in its socket is controlled by six extra-ocular muscles arranged in opposing pairs. The pattern of eye movements used when examining a visual scene consists of a series of fixations and saccades. Fixations are attempts to keep the retinal image of the object of interest on the fovea. Figure 2.2 shows a pattern of fixation points for two people examining the seams on a pair of briefs. 16

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C

C

C E

Chapter Two: Vision

S

S

C

E

M

D

Figure 2.2 The pattern of fixations made by two inspectors examining men’s briefs held on a frame. S = start of scan path, C = end of scan of front and one side, rotation of frame and continuation of scan across back and sides, E = end of scan. Inspector M examines only the seams while Inspector D examines the fabric as well (after Megaw and Richardson, 1979). Movement between the fixation points is made by saccades. Saccades are very fast, velocities ranging up to 1000 degrees/second depending upon the distance moved. Saccadic eye movements have a latency of about 200 ms, which limits how frequently the line of sight can be moved to about five movements per second. Visual functions are substantially limited during saccadic movements. Fixations and saccades both occur in a single eye, but movements in the two eyes are not independent. Rather, they are coordinated so that the lines of sight of the two eyes are both pointed at the same target at the same time. Movements of the two eyes that keep the primary lines of sight converged on a target, or which may be used to switch fixation from a target at one distance to a new target in the same direction but at a different distance, are called vergence movements. These movements are very slow, up to 10 degrees/second, and can occur as a jump movement or can smoothly follow a target moving in a fore-and-aft direction. Both types of movement involve a change in the angle between the two eyes. 2.1.3 Optics of the eye Figure 2.3 shows a section through the eye, the upper and lower halves being adjusted for focus at near and far distances, respectively. The eye is basically spherical with a diameter of about 24 mm. The sphere is formed from three concentric layers. The outermost layer, called the sclera, protects the contents of the eye and maintains its shape under pressure. Over most of the eye’s surface, the sclera looks white but at the front of the eye the sclera bulges up and becomes transparent. It is through this area, called the cornea, that light enters the eye. The next layer is the vascular tunic, or choroid. This layer contains a dense network of small blood vessels that provide oxygen and nutrients to the next layer, the retina. As the choroid approaches the front of the eye it separates from the sclera and forms the ciliary body. This element produces the watery fluid that lies between the cornea and the lens, called the aqueous humor. The aqueous humor provides oxygen and nutrients to the cornea and the lens, and takes away their waste products. Elsewhere in the eye this is done by blood but on the optical pathway through the eye, a transparent medium is necessary. 17

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Chapter Two: Vision

As the ciliary body extends further away from the sclera, it becomes the iris. The iris forms a circular opening, called the pupil, that admits light into the eye. Pupil size varies with the amount of light reaching the retina but it is also influenced by the distance of the object from the eye, the age of the observer and by emotional factors such as fear, excitement and anger.

Near vision

Sclera Lens rounded

Cornea

Retina Fovea

Iris contracted

Blind spot Pupil

Cilary muscle Optic nerve

Iris opened

Distant vision Lens flattened

Figure 2.3 A section through the eye adjusted for near and distant vision After passing through the pupil, light reaches the lens. The lens is fixed in position, but varies its focal length by changing its shape. The change in shape is achieved by contracting or relaxing the ciliary muscles. For objects close to the eye, the lens is fattened. For objects far away, the lens is flattened. The space between the lens and the retina is filled with another transparent material, the jelly-like vitreous humor. After passing though the vitreous humor, light reaches the retina, the location where light is absorbed and converted to electrical signals. The retina is a complex structure, as can be seen from Figure 2.4. It can be considered as having three layers: a layer of photoreceptors, which can be divided into four types; a layer of collector cells which provide links between multiple photoreceptors, and a layer of ganglion cells. The axons of the ganglion cells form the optic nerve which produces the blind spot where it passes through the retina out of the eye. Light reaching the retina, passes through the ganglion and collector cell layers before reaching the photoreceptors, where it is absorbed. Any light that gets through the photoreceptor layer is absorbed by the pigment epithelium mounted on Bruch’s membrane. 18

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Chapter Two: Vision

Light

Vitreous Retinal ganglion cell axons

Retinal ganglion cells

Collector cells

Receptors

Pigment epithelium Bruch’s membrane Choroid

Figure 2.4 A section through the retina (after Sekular and Blake, 1994) 2.1.4 The structure of the retina The retina is an extension of the brain. The visual system has four photoreceptor types in the retina, each containing a different photopigment. These four types are conventionally grouped into two classes, rods and cones. All the rod photoreceptors are the same, containing the same photopigment and hence having the same spectral sensitivity. The relative spectral sensitivity of the rod photoreceptors is shown in Figure 2.5. The other three photoreceptor types are all cones, each with a different photopigment. Figure 2.6 shows the relative spectral sensitivity functions of the three cone photoreceptor types, called short (S), medium (M) and long (L) wavelength cones.

19

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Log relative luminous efficiency 0 –0.5 –1.0 –1.5 –2.0 –2.5 –3.0 –3.5 –4.0 –4.5 –5.0 400

420

440

460

480

500

520

540

560

580

600

620

640

660

680

700

Wavelength (nm)

Figure 2.5 Log relative luminous efficiency of the rod photoreceptor Relative sensitivity 0.7 Long wavelength cones Medium wavelength cones

0.6

Short wavelength cones 0.5

0.4

0.3

0.2

0.1

0 350

400

450

500

550

600

650

700

750

Wavelength (nm)

Figure 2.6 The relative spectral sensitivities of long wavelength (L), medium wavelength (M) and short wavelength (S) cone photoreceptors (after Kaiser and Boynton, 1996)

20

Rods and cones are distributed differently across the retina (Figure 2.7). Cones are concentrated in one small area that lies on the visual axis of the eye, called the fovea, although there is a low density of cones across the rest of the retina.

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Chapter Two: Vision

Density (thousands /mm2) 200

Cones Rods 150

100

50

0 100

80

60 nasal retina

40

20

0 fovea

20

40

60

80

temporal retina

Eccentricity (degrees)

Figure 2.7 The distribution of rod and cone photoreceptors across the retina. The 0 degree indicates the position of the fovea. The three cone types are also not distributed equally across the retina. The L- and M-cones are concentrated in the fovea, their density declining gradually with increasing eccentricity. The S-cones are largely absent from the fovea; reach a maximum concentration just outside the fovea and then decline gradually in density with increasing eccentricity. Over the whole retina there are approximately 120 million rods and 8 million cones. The fact that there are many more rod than cone photoreceptors should not be taken to indicate that human vision is dominated by the rods. It is the fovea that allows resolution of detail and other fine discriminations and the fovea is entirely inhabited by cones. There are three other anatomical features that emphasise the importance of the fovea. The first is the absence of blood vessels. The second is that the collector and ganglion layers of the retina are pulled away over the fovea. The third is the fact that the outer limb of the cone photoreceptor can act as a waveguide, making cones most sensitive to light rays passing through the centre of the lens. This last characteristic, known as the Stiles-Crawford effect, compensates to some extent for the poor quality of the eye’s optics by making the fovea less sensitive to light passing through the edge of the lens or scattered in the optic media. The fovea is populated only with cones. Rod photoreceptors, which dominate the population of the rest of the retina, do not show a StilesCrawford effect. 21

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Chapter Two: Vision

2.1.5 The functioning of the retina The retina is where the processing of the retinal image begins. Recordings of electrical output from single ganglion cells have shown a number of important characteristics. The first is that the electrical discharge is a series of voltage spikes of equal amplitude. Variations in the amount of light falling on the photoreceptors supplying signals to the ganglion cell through the network of collector cells, produce changes in the frequency with which these voltage spikes occur but not in their amplitude. The second is that there is a level of electrical discharge present even when there is no light falling on the photoreceptors, called the spontaneous discharge. The third is that illuminating photoreceptors with a spot of light, can produce either an increase or a decrease in the frequency of electrical discharges, relative to the level of frequency of discharges present when light is absent. Further studies of the pattern of electrical discharges from a single ganglion cell have revealed two other important aspects of the operation of the retina. The first is the existence of receptive fields. A receptive field is the area of the retina that determines the output from a single ganglion cell. A receptive field always represents the activity of a number of photoreceptors, and often reflects input from different cone types as well as from rods. The sizes of receptive fields vary systematically with retinal location. Receptive fields around the fovea are very small. As eccentricity from the fovea increases, so does receptive field size. Within each receptive field there is a specific structure. Receptive fields consist of a central circular area and a surrounding annular area. These two areas have opposing effects on the ganglion cell’s electrical discharge. Either the central area increases and the annular surround decreases the rate of electrical discharge, or, in other receptive fields, the reverse occurs. These types of receptive fields are known as on-centre/off-surround and off-centre/on-surround fields, respectively. If either of these two types of retinal receptive fields is illuminated uniformly, the two types of effect on electrical discharge cancel each other, a process called lateral inhibition. However, if the illumination is not uniform across the two parts of the receptive field, a net effect on the ganglion cell discharge is evident. This pattern of response makes the retinal fields well suited to detect boundaries in the retinal image. While every retinal ganglion cell has a receptive field, not every ganglion cell is the same. In fact, there are two types of ganglion cell, called magnocellular (M) cells and parvocellular (P) cells. There are a number of important differences between the M-cells and P-cells. First, the axons of the M-cells are thicker than the axons of the P-cells, indicating that signals are transmitted more rapidly from the M-cells than from the P-cells. Second there are many more P-cells than M-cells and they are distributed differently across the retina. The P-cells dominate in the fovea and parafovea and the M-cells dominate in the periphery. Third, for a given eccentricity, the P-cells have smaller receptive fields than the M-cells. Fourth, the M-cells and P-cells are sensitive to different aspects of the retinal image. The M-cells are more sensitive to rapidly varying stimuli and to small differences in illumination but are insensitive to differences in colour. The P-cells are more sensitive to small areas of light and to colour. This brief description shows that the retina extracts information on boundaries in the retinal image and then extracts specific aspects of the stimulus within the boundaries, such as colour. These aspects are then transmitted up the optic nerve, formed from the axons of the retinal ganglion cells, along different channels.

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Lateral geniculate nucleus

Superior colliculus

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2.1.6 The central visual pathways Signals from the retina are transmitted to the visual cortex of the brain over the central visual pathways (Figure 2.8).

Visual cortex

Optic chiasm

Retina

Optic nerve

Optic tract

Cortical cells

Figure 2.8 A schematic diagram of the pathways from the eyes to the visual cortex (from the IESNA Lighting Handbook) The optic nerves leaving the two eyes are brought together at the optic chiasm where the nerves from each eye are split and parts from the same side of the two eyes are combined. This arrangement ensures that the signals from the same side of the two eyes are received together on the same side of the visual cortex. The pathways then proceed to the lateral geniculate nuclei. Somewhere between leaving the eyes and arriving at the lateral geniculate nuclei, some optic nerve fibers are diverted to the superior colliculus, responsible for controlling eye movements, and to the suprachiasmatic nucleus which is concerned with entraining circadian rhythms. After the lateral geniculate nuclei, the two optic nerves spread out to supply information to various parts of the visual cortex, the part of the brain where vision occurs. The visual cortex is located at the back of cerebral hemispheres. About 80% of the cortical cells are devoted to the central ten degrees of the visual field, the centre of which is the fovea, a phenomenon that again emphasises the importance of the fovea. 2.1.7 Colour vision Human colour vision is trichromatic. It is based on the L, M and S cone photoreceptors. Figure 2.9 shows how the outputs from the three cone photoreceptor types are believed to be arranged. The achromatic channel combines inputs from the M- and L-cones only. Its output is related to luminance. The other two channels are opponent channels in that they produce a difference signal. These opponent channels are responsible for the perception of colour. The red-green opponent channel produces the difference between the output of the M-cones and the sum of the outputs of the L- and S-cones. The blue-yellow opponent channel produces the difference between the S-cones and the sum of the M- and L-cones. 23

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S cones M cones L cones

Achromatic channel M+L

S cones M cones

Blue/yellow channel [(M+L) vs. S]

L cones

S cones M cones

Red/green channel [(L+S) vs. M]

L cones

Figure 2.9 The organisation of the human colour system showing how the three cone photoreceptor types are believed to feed into one achromatic, non-opponent channel and two chromatic, opponent channels (after Sekular and Blake, 1994) The ability to discriminate the wavelength content of incident light makes a dramatic difference to the information that can be extracted from a scene. Creatures with only one type of photopigment, i.e. creatures without colour vision, can only discriminate shades of grey, from black to white. Approximately 100 such discriminations can be made. Having three types of photopigment increases the number of discriminations to approximately 1,000,000. Thus, colour vision is a valuable part of the visual system, and not a luxury that adds little to utility.

2.2 Continuous adjustments of the visual system 2.2.1 Adaptation To cope with the wide range of luminances to which it might be exposed, from a very dark night (10–6 cd/m2) to a sunlit beach (106 cd/m2), the visual system changes its sensitivity through a process called adaptation. Adaptation is a continuous process involving three distinct changes. Change in pupil size: the iris constricts and dilates in response to increased and decreased levels of retinal illumination. The maximum change in retinal illumination that can occur through pupil changes is 16 to 1. As the visual system can operate over a range of about 1,000,000,000,000 to 1, this indicates that the pupil plays only a minor role in the adaptation of the visual system.

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Neural adaptation: this is a fast (less than 200 ms) change in sensitivity produced in the retina. Neural processes account for virtually all the transitory changes in sensitivity of the eye at luminance values commonly encountered in electrically lighted environments, i.e. below luminances of about 600 cd/m2. The facts that neural adaptation is fast, is operative at moderate light levels, and is effective over a luminance range with a maximum to minimum ratio of 1000:1 explain why it is possible to look around most lit interiors without being conscious of being misadapted.

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Photochemical adaptation: the sensitivity of the eye to light is largely a function of the percentage of unbleached pigment in each photoreceptor. Under conditions of steady retinal illumination, the concentration of photopigment produced by the competing processes of bleaching and regeneration is in equilibrium. When the retinal irradiance is changed, pigment is bleached and regenerated so as to re-establish equilibrium. Because the time required to accomplish the photochemical reactions is of the order of minutes, changes in the sensitivity can lag behind the irradiance changes. The cone photoreceptors adapt much more rapidly than do the rod photoreceptors. Exactly how long it takes to adapt to a change in retinal illumination depends on the magnitude of the change, the extent to which it involves different photoreceptors and the direction of the change. For changes in retinal illumination of about 2–3 log units, neural adaptation is sufficient so adaptation should be complete in less than a second. For larger changes photochemical adaptation is necessary. If the change in retinal illumination lies completely within the range of operation of the cone photoreceptors, a few minutes will be sufficient for adaptation to occur. If the change in retinal illumination covers from cone photoreceptor operation to rod photoreceptor operation, tens of minutes may be necessary for adaptation to be completed. As for the direction of change, once the photochemical processes are involved, changes to a higher retinal illuminance can be achieved much more rapidly than changes to a lower retinal illuminance. When the visual system is not completely adapted to the prevailing retinal illumination, its capabilities are limited. This state of changing adaptation is called transient adaptation. Transient adaptation is unlikely to be noticeable in interiors in normal conditions but can be significant where sudden changes from high to low retinal illumination occur, such as on entering a long road tunnel on a sunny day or in the event of a power failure in a windowless building. 2.2.2 Photopic, scotopic and mesopic vision This process of adaptation can change the spectral sensitivity of the visual system because at different retinal illuminances, different combinations of retinal photoreceptors are operating. The three states of sensitivity are conventionally identified as follows. Photopic vision: this occurs at luminances higher than approximately 3 cd/m2. For these luminances, the retinal response is dominated by the cone photoreceptors so both colour vision and fine resolution of detail are available. Scotopic vision: this occurs at luminances less than approximately 0.001 cd/m2. For these luminances only the rod photoreceptors respond to stimulation so colour is not perceived and the fovea of the retina is blind. Mesopic vision: this is intermediate between the photopic and scotopic states, i.e. between about 0.001 cd/m2 and 3 cd/m2. In the mesopic state both cones and rod photoreceptors are active. As luminance declines through the mesopic region, the fovea, which contains only cone photoreceptors, slowly declines in absolute sensitivity without significant change in spectral sensitivity, until vision fails altogether as the scotopic state is reached. In the periphery, the rod photoreceptors gradually come to dominate the cone photoreceptors, resulting in gradual deterioration in colour vision and resolution and a shift in spectral sensitivity to shorter wavelengths. The relevance of the different types of vision for lighting practice varies. Scotopic vision is largely irrelevant. Any lighting installation worthy of the name provides enough light to at least move the visual system into the mesopic state. Most interior lighting ensures the visual system is operating in the photopic state. Current practice in exterior lighting ensures the visual system is often operating in the mesopic state.

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All photometric quantities used by the lighting industry are based on the CIE Standard Photopic Observer, i.e. photopic vision. Therefore, it should not come as a surprise when light sources with different spectral content do not have the same effects when used to provide mesopic vision despite being matched photometrically. 2.2.3 Accommodation There are three optical components involved in the ability of the eye to focus an image on the retina, the thin film of tears on the cornea, the cornea itself, and the crystalline lens. The ciliary muscles have the ability to change the curvature of the lens and thereby adjust the power of the eye’s optical system in response to changing target distances; this change in optical power is called accommodation. Accommodation is a continuous process, even when fixating, and is always a response to an image of the target located on or near the fovea rather than in the periphery of the retina. Any condition that handicaps the fovea, such as a low light level, will adversely affect accommodative ability. As adaptation luminance decreases below 0.03 cd/m2, the range of accommodation narrows so that it becomes increasingly difficult to focus objects near and far from the observer. When there is no stimulus for accommodation, as in complete darkness or in a uniform luminance visual field such as occurs in a dense fog, the visual system typically accommodates to approximately 70 cm away.

2.3 Capabilities of the visual system The human visual system has a limited range of capabilities. These limits, conventionally called thresholds, are mainly of interest for determining what will not be seen rather than how well something will be seen. For the threshold measurements shown here the observers were all fully adapted, the target was presented on a field of uniform luminance and the observers’ accommodation was correct. 2.3.1 Threshold measures The threshold capabilities of the human visual system can conveniently be divided into spatial, temporal and colour classes. Spatial threshold measures Spatial threshold measures relate to the ability to detect a target against a background or to resolve detail within a target. Common spatial threshold measures are threshold luminance contrast and visual acuity. The luminance contrast of a target quantifies its visibility relative to its immediate background. The higher is the luminance contrast, the easier it is to detect the target. There are three different forms of luminance contrast. For uniform targets seen against a uniform background, luminance contrast is defined as

C=

Lt – Lb Lb

where: C = luminance contrast Lb = luminance of the background Lt = luminance of the target 26

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Another form of luminance contrast for a uniform targets seen against a uniform background is defined as C = Lt / Lb

Chapter Two: Vision

This formula gives luminance contrasts which range from 0 to 1 for targets which have details darker than the background and from 0 to infinity for targets which have details brighter than the background. It is widely used for the former, e.g. printed text on white paper.

where: C = luminance contrast Lb = luminance of the background Lt = luminance of the target This formula gives luminance contrasts that can vary from 0, when the target has zero luminance, to infinity, when the background has zero luminance. It is often used for selfluminous displays, e.g. computer monitors. For targets that have a periodic luminance pattern, e.g. a grating, the luminance contrast is given by C = (Lmax – Lmin) / (Lmax + Lmin) where: C = luminance contrast Lmax = maximum luminance Lmin = minimum luminance This formula gives luminance contrasts that range from 0 to 1, regardless of the relative luminances of the target and background. It is sometimes called the luminance modulation. Given the different forms of luminance contrast measure, it is always important to understand which is being used. Visual acuity is a measure of the ability to resolve detail for a target with a fixed luminance contrast. Visual acuity is most meaningfully quantified as the angle subtended at the eye by the detail that can be resolved on 50 percent of the occasions the target is presented. This angle is usually expressed in minutes of arc. Using this measure, the visual acuity corresponding to ‘normal’ vision is taken to be 1 min arc. Unfortunately for simplicity, there are several other measures used to quantify visual acuity. One is the reciprocal of the angle subtended at the eye by the detail that can be resolved on 50 percent of the occasions the target is presented. A relative measure is used by the medical profession. This is the distance at which a patient can read a given size of letter or symbol relative to the distance an average member of the population with normal vision could read the same letter or symbol. For example, if the patient is said to have 20/200 vision it means that the patient can only read a given letter at 20 feet that an average member of the population with normal vision can read from 200 feet. Again, given the different forms of visual acuity that are used by different professions, it is important to be sure which metric is being used.

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Temporal threshold measures Temporal threshold measures relate to the speed of the response of the human visual system and its ability to detect fluctuations in luminance. The ability of the human visual system to detect fluctuations in luminance can be measured as the frequency of the fluctuation, in hertz, and the amplitude of the fluctuation, for the stimulus that can be detected on 50 percent of the occasions it is presented. The amplitude is expressed as M = (Lmax – Lmin) / (Lmax + Lmin) where: M = modulation Lmax = maximum luminance Lmin = minimum luminance This formula gives modulations that range from 0 to 1. Sometimes, modulation is expressed as a percentage modulation, calculated by multiplying the modulation by 100. Colour threshold measures Colour threshold measures are based on the separation in colour space of two colours that can just be discriminated, usually measured on the CIE 1931 chromaticity diagram and the related uniform chromaticity scale diagrams. 2.3.2 Factors determining visual threshold There are three distinct groups of factors that influence the measured threshold; visual system factors, target characteristics and the background against which the target appears. Important visual system factors are the luminance to which the visual system is adapted, the position in the visual field where the target appears, and the extent to which the eye is correctly accommodated. As a general rule, the lower the luminance to which the visual system is adapted, the further the target is from the fovea, and the more mismatched the accommodation of the eye is to the viewing distance, the larger will be the threshold values. Important target characteristics are the size and luminance contrast of the target and the colour difference between the target and the immediate background. All three factors interact. For example, the visual acuity for a low luminance contrast, achromatic target will be much larger than for a high luminance contrast, achromatic target when expressed as minutes of arc but will be reduced if there is a colour difference between the target and the background. As for the effect of the background against which the target appears, the important factors are the area, luminance and colour of the background. As a general rule, the larger the area around the target that is of a similar luminance to the target and neutral in colour, the smaller will be the threshold measure. 2.3.3 Spatial thresholds One of the simplest visual tasks is the detection of a spot of light presented continuously against a uniform luminance background. For such a target the visual system demonstrates spatial summation, i.e. the product of target luminance and target area is a constant. This relationship between target luminance and target area is known as Ricco’s Law. It implies that the total amount of energy required to stimulate the visual system so that the target can be detected is the same, regardless of whether it is concentrated in a small spot or distributed over a larger area.

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Given that the size of the target is above the critical size, the detection of the presence of a spot of light is determined simply by the luminance contrast. For the luminance of the surround in the photopic range, there is a constant relationship between the luminance difference of the target and the background and the background luminance known as Weber’s Law. This relationship takes the form

Chapter Two: Vision

Spatial summation breaks down when the target is above about 6 min arc diameter for the fovea, above about 0.5 degree at 5 degrees from the fovea, and above about 2 degrees at 35 degrees from the fovea.

(Lt – Lb) / Lb = k where: Lt = luminance of the target Lb = luminance of the background k = constant A more general picture of the effect of adaptation luminance on threshold contrast for targets of different size is shown in Figure 2.10. The increase in threshold contrast as adaptation luminance decreases is obvious, as is the increase in threshold contrast with decreasing target size. These data were obtained using a disc of different sizes presented for 1 second in the fovea. Decreasing the presentation time and moving the target away from the fovea increases the threshold contrast, for all sizes, particularly at lower adaptation luminances.

Threshold contrast 1000

= 1 min arc = 2 min arc = 4 min arc

100

= 10 min arc = 60 min arc

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Figure 2.10 Threshold contrast plotted against background luminance for disc targets of various diameters, viewed foveally. The discs were presented for 1 second (after Blackwell, 1959). 29

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Threshold luminance contrast is relevant to the detection of targets on a background. Targets with a luminance contrast close to or below the threshold value are unlikely to be seen and targets with a luminance contrast more than twice the threshold value are likely to be seen every time. Figure 2.11 shows the variation in visual acuity with luminance for foveal viewing of the target. As luminance increases, visual acuity, measured as the reciprocal of the minimum gap size, improves, approaching an asymptote at very high luminances corresponding to about 0.45 min arc. Visual acuity deteriorates with increasing deviation from the fovea and improves as the area around the target that has the same luminance increases.

Reciprocal of gap size (min arc)–1 2.5

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Figure 2.11 Visual acuity, expressed as the reciprocal of the minimum gap size, for a Landolt ring, plotted against log background luminance (after Shlaer, 1937) 2.3.4 Temporal thresholds The simplest possible form of temporal visual task is the detection of a spot of light briefly presented against a uniform luminance background, i.e. a flash of light. For such a target the visual system demonstrates temporal summation, i.e. the product of target luminance and the duration of the flash is a constant. This relationship between target luminance and duration is known as Bloch’s Law. It implies that the total amount of energy required to stimulate the visual system so that the target can be detected is the same, regardless of the time for which the target is presented. Temporal summation breaks down above a fixed duration, ranging from 0.1 s for scotopic luminances to 0.03 s for photopic luminances. For presentation times longer than the critical duration, presentation time has no effect, the ability to detect the flash being determined by the difference in luminance between the flash and the background. 30

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An aspect of temporal thresholds relevant to lighting is the ability to detect flicker. Figure 2.12 shows the maximum frequency of a sinewave fluctuation at 100 percent modulation that is visible at different retinal illuminations, for visual fields of different sizes. Retinal illumination is measured in trolands which are the product of the luminance of the stimulus and the associated pupil area. For large field sizes, such as might occur when using indirect lighting, the maximum frequency increases linearly with retinal illumination in the scotopic state, shows little change in the mesopic state and increases linearly in the photopic state until saturation occurs.

Critical fusion frequency (Hz) 60 = 19˚ 50

= 16˚ = 0.3˚

40

30

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–2

–1

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Log retinal illumination (trolands)

Figure 2.12 Critical fusion frequency plotted against log retinal illumination, for three different test field sizes (after Hecht and Smith, 1936) 2.3.5 Colour thresholds Figure 2.13 shows the MacAdam ellipses, ten times enlarged, plotted in the CIE chromaticity diagram. Each ellipse represents the standard deviation in the chromaticity coordinates for colour matches made between the two parts of a 2–degree bipartite field with the reference field having the chromaticity of the centre point of the ellipse. The lighting industry uses four-step MacAdam ellipses as its tolerance limits for quality control in lamp manufacture.

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530

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Figure 2.13 The CIE 1931 chromaticity diagram with the MacAdam ellipses displayed, multiplied ten times (after MacAdam, 1942, from the IESNA Lighting Handbook) 2.3.6 Light spectrum and movement Adaptation luminance and position relative to the fovea are major factors in determining thresholds. Other factors, such as light spectrum and movement of the target are also important. Visual acuity is only slightly influenced by light spectrum, light sources with greater energy at short wavelengths enhance visual acuity. As for movement, as long as the movement is slow enough and smooth enough to allow the retinal image of the target to be kept on the fovea, visual acuity is only slightly worsened. However, smooth movements faster than 40 degrees per second or erratic movement at slower speeds will lead to a dramatic deterioration in visual acuity.

2.4 Suprathreshold performance Threshold measurements are used to define whether or not a target will be seen. When the target can be seen every time, it is said to be suprathreshold and the question of interest becomes how quickly and accurately the work of which the target is a part can be done. The answer to this question depends on the structure of the task. Most apparently visual tasks actually have three components; visual, cognitive and motor. The effect of lighting on task performance depends on the place of the visual component relative to the cognitive and motor components. Tasks in which the visual component is large or limiting will be more sensitive to changes in lighting conditions than tasks where the visual component is small or unimportant. 32

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The impact of lighting conditions on visual performance is determined by the size, luminance contrast and colour difference of the task and the amount, spectrum and distribution of the lighting. An analytical approach using a standard task measured over a wide range of conditions has served to demonstrate, qualitatively, the effects of increasing illuminance on visual performance (Figure 2.14). They are that increasing illuminance follows a law of diminishing returns, i.e. that equal increments in illuminance lead to smaller and smaller changes in visual performance until saturation occurs; that the point where saturation occurs is different for different sizes and contrasts of critical detail; that larger improvements in visual performance can be achieved by changing the task than by increasing the illuminance, at least over any illuminance range of practical interest; and, that it is not possible to make a visually difficult task reach the same level of performance as a visually easy task simply by increasing the illuminance over any reasonable range.

Chapter Two: Vision

It is important to distinguish between task performance and visual performance. Task performance is the performance of the whole task. Visual performance is the performance of the visual component of the task. Task performance is what is needed to measure productivity and estimate cost benefit ratios for lighting. Visual performance is all that lighting conditions can influence directly. Every task has a different relationship between visual performance and task performance depending on the structure of the task. This makes it impossible to generalise from measurements of visual performance to the performance of all tasks.

Mean performance score 0.6

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Figure 2.14 Mean performance scores for Landolt ring charts of different critical size and contrast, plotted against illuminance (after Weston, 1945) 33

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Chapter Two: Vision

While this understanding is useful, it is not enough to make quantitative predictions of the effect of lighting conditions on visual performance for all tasks although it is possible for some. Specifically, the relative visual performance (RVP) model of visual performance (Rea and Ouellette, 1991) has been shown to make accurate predictions for tasks that are dominated by the visual component, that do not require the use of peripheral vision to any extent, that present stimuli to the visual system that can be completely characterised by their visual size, luminance contrast and background luminance only, and that are seen in photopic conditions e.g. reading and doing data-entry work. Figure 2.15 shows the form of relative visual performance produced by this model for a fixed size but variable luminance contrast target and a range of background luminances. This form has been described as the plateau and escarpment of visual performance, the point being that over a wide range of luminance contrasts and background luminances the change in relative visual performance is slight but at some point either contrast or luminance will be so low that performance will start to deteriorate rapidly. The objective of functional lighting is to keep performance on the plateau and well away from the escarpement.

Relative visual performance

1.0

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1 100

Luminance (cd⋅m–2)

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Figure 2.15 The form of relative visual performance plotted against target luminance contrast and background luminance for a fixed target size (after Rea, 1986)

2.5 Visual search

34

One type of work that is outside the RVP model is visual search. Visual search proceeds via a series of fixations joined together by saccades (Figure 2.2). This implies that the target is most likely to be seen first, away from the fovea. For a uniform field, where any departure from uniformity is a target, the probability of off-axis detection can be related to the visibility of the defect. The concept used to model the effect of lighting conditions on search time is the visual detection lobe, i.e. a surface centred on the fovea that defines the probability of detecting the target at different deviations from the fovea within a single fixation pause (Figure 2.16).

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Probability of detection 1.0

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Figure 2.16 The probability of detection of targets of (a) contrast = 0.058, size = 19 min arc; (b) contrast = 0.08, size = 10 min arc; (c) contrast = 0.044, size = 10 min arc; within a single fixation pause, plotted against deviation from the visual axis. Each curve can be used to form a visual detection lobe for each target by assuming radial symmetry about the visual axis. The visual detection lobe has a maximum at the fovea; the probability of detecting the target decreasing as the target is located further off-axis. Different targets have different visual detection lobes. A large-area, high-contrast hole in some sheet material will have a large visual detection lobe while a small-size, low-contrast hole will have a small lobe. The size of the visual detection lobe matters because, provided the interfixation distance is related to it and the total search area is fixed, the total time taken to cover the search area is inversely proportional to the size of the visual detection lobe. Other important factors for determining visibility are the luminance contrast, and the colour of the target relative to the background. There is also the question of what happens when the area to be searched contains other items. For searching uniform, empty fields, it is the visibility of the target off-axis that determines the search time. Where there are other items present, the visibility of the target alone is not enough to predict the search time. The other factor that must be considered is the conspicuity of the target, i.e. how easy it is to distinguish the targets from the other items. For high conspicuity, the defects should differ from the other items in the field on as many dimensions as possible, e.g. size, contrast, shape, colour and movement. Many of the lighting techniques used for visual search are aimed at either increasing the visual size or luminance contrast of the defect, either by casting shadows (Figure 2.17) or by using specular reflections (Figure 2.18). Probably the most widely applicable aspect of lighting which aids visual search is to increase the illuminance on the search area. While illuminance is generally a useful method of reducing search times, it should not be used without thought. If the effect of increasing illuminance is to decrease the luminance contrast, or effective visual size of the targets or to produce confusing visual information in the search area, visual search performance will be worsened. 35

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Chapter Two: Vision Figure 2.17 A cut in textured material lit by directional lighting delivered at a glancing angle to the surface of the material. The cut is visible under the directional lighting because of the high luminance contrast. The high luminance contrast occurs because of the highlights on the sides of the cut and the deep shadow in the cut.

Figure 2.18 A specular aluminium surface with a cross scribed into it, lit by directional lighting from above and behind the camera. The scribed cross is easily seen because the scribed marks cut into the surface and thereby alter the reflection characteristics of the surface. The result is a high luminance reflection towards the camera for the cut and a high luminance reflection away from the camera for the undamaged surface. 36

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There are four situations in which lighting installations may cause visual discomfort. They are: visual task difficulty, in which the lighting makes the required information difficult to extract, under- or over-stimulation, in which the visual environment is such that it presents too little or too much information,

Chapter Two: Vision

2.6 Visual discomfort

distraction, in which the observer’s attention is drawn to objects that do not contain the information being sought, perceptual confusion, in which the pattern of illuminance can be confused with the pattern of reflectance in the visual environment. The occurrence of visual discomfort is made manifest by the occurrence of red, itchy eyes, headaches and aches and pains associated with poor posture. The most common aspects of lighting that cause visual discomfort are insufficient light, too much variation in illuminance between and across working surfaces, glare, veiling reflections, shadows and flicker. 2.6.1 Insufficient light There is insufficient light when the worker approaches the escarpment of the relative visual performance surface for the task (see Figure 2.15). Behavioural signs that there is insufficient light are attempts to move the work to get more light or movements by the worker to get closer to the task. Discomfort caused by insufficient light can be avoided by following the recommendations in the SLL Code for lighting and the guidance given in the application chapters of this Handbook. 2.6.2 Illuminance uniformity Lighting recommendations almost always include an illuminance uniformity criterion. These criteria can be direct or indirect. Direct criteria are ratios of illuminance, typically minimum/maximum or minimum/average measured on the relevant working plane. Indirect criteria are selected to produce a minimum illuminance uniformity ratio, e.g. spacing/mounting height ratio. Such criteria can be considered on different scales. For a whole room where tasks can be anywhere in the room, the minimum/average illuminance ratio on the working plane should not be less than 0.7. This criterion probably only applies where the lighting installation is perceived by the occupants to be intended to produce a uniform distribution of illuminance. In rooms with large windows, the illuminance on a desk close to the window will be much greater than on a desk well back from the window so the illuminance uniformity ratio will be much less than 0.7, but few complaints are heard. Similarly, studies in offices where the luminaires can be individually switched or dimmed have shown that wide variations in the illuminance on desks can be tolerated, without complaint. This suggests that illuminance uniformity limitations are more a design requirement adopted to ensure that no one has insufficient illuminance for their work rather than an intrinsic requirement of the visual system. On the scale of an individual work surface, there are two potential sources of discomfort. Distraction can occur where there are areas of high illuminance adjacent to the work area. 37

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Chapter Two: Vision

Perceptual confusion can occur when the illuminance pattern has a sharp edge so that it could be mistaken for a change in reflectance. The most preferred form of work surface lighting is one that provides a uniform illuminance over the area where the work is to be done (minimum/maximum illuminance ratio > 0.7) and lower illuminances outside that area. 2.6.3 Glare The presence of a luminance much above the average for the visual field will produce discomfort and is called glare. There are five forms of glare associated with lighting installations. Saturation glare This occurs when a large part of the visual field is at a very high luminance for a long time, e.g. sunlight on snow. Saturation glare is painful and the behavioural response is to shield the eyes in some way, e.g. by wearing low transmittance glasses. Adaptation glare This occurs when the visual system is exposed to a sudden, large increase in luminance of the whole visual field, e.g. on exiting a long road tunnel into bright sunlight. The perception of glare is due to the visual system being oversensitive. Adaptation glare is temporary in that visual adaptation will soon adjust the visual sensitivity to the new conditions. It can be avoided by providing a transition zone of intermediate luminance, the transition zone being large enough to allow the visual system time to adapt to the new conditions. Disability glare This occurs when high luminance is present in a low luminance scene. Light from the source is scattered in the eye thereby forming a luminous veil over the retinal image of parts of the scene adjacent to the source. This luminous veil reduces the luminance contrast and desaturates any colours in the retinal image of the adjacent parts of the scene. The magnitude of disability glare is quantified by the equivalent veiling luminance. For glare sources within an angular range of 0.1 to 30 degrees, this is given by the equation:

Lv = 10

∑

En Θ n2

where: Lv = equivalent veiling luminance (cd/m2) En = illuminance at the eye from the nth glare source (lx) Θn = angle of the nth glare source from the line of sight (degrees) The effect of the equivalent veiling luminance on the luminance contrast of an object can be estimated by adding it to the luminance of both the object and the immediate background. Disability glare can be associated with point sources and large area sources. The disability glare formulae can be applied directly to point sources but for large area sources, the area has to be broken into small elements and the overall effect integrated. Disability glare from point sources is experienced most frequently on the roads at night when facing an oncoming vehicle. Disability glare from an extended source can occur when looking at an object on a wall adjacent to a window. The sky seen through the window is the glare source.

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UGR = 8 log10

0.25 Lb

Ls ω p2 2

∑

where: UGR = Unified Glare Rating Lb = background luminance (cd/m2), excluding the contribution of the glare sources. This is numerically equal to the indirect illuminance on the plane of the observer’s eye, divided by π Ls = luminance of the luminaire (cd/m2) ω = solid angle subtended at the observer’s eye by the luminaire (steradians) p = Guth position index

Chapter Two: Vision

Discomfort glare This occurs when people complain about visual discomfort in the presence of bright light sources, luminaires or windows. Discomfort glare is quantified by the Unified Glare Rating (UGR), derived from the equation:

UGR values typically range from 13 to 30, the lower the value, the less the discomfort. Luminaire manufacturers publish UGR values for regular arrays of their luminaires in a number of standardised rooms. This enables comparisons to be made between different luminaire types. When making such a comparison the smallest meaningful difference is one whole unit in UGR. Where a luminous ceiling or uniform indirect lighting is used, discomfort glare is limited by setting a maximum average illuminance. Specifically, if a UGR value of 13 is desired then the average illuminance provided should not exceed 300 lx, for UGR = 16, the maximum average illuminance should not exceed 600 lx and for UGR = 19, the maximum average illuminance should not exceed 1,000 lx. Overhead glare A high luminance immediately overhead can also cause discomfort, even though it cannot be seen when looking directly ahead. The cause of the discomfort is distraction, caused by high luminance reflections from eyebrows, glasses and facial features. The UGR system can be applied to overhead glare to predict the magnitude of the discomfort. 2.6.4. Veiling reflections Veiling reflections are luminous reflections from specular surfaces that physically change the contrast of the visual task and therefore change the stimulus presented to the visual system (Figure 2.19). The two factors that determine the nature and magnitude of veiling reflections are the specularity of the surface being viewed and the geometry between the observer, the surface, and any sources of high luminance. If the surface is a perfectly diffuse reflector, no veiling reflections can occur. If the surface has a specular reflection component, veiling reflections can occur. Veiling reflections occur at positions where the geometry between the observer, the surface and any sources of high luminance is such that the angle of incidence between the surface and the source of high luminance equals the angle of reflection between the surface and the observer. The effect of veiling reflections on the luminance contrast of a specific target may be quantified by adding the luminance of the veiling reflection to the appropriate components of the luminance contrast formulae. What the appropriate components are depends on the reflection properties of the material being viewed. For glossy ink writing on matte paper, the luminance of the veiling reflections should only be added to the luminance of the ink. 39

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Chapter Two: Vision

For a glossy magazine page or a VDT screen veiling reflections occur over the whole surface. In this case the luminance of the veiling reflections should be added to all terms in the luminance contrast formula.

Figure 2.19 A glossy book, with and without veiling reflections Although veiling reflections are usually considered a negative outcome of lighting that can cause discomfort, they can be used positively, but when they are, they are conventionally called highlights. Physically, veiling reflections and highlights are the same thing. Display lighting of specularly reflecting objects is all about producing highlights to reveal the specular nature of the surface. 2.6.5 Shadows Shadows are cast when light coming from a particular direction is intercepted by an opaque object. If the object is big enough, the effect is to reduce the illuminance over a large area. This is typically the problem in industrial lighting where large pieces of machinery cast shadows in adjacent areas. The effect of these shadows can be overcome either by increasing the proportion of inter-reflected light by using high reflectance surfaces or by providing local lighting in the shadowed area. If the object is smaller, the shadow can be cast over a meaningful area which in turn can cause perceptual confusion, particularly if the shadow moves. An example of this is the shadow of a hand cast on a blueprint. This problem can also be reduced by increasing the inter-reflected light in the space or by providing local lighting which can be adjusted in position. 40

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The number and nature of shadows produced by a lighting installation depends on the size and number of light sources and the extent to which light is inter-reflected around the space. The strongest shadow is produced from a single point source in a black room. Weak shadows are produced when the light sources are large in area and the degree of inter-reflection is high.

Chapter Two: Vision

Although shadows can cause visual discomfort, it should be noted that they are also an essential element in revealing the form of three-dimensional objects. Techniques of display lighting are based around the idea of creating highlights and shadows to change the perceived form of the object being displayed. Many lighting designers insist that the distribution of shadows is as important as the distribution of light in achieving an attractive and meaningful visual environment.

2.6.6 Flicker Virtually all electric light sources that operate from an alternating-current supply produce regular fluctuations in the amount of light emitted. When these fluctuations become visible they are called flicker. The probability that a lighting installation will be seen to flicker can be minimised by ensuring a stable supply voltage and by the use of high-frequency electronic control gear for discharge lamps. Incandescent light sources do not require control gear but they are particularly sensitive to fluctuations in supply voltage. Where the local electricity network has equipment attached to it that can impose sudden large loads, e.g. the motors of a steel rolling mill, local fluctuations in supply voltage are likely and, in consequence, so are fluctuations in light output of incandescent light sources. These can be minimised by using a voltage regulator between the electricity supply and the light source. Discharge lamps are less sensitive to supply voltage fluctuations than incandescent lamps because the electricity supply is filtered through the control gear. Electromagnetic control gear typically produces an output at the same frequency as the electricity supply. Electronic control gear for fluorescent lamps typically produces an output at much higher frequencies. Given the time constants of the light producing processes in most discharge lamps, this increase in supply frequency not only produces a higher frequency but also a smaller percentage modulation in light output. Another approach used to reduce the probability of flicker is to combine light from lamps powered from different phases of the electricity supply on the working plane. This results in an increased frequency and a reduced percentage modulation and hence a decrease in the probability of flicker being seen when looking at the working plane. Obviously, it does nothing for the probability of flicker being seen when looking directly at an individual light source. Although flicker occurring over a large area is almost always disturbing, localised flicker does have its uses. Localised flicker is a potent means of attracting attention because peripheral vision is sensitive to changes in the retinal illumination pattern, either in space or time. Localised flicker can also create a stroboscopic effect (see Section 10.2.8).

2.7 Perception through the visual system 2.7.1 The constancies When considering how we perceive the world, the overwhelming impression is one of stability in the face of continuous variation. This invariance of perception is called perceptual constancy. 41

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Chapter Two: Vision

There are four fundamental attributes of an object that are maintained constant over a wide range of lighting conditions. Lightness: lightness is the perceptual attribute related to reflectance. In most lighting situations, it is possible to distinguish between the illuminance on a surface and its reflectance, i.e. to perceive the difference between a low-reflectance surface receiving a high illuminance and a highreflectance surface receiving a low illuminance, even when both surfaces have the same luminance. It is this ability perceptually to separate the luminance of the retinal image into its components of illuminance and reflectance that makes the use of luminance as the basis of lighting design criteria problematical. Colour: physically, the stimulus a surface presents to the visual system depends on the spectral content of the light illuminating the surface and the spectral reflectance of the surface. However, quite large changes in the spectral content of the illuminant can be made without causing any changes in the perceived colour of the surface, i.e. colour constancy occurs. Colour constancy is similar in many ways to lightness constancy. There are two factors that need to be separated; the spectral distribution of the incident light and the spectral reflectance of the surface. As long as the spectral content of the incident light can be identified the spectral reflectance of the surface, and hence its colour, will be stable. Size: as an object gets further away, the size of its retinal image gets smaller but the object itself is not seen as getting smaller. This is because by using clues such as texture and masking, it is usually possible to estimate the distance and then to compensate unconsciously for the increase in distance. Shape: as an object changes its orientation in space, its retinal image changes. Nonetheless, in most lighting conditions the distribution of light and shade across the object makes it possible to determine its orientation in space. This means that in most lighting conditions a circular plate that is tilted will continue to be seen as a tilted circular plate even though its retinal image is elliptical. These constancies represent the application of everyday experience and the integration of all the information about the lighting available in the whole retinal image to the interpretation of a part of the retinal image that bears several alternative interpretations. Constancy is likely to break down whenever there is insufficient or misleading information available from the surrounding parts of the visual field. The constancies are most likely to be maintained when there is enough light for the observer to see the object and the surfaces around it clearly, the light being provided by an obvious but not necessarily visible light source, there are a variety of surface colours, including some small white surfaces and there are no large glossy areas. Lighting conditions used in display lighting sometimes set out to break the constancies, particularly lightness constancy, in order to give the display some drama. 2.7.2 Attributes and modes of appearance While lighting has an important role in preserving or eliminating constancy, it also has a role in determining the perceived visual attributes of objects. Objects can have five different attributes: brightness, lightness, hue, saturation, transparency and glossiness, depending on their nature and the way they are lit. These attributes are defined as follows. Brightness: an attribute based on the extent to which an object is judged to be emitting more or less light.

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Hue: an attribute based on the classification of a colour as reddish, yellowish, greenish, bluish or their intermediaries or as having no colour. Saturation: an attribute based on the extent to which a colour is different from no colour of the same brightness or lightness.

Chapter Two: Vision

Lightness: an attribute based on the extent to which an object is judged to be reflecting a greater or lesser fraction of the incident light.

Transparency: an attribute based on the extent to which colours are seen behind or within an object. Glossiness: an attribute based on the extent to which a surface is different from a matte surface with the same lightness, hue, saturation and transparency. Not all these attributes occur in every situation. Rather, different combinations of attributes occur in different modes of appearance. The four modes of appearance are as follows. Aperture mode: this occurs when an object or surface has no definite location in space, as occurs when a surface is viewed through an aperture. Illuminant mode: this occurs when an object or surface is seen to be emitting light. Object mode (volume): this occurs when a three-dimensional object has a definite location in space with defined boundaries. Object mode (surface): this occurs when a two-dimensional surface has a definite location in space with defined boundaries. Table 2.1 shows which of the attributes can be associated with each mode of appearance. Of particular interest to the perception of lighting is the shift between the attributes of brightness and lightness in different modes of appearance. An object which appears in the self-luminous mode, such as a VDT screen or a light source, is perceived to have a brightness but not a lightness. In this mode of appearance, the concept of reflectance is perceptually meaningless. However, an object that appears in the volume mode, such as a VDT screen or a light source that is turned off, does not have an attribute of brightness but does have a lightness in that its reflectance can be estimated. Table 2.1 The visual attributes that can occur with each mode of appearance Attribute

Aperture

Illuminant

Brightness

*

*

Lightness

Volume

Surface

*

*

Hue

*

*

*

*

Saturation

*

*

*

*

*

*

*

Transparency Glossiness

* 43

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Chapter Two: Vision

A similar transformation occurs between the volume or surface modes of appearance and the aperture mode. Even non-self-luminous objects seen in the aperture mode are perceived as having a brightness but not a lightness. When seen in the object mode they have a lightness but not a brightness. This is important because lighting can be used to change the mode of appearance. For example, a painting hung on a wall has a lightness attribute when lighted so that both it and the wall appear in the object mode (surface). However, if the painting is illuminated solely with a carefully aimed framing spot so that the edge of the beam coincides with the edges of the painting, the painting is seen in the aperture mode and takes on a selfluminous quality with a brightness attribute. Adjusting the modes of appearance is an important technique in display lighting, both indoors and outdoors.

2.8 Anomolies of vision All the capabilities of human vision discussed above assume normal vision. However, there are a number of forms of defective vision that occur due to either genetics or ageing. 2.8.1 Defective colour vision About 8 percent of males and 0.4 percent of females have some form of defective colour vision. People with defective colour vision are classified into three categories: monochromats, dichromats, and anomalous trichromats, according to the number of photoreceptors present and the nature of the photopigments present in the photoreceptors. Monochromats, although very rare, occur in two forms: rod monochromats, where there are no cone photoreceptors, only rod photoreceptors; and cone monochromats, where there are rod photoreceptors and only one type of cone photoreceptor, usually the short-wavelength cone. Rod monochromats are truly colour-blind and see only differences in brightness. Cone monochromats have a very limited form of colour vision in the luminance range where both rod and short-wavelength cones are operating. Dichromats have two cone photoreceptors. They see a more limited range of colours than people with normal colour vision and have a different spectral sensitivity, depending on which cone photoreceptor is missing. Dichromats with the long-wavelength cone missing are called protanopes. Dichromats with the medium-wavelength cone missing are called deuteranopes, while dichromats with short-wavelength cones missing are called tritanopes. Anomalous trichromats have all three cone photopigments present, but one of the cones contains a photopigment that does not have the usual spectral sensitivity. Anomalous trichromats who have a defective long-wavelength photopigment are called protonamalous. Anomalous trichromats who have a defective medium-wavelength photopigment are called deteuranomalous, while anomalous trichromats who have a defective short-wavelength photopigment are called tritanomalous. The colour vision of anomalous trichromats can vary widely from almost as bad as a dichromat to little different from someone with normal colour vision. People with defective colour vision have trouble with some everyday tasks (see Table 2.2) and are prohibited from some occupations. Defective colour vision is usually inherited, although it can also be acquired through age, disease, injury or exposure to some chemicals.

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Activity

Dichromats

Anomalous trichromats

Normal

Selecting clothes, cosmetics etc.

86

66

0

Distinguishing the colours of wires, paints etc.

68

23

0

Identifying plants and flowers

57

18

0

Determining when fruits and vegetables are ripe, by colour

41

22

0

Determining when meat is cooked, by colour

35

17

0

Difficulties in participating or watching sports, because of colour

32

18

0

Adjusting the colour balance of a television satisfactorily

27

18

2

Recognising skin conditions such as a rash or sunburn

27

11

0

Taking the wrong medication because of difficulties with colour

0

3

0

Chapter Two: Vision

Table 2.2 Percentage of people with different types of colour vision reporting difficulties with everyday tasks (from Steward and Cole, 1989)

2.8.2 Low vision As the visual system ages, the ability to focus close up is diminished, the amount of light reaching the retina is reduced, more of the light reaching the retina is scattered, the spectrum of the light reaching the retina is changed and more straylight is generated inside the eye. These changes start in early adulthood and continue at a steady rate with increasing age. The consequences of these changes with age for the capabilities of the visual system are many and varied. At the threshold level, old age is characterised by reduced absolute sensitivity to light, reduced visual acuity, increased contrast threshold, reduced colour discrimination and greater sensitivity to glare. In practice, the elderly have difficulty seeing in dim light, moving from bright to dark conditions suddenly, reading small print and distinguishing dark colours. With increasing age comes a greater likelihood of pathological changes leading to low vision and eventual blindness. The World Health organisation (WHO) defines classes of vision based on visual acuity and visual field size (Table 2.3).

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Chapter Two: Vision

Table 2.3

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The WHO classification of vision (after Tielsch et al, 1990) Grade

Criteria

Normal vision

0

20/25 or better

Near normal vision

0

20/30 to 20/60

Moderate visual impairment

1

20/70 to 20/160

Severe visual impairment

2

20/200 to 20/400

Profound visual impairment

3

20/500 to 20/1000 or a visual field less than 10°

Near-total visual impairment

4

Worse than 20/1000 or a visual field less than 5°

5

No light perception

Category

Low vision

Blindness

Total visual impairment

The prevalence of low vision and blindness increases dramatically after 70 years of age (Table 2.4) The four most common causes of low vision in developed countries are cataract, macular degeneration, glaucoma and diabetic retinopathy. Table 2.4 Percentage prevalence of blindness and low vision for different age groups and races. In this case, blindness is defined as a visual acuity of 20/200 or worse, and low vision is defined as a visual acuity of from 20/40 to 20/200 (after Tielsch et al, 1990) Age range (years)

Blindness (Caucasian)

Blindness (Afro-American)

Low vision (Caucasian)

Low vision (Afro-American)

40–49

0.6

0.6

0.2

0.6

50–59

0.5

0.7

0.7

1.3

60–69

0.2

1.6

1.1

3.4

70–79

0.6

2.9

5.2

8.1

80+

7.3

8.0

14.6

18.0

Cataract is an opacity developing in the lens. The effect of cataract is to absorb and scatter more light on passage through the lens. This results in reduced visual acuity and increased contrast thresholds over the entire visual field, as well as greater sensitivity to glare. Macular degeneration occurs when the macular, which covers the fovea, becomes opaque. An opacity immediately in front of the fovea implies a serious reduction in foveal vision so seeing detail becomes difficult if not impossible. However, peripheral vision is unaffected so the ability to orient oneself in space and to find ones way around is little changed. 46

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Diabetic retinopathy is a consequence of chronic diabetes mellitus and effectively destroys parts of the retina. The effect this has on visual capabilities depends on where on the retina the damage occurs and the rate at which it progresses.

Chapter Two: Vision

Glaucoma is shown by a progressive narrowing of the visual field. Glaucoma is due to an increase in intraocular pressure which damages the blood vessels supplying the retina. Glaucoma will continue until complete blindness occurs unless the intraocular pressure is reduced.

These changes with age can be compensated, to some extent. The limited range of focus of the elderly can be overcome by the use of lenses. The tasks they have difficulty with can be redesigned to make them visually easier, usually by increasing the luminance contrast of the task details, making the task details bigger and using more saturated colours. Lighting can also be used to compensate for aging vision. The elderly benefit more from higher illuminances than do the young, but simply providing more light may not be enough. The light has to be provided in such a way that both disability and discomfort glare are carefully controlled and veiling reflections are avoided. Where elderly people are likely to be moving from a well-lit area to a dark area a transition zone with a gradually reducing illuminance is desirable. People with low vision may or may not benefit from such changes in lighting depending on the specific cause of the low vision. However, there is one approach that is generally useful. This approach is to simplify the visual environment and to make its salient details more visible by attaching high luminance contrast to those details, and only to those details. Figure 2.20 shows an interior where this principle has been applied. Figure 2.20 Contrast in the visual environment

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Chapter Three: Light sources

PART 2. TECHNOLOGY Chapter 3: Light sources 3.1 Production of radiation 3.1.1 Incandescence When an object is heated to a high temperature, the atoms within the material become excited by the many interactions between them and energy is radiated in a continuous spectrum. The exact nature of the radiation produced by an idealised radiator, known as a black body, was studied by Max Planck at the end of the 19th century and he developed the following formula to predict the radiation produced c1 th = M eλ λ5 [exp(c2 / λT) –1] where:

M eλth is the spectral radiant exitance, c1 and c2 are constants, with values

of 3.742 × 10–16 W/m2 and 1.439 × 10–2 m⋅K respectively. λ is the wavelength in metres T the temperature in kelvins.

The values of the spectral radiant exitance are plotted for different temperatures in Figure 3.1. 4500 20 × 106

15 × 106 T (K) 4000

10 × 106

th

Spectral radiant excitance M eλ (W⋅m–2 per µm waveband)

Wien’s Displacement Law

λmax 5 × 106 3500

20 × 106 3000

2500 2000 500

1000

1500

Wavelength (nm)

48

Figure 3.1 Spectral power distribution of radiation according to Planck’s Law

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λmax =

c3 T

c3 has a value of 2.90 × 10–3 mK. The result of the application of this formula is that if an object is heated to a high enough temperature (in excess of 2,000 ˚C) a reasonable amount of light is produced; this provides the basic operating principle of the incandescent lamp.

Chapter Three: Light sources

The wavelength for maximum power (λmax) is inversely proportional to the temperature (T). The following formula was developed by Planck’s co-worker at the University of Berlin and is known as Wien’s Displacement Law.

In practice many materials when heated radiate energy at slightly different rates to that predicted by Planck. This property can be exploited by light source makers. For example tungsten emits about a third more energy as light than would be predicted by Planck’s formula. 3.1.2 Electric discharges An electric discharge is an electric current that flows through a gas. These discharges generally take a high voltage to initiate but once started they can carry considerable currents with very little voltage drop. A good example of such a discharge is the natural phenomenon of lightning. In an electric discharge the electric current is carried by electrons that have been removed from the gas atoms and ions that are gas atoms with one or more electrons removed. This is shown in Figure 3.2.

Electron

+ Cathode

+

+

+ Positive Ion

Current direction

Anode

Figure 3.2 Electric discharge through an ionised gas The negatively charged electrons tend to drift towards the anode whilst the positively charged ions drift towards the cathode. As the ions are several thousand times heavier than the electrons they tend to be less mobile. When an electron collides with an atom, one of three things may happen: (a) The electron rebounds with only a small change in energy – elastic collision (b) The impact excites the atom and the electron loses energy – excitation (c) The impact removes an electron from the atom – ionisation Elastic collisions just heat the gas. Excitation raises the energy state of the atom so that it may radiate light. Ionisation generates more free electrons so that the discharge is maintained. 49

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Excitation

Page 64

Radiation Ion

10.43

Green 546.1

Energy (electron volts)

Chapter Three: Light sources

Ionisation

25/3/09

Blue 435.8

7.73

Violet 404.7

nm

Excited states 5.46 4.89 4.67

Ultraviolet resonance radiation 253.7 nm

Ground state

Figure 3.3 Simplified energy level and transition diagram for mercury The result of any collision between an electron and an atom is largely dependent on the energy of the electron. If the energy of the electron is less than that necessary to raise the atom to the first excited state then the collision will be elastic. The most common transition is between the ground state of the atom and the first excited state. Radiation from the atom returning to ground state tends to dominate the output of the discharge; this radiation is known as resonance radiation. In low pressure discharges, such as low pressure sodium, the light output tends to be at a series of discrete wavelengths, each corresponding to a particular energy transition in the atoms of the gas. In high pressure discharges the atoms of the gas interact with one another and this coupled with the higher electric and magnetic fields in the discharge cause the individual wavelengths found in the low pressure discharge to broaden into wider bands of radiation output. In developing lamps the selection of atoms or molecules that have energy transitions that correspond to radiation in the visible and ultra-violet is important (Figure 3.3). Starting a discharge can be difficult because if there are no ions and free electrons present, the gas will not conduct a current. Most lamps use either a high voltage pulse or heated electrodes covered in special powders to get started. The electrical properties of the discharge are unusual and in general discharges do not obey Ohm’s Law. This is because the current in a discharge is carried by electrons and ions and their number is generally a function of the current, thus at higher currents it is easier for the charge to pass through the discharge and the voltage drops. In order to maintain a steady current through a lamp most discharge lamps require control gear. 50

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Pure semi-conductors have intrinsically a very high resistivity and it is only when they are doped with other materials that it is possible to pass electricity through them. Some materials induce conduction by negatively charged carriers (n-type) and some by positively charged carriers (p-type). When charged carriers of different types recombine the energy released may be emitted as light. See section 3.3.9 for more information on light emitting diodes. Some phosphors can be excited by electrical fields (usually an alternating field) to produce light. The most common material used is zinc sulphide generally doped with another metal such as copper. The process by which the radiation is created is not fully understood. However this has not stopped the process being used to make self luminous signs. For more information on electroluminescent light sources see section 3.3.10.

Chapter Three: Light sources

3.1.3 Electroluminescence Some materials will convert electricity into light directly. Two major physical processes account for the majority of the various electroluminescence phenomena. They are the recombination of current carriers in certain semi-conductors and via the excitation of luminescent centres in certain phosphors.

3.1.4 Luminescence The term luminescence is sometimes also known as fluorescence, or photoluminescence. The process involves a material absorbing radiation and then re-emitting light. The energy may be re-radiated almost immediately or it may take several hours. There are a number of ways that the material can hold the energy and this impacts on length of the time the energy is stored and the amount of energy that is re-radiated.

a

b

c

Figure 3.4 Simplified representations of energy level schemes in luminescence In Figure 3.4 image (a) represents simple luminescence where the material absorbs the energy and the next transition is to re-radiate the energy. In (b) the some of energy in the material is lost via another process before re-radiation takes place. In (c) some of the energy is dissipated and the material falls into a state where it can not re-radiate until it is restored to the higher energy level. This process can lock energy into materials and is the basis of some ‘glow in the dark’ materials. 3.1.5 Radioluminescence This occurs in a similar manner to luminescence but the primary source of the activation energy is particles or gamma rays emitted by a decaying nucleus of a radioactive atom. 51

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Chapter Three: Light sources

3.1.6 Cathodoluminescence In cathodoluminescence the energy driving the phosphor is an electron that has been accelerated away from a cathode. This process is the means by which light is generated in a cathode ray tube. 3.1.7 Chemiluminescence Some chemical reactions can produce light directly, not via the heat the reaction creates. The process is used by some living organisms to generate light; the best known example being the glow worm. 3.1.8 Thermoluminescence This is exhibited by some materials when they are heated. The materials give out much more light than would be expected due to black body radiation. The best known practical use of the method of light production is the mantle used in some types of gas lamps.

3.2 Daylight The sun is a large cloud of high temperature hydrogen gas. It is held together by its own gravitational force. As the atoms of hydrogen are held together at such pressure and high temperature it is possible for nuclear fusion to take place and the hydrogen is converted into heavier elements, mainly helium. This process releases a lot of energy which keeps the sun hot; because the sun is so hot it radiates energy by incandescence. The sun is the biggest source of light on earth. Light from the sun not only gives us light so that we can see, it also powers the whole ecosystem on earth. Light from the sun can reach the earth in two ways: directly as sunlight, and, after it has been modified and redistributed by the atmosphere, as skylight. 3.2.1 Sunlight The key to the understanding of sunlight is knowing where the sun will be in the sky at any given time or date relative to the site in question. On any given day the sun will rise in the east. In the northern hemisphere the sun then rises through the southern sky; reaching its highest altitude at due south at solar noon and passes through the southern sky before setting in the west.

W

S

Figure 3.5 The daily sun path 52

N

E

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Summer solstice

Equinox Winter solstice

Chapter Three: Light sources

At different times of the year the sun follows different paths in the sky (Figure 3.6). The angle of inclination of the path of the sun from the perpendicular is equal to the latitude of the site.

W

S

N

E

Figure 3.6 Annual variations in the sun path To understand the reasons behind these sun paths and to be able to predict the position of the sun at any time it is necessary to consider the relative motions of the sun and the earth. The earth rotates on its axis in approximately 23 hours and 56 minutes and it orbits the sun once per year. The reason that days last approximately 24 hours is that due to its motion around the sun, the earth has to turn a little bit more than one rotation before the same point on its surface is facing the sun again. The orbit of the of the earth around the sun is shown in Figure 3.7.

23˚ 27’ March

Sun June

December

September

Figure 3.7 Orbit of the earth around the sun 53

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The tilt of the earth’s axis away from the normal to the plane of the orbit is what causes the seasonal variation in the sun path. The angle that the sun makes to the earth normal to the equator is known as the angle of declination. To be able to predict the sun’s position in the sky it is first necessary to define a system of angular co-ordinates by which the sun’s position may be described.

W

a

S

N

g

E

Figure 3.8 Angular co-ordinates used to describe the sun’s position There are formulae available to calculate the position of the sun at any time and any date, however care needs to be used in the calculations as they rely on inverse trigonometric functions and it is quite easy to confuse the results as most of the functions may take the same value for more than one angle. 3.2.2 Skylight Whilst equations mentioned in the previous section can predict the position of the sun in the sky, they tell us very little about the distribution of light throughout the sky. This is because the light from the sun is scattered by the atmosphere, and the distribution and amount of light received at ground level is dependent on atmospheric conditions. To calculate the distribution of luminance under various atmospheric conditions the standard BS EN 15469: 2004: Spatial distribution of daylight — CIE standard general sky may be used. This lists a series of 15 sky distributions and gives a formula that may be used for calculating the relative luminance distribution of the sky.

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Type Number

Description of luminance distribution

1

CIE Standard Overcast Sky, steep luminance gradation towards zenith, azimuthal uniformity

2

Overcast, with steep luminance gradation and slight brightening towards the sun

3

Overcast, moderately graded with azimuthal uniformity

4

Overcast, moderately graded and slight brightening towards the sun

5

Sky of uniform luminance

6

Partly cloudy sky, no gradation towards zenith, slight brightening towards the sun

7

Partly cloudy sky, no gradation towards zenith, brighter circumsolar region

8

Partly cloudy sky, no gradation towards zenith, distinct solar corona

9

Partly cloudy, with the obscured sun

10

Partly cloudy, with brighter circumsolar region

11

White-blue sky with distinct solar corona

12

CIE Standard Clear Sky, low luminance turbidity

13

CIE Standard Clear Sky, polluted atmosphere

14

Cloudless turbid sky with broad solar corona

15

White-blue turbid sky with broad solar corona

Chapter Three: Light sources

Table 3.1 CIE standard sky types

The standard helps with the distribution of daylight but it gives no information on the actual amount of daylight available at any particular time. There are a number of stations that record the global and diffuse (not including light direct from the sun) horizontal plane illuminance values on an unobstructed site and these data can be used to predict daylight availability. Whilst data are logged every five minutes or so at most measuring stations it is usually presented as a chart showing monthly averages of hourly values. Figure 3.9 shows a typical chart giving data on daylight availability.

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Chapter Three: Light sources

24

1 klx = 1000 lux

5 klx 18

10 klx

20 klx

12

35 klx

Time of day (GMT)

6

Time of year

0 Winter

Spring

Summer

Autumn

Winter

Figure 3.9 A typical daylight availability chart The colour of the light from the sun and sky depends not only on the colour of the light from the sun but also on the way that light is absorbed and scattered by the atmosphere.

300

350

400

450

500

550

600

650

700

750

800

850

Wavelength (nm)

Figure 3.10 The spectrum of daylight with a colour temperature of 6500 K (from CIE Publication 15.2) Figure 3.10 shows the standardised spectrum of daylight from the CIE Publication 15.2 which gives formulae for the calculation of daylight spectra of different colour temperatures. In practice the sky condition is constantly changing so it is difficult to give exact values of the colour of the sky, however, Table 3.2 lists approximate values of correlated colour temperature for various sky conditions. 56

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Chapter Three: Light sources

Table 3.2 Correlated colour temperatures of the sky Sky condition

CCT (K)

Bright midday sun

5,200

Lightly overcast sky

6,000

Heavily overcast sky

6,500

Hazy sky

8,000

Deep blue clear sky

20,000

3.3 Electric light 3.3.1 Incandescent The incandescent lamp is operated by heating a filament in the lamp to a high temperature, so that it emits light. The basic principle of the lamp may be simple but the technology required to maintain a filament at a high enough temperature to give significant amount of light whilst ensuring the lamp has a reasonable life is highly complex. The basic and most popular form of the lamp is the General Lighting Service (GLS) lamp.

Glass bulb

Tungsten filament

Lead wire

Molybdenum filament supports

Dumet wire

Glass pinch

Balotini filled fuse sleeve

Exhaust tube

Fuse

Cement

Lead wire

Cap

Figure 3.11 The construction of a GLS incandescent lamp

Contacts

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The filament design is critical in setting up the operating characteristics of the lamp. The length of the filament wire is largely determined by the supply voltage, whilst the thickness of the wire is determined by the operating current of the lamp. The filament is coiled to reduce heat convection to the filling gas. There are various forms of filament coiling with the coiled coil being one of the most common (see Figure 3.12).

Figure 3.12 A coiled coil filament The filament must be robust enough to withstand the shocks and vibration that the lamp receives during its life and at the same time be rigid enough so that it does not droop. Support wires can help prevent the filament from drooping but they conduct heat away from the filament and thus reduce the efficiency of the lamp. Therefore normal service lamps are made with hard brittle filaments that only need a few support wires. Lamps for rough service are made with a softer more malleable filament but have several support wires. The bulb is generally made of a soft soda glass and its size is set so that it does not get too hot and the tungsten that evaporates from the filament during the life of the lamp does not blacken the bulb too much. The gas filling of the lamp is present to reduce the rate at which the tungsten evaporates and thus make the lamp last longer. To minimise the heat losses from the filament noble gasses are used as the primary fill gases. Most lamps have argon based filling but some high performance lamps use krypton. In addition to the noble gas filling most mains voltage lamps have a small percentage of nitrogen added to the filling to help suppress arcing at the end of life. There are many variations on this basic lamp type. They are designed to run on voltages between 1.5 and 415 volts at wattages between 1 and 1,000 watts. There is also a wide variety of bulb shapes including lamps with built in reflectors.

Figure 3.13 Forms of incandescent lamp 58

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Lamp wall temperature 250 ˚C minimum

Chapter Three: Light sources

3.3.2 Tungsten halogen The applications of conventional incandescent lamps are limited by their physical size and luminous efficiency. Raising the filament temperature to increase the luminous output has the effect of increasing the rate of blackening of the glass envelope, blackening which is a result of the evaporation of tungsten from the filament. By adding a halogen to the gas fill a chemical transport cycle involving the reaction of tungsten reduces the amount of blackening of the envelope. It is then possible to reduce the size of lamp, increase the pressure of the filling gas and thereby limit the loss of the tungsten from the filament.

Ceramic

Fil seal 350 ˚C maximum

Filament support

Zone 1

Zone 2

Zone 3

Tungsten filament

Figure 3.14 A representation of the tungsten halogen cycle The chemistry of the tungsten halogen cycle is highly complex. However the key stages are: the halogen combining with the tungsten on the wall of the lamp (zone 3) the tungsten halide vapour mixing with the fill gas of the lamp (zone 2) the tungsten halide dissociating close to the filament of the lamp, leaving the halogen free to migrate though the fill gas to the lamp wall again and the tungsten being deposited on the filament (zone 1). To enable an efficient cycle it is necessary for the wall of the lamp to run at a temperature above 250 ˚C; this means that the bulb has to be made from quartz or hard glass. 59

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Chapter Three: Light sources Figure 3.15 Forms of tungsten halogen lamps Tungsten halogen lamps are more efficient and have longer lives compared with standard tungsten lamps. Also they are more compact than standard lamps. However they are more expensive as it is hard to make the quartz outer bulb and it is harder to introduce the gas fill into the lamp due to the high filling pressure. 3.3.3 Fluorescent Fluorescent lamps are the most commonly used form of discharge lamp. They come in a variety of shapes and sizes and are available in a wide range of colours. The original form of the lamp was a long straight tube. New forms of the lamp known as compact fluorescent lamps have been developed where the lamp tube is bent or folded to produce a smaller light source. Fluorescent lamps work by generating ultraviolet radiation in a discharge in low pressure mercury vapour. This is then converted into visible light by a phosphor coating on the inside of the tube. The electric current supplied to the discharge has to be limited by control gear to maintain stable operation of the lamp. Traditionally this is done with magnetic chokes but most circuits now use high frequency electronic control gear. Electronic control gear has a number of advantages: first, driving the lamp at high frequency maintains the ions in the gas and thus makes the lamp run more efficiently. Secondly, it reduces the amount of flicker in the lamp and, finally, electronic gear consumes less power than a magnetic choke.

Ultraviolet radiation

Visible radiation

Fluorescent powder

Mercury atom

Figure 3.16 Working principle of a fluorescent lamp 60

Electrons

Electrode

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The tube: this is made from a glass with a high iron content so that any short wave UV radiation that gets through the phosphor coating is absorbed by the glass The phosphor coating: there are a wide variety of phosphors available. Each produces a different spectrum of light and by careful blending of the various phosphors lamp makers can tailor a wide range of lamp colours. The lumen output of the lamp also depends on the choice of phosphor mix. It is also important to control the particle size of the phosphor powders and the thickness of the coating. There are three main types of phosphor mixes currently used in fluorescent lamps:

Chapter Three: Light sources

The lamps are made from the following main components.

Halophosphates: this range of phosphors tend to emit light in a relatively wide band and it is normal to use only one phosphor of this type at any one time. Halophosphates are only reasonably efficient as phosphors and generally have poor colour rendering. Tri-phosphors: are mixes of three narrow band phosphors. They generally achieve CIE general colour rendering indices greater than 80 and have a high efficacy and good lumen maintenance. Multi-phosphors: are mixes of a number, usually five, phosphors. These mixes usually give a CIE general colour rending index higher than 90, however the efficacy is normally lower than a tri-phosphor mix. The electrodes: generally coils of tungsten wire that are coated in a material that when heated will give off electrons readily. To start the lamp a current is passed through the coil to heat the emissive coating. However, once the lamp is running the ionised gas atoms hitting the electrode provide enough energy to keep the cathode hot. The electrodes are generally surrounded by a shield as some of the material used to coat the electrode evaporates during the life of the lamp. If the shield was not there the material would be deposited on the wall of the lamp causing a black ring and reducing the light output. The gas fill: the lamp fill is made up of two components; a noble gas mixture and the mercury vapour. The noble gas in the lamp has three main functions. First, it reduces the mobility of the free electrons in the lamp and by careful control of the pressure; it optimises the number of electrons with the right amount of energy to excite the mercury atoms. Secondly, the gas reduces the rate at which the coatings on the electrodes evaporate and thus prolongs the life of the lamp. Finally it lowers the breakdown voltage of the lamp and thus makes starting easier. Most lamps use either a mixture of argon and krypton or neon and argon. The use of the heaver krypton gas makes the lamps slightly more efficient but it is significantly more expensive. The vapour pressure of mercury in the lamp is significantly lower than the pressure of the noble gas mixture and it is controlled by the temperature of the coolest part of the lamp. At the cold spot of the lamp the mercury condenses to form liquid mercury. At this point the liquid and gaseous mercury are in equilibrium and the vapour pressure is determined by the temperature. As the vapour pressure of mercury is critical to the operation of the lamp, the light output of the lamp varies with temperature. Most lamps are optimised to run in an environment with an ambient temperature of 25 ˚C. 61

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100

Relative luminous flux (%)

Chapter Three: Light sources

However, some of the new types of lamp are set up to run in an ambient temperature of 35 ˚C. In some lamp types the mercury dose is mixed with other metals such as bismuth or indium. These metals form an amalgam with the mercury and this reduces the vapour pressure of the mercury at any given temperature. This enables the lamp to operate at higher temperatures but has the drawback that the lamp takes a long time to reach full output.

90

Amalgam lamps

80 Standard lamps

70

60

50 10

20

30

40

50

60

70

80

Ambient temperature (˚C )

Figure 3.17 Luminous flux as a function of temperature for standard and amalgam fluorescent lamps. 100 percent corresponds to the maximum luminous flux. There are two main types of fluorescent lamps; the traditional linear lamps and the compact fluorescent lamps. Linear lamps come in variety of diameters and lengths. The main diameters of lamp are the T12 lamps which are 38 mm in diameter, T8 lamps which are 25 mm and the T5 types which are 16 mm. All of these families of lamps come in a variety of lengths and wattages. Linear fluorescent lamps are generally efficient light sources with some of the lamps approaching 100 lumens per watt. They also come in a wide variety of colours with a range of colour rendering properties. Table 3.3 gives a summary of the main lamp colours.

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Colour appearance

Triphosphor colour rendering group 1b

Multi-phosphor colour rendering group 1a

Northlight (6000–6500 K)

Colour 865 Lumilux Plus ECO 860 Luxline Plus ECO 860 Polylux XLR 860

Colour 965

Daylight (5000–5500 K)

Colour 950 Lumilux De Luxe 950 Colour 840 Lumilux Plus ECO 840 Luxline Plus ECO 840 Polylux XLR 840

Cool White (4000 K) Intermediate (3500 K) White

Colour 835 Lumilux Plus ECO 835 Luxline Plus ECO 835 Polylux XLR 835

Warm White (3000 K)

Colour 830 Lumilux Plus ECO 830 Luxline Plus ECO 830 Polylux XLR 830

Very Warm (2700 K)

Colour 827 Lumilux Plus ECO 827 Luxline Plus ECO 827 Polylux XLR 827

Colour 940 Lumilux De Luxe 940 Polylux Deluxe 940

Chapter Three: Light sources

Table 3.3 Colours of fluorescent lamps

Colour 930 Lumilux De Luxe 930 Polylux Deluxe 930

There is a large variety of compact fluorescent lamp types. Figure 3.18 below illustrates the range.

Quad-lamp

Twin-tube

Triple-twin

Circline

F-lamp

Oct lamp

2-D

Helical

Figure 3.18 Types of compact fluorescent lamp 63

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In general compact fluorescent lamps are less efficient than linear lamps, but because of their small size, they are suited to many applications where a smaller lamp is needed. Some of the lamps have the control gear built into them and can be retro-fitted into GLS lamp sockets. 3.3.4 High pressure mercury In this type of lamp a discharge takes place in a quartz discharge tube containing mercury vapour at high pressure (2 to 10 atmospheres). Some of the radiation from the discharge occurs in the visible spectrum but part of the radiation is emitted in the ultraviolet. The outer bulb of the lamp is coated internally with a phosphor that converts this UV radiation into light. The general construction of the lamp is shown in Fgure 3.19 below.

Support for discharge tube

Discharge tube

Outer bulb

Main electrode Auxiliary electrode

Resistor

Lamp cap

Figure 3.19 Construction of a high pressure mercury lamp 64

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First ignition; when power is first applied to the lamp the voltage is not high enough to strike an arc between the two main electrodes. Ignition is achieved using an auxiliary electrode placed close to one of the main electrodes. The auxiliary electrode is connected via a resistor (typically 25,000 ohms). This limits the size of the current in the arc formed by the auxiliary electrode so the voltage across the starting arc is reduced as the current increases. This means that the ions in the arc are drawn towards the main electrode at the other end of the lamp and these ions allow the main arc to start. The next stage is the run-up. Once the arc has started between the main electrodes very little light is given out because the mercury pressure is too low as the tube is cool. The arc in the gas slowly warms up the tube and so the mercury vapour pressure rises and the light output increases. Typically it takes about 4 minutes for the lamp to achieve 80% of the final light output.

Chapter Three: Light sources

The operation of the lamp is quite complex and needs to be considered in three phases: ignition, run-up and stable running.

When the lamp reaches stable running and normal operating pressure all the mercury in the lamp is in the vapour phase. This means that the vapour pressure of the mercury is controlled by the amount of mercury put into the lamp rather than the temperature of the lamp. High pressure mercury lamps are made from the following main components. The discharge tube is generally made of quartz and has the main electrodes and the starting electrode sealed into it. The main electrodes are usually made of tungsten rods which have coil of tungsten wire wrapped round them. This coil is usually impregnated with emitter material similar to that used in fluorescent lamps. The auxiliary electrode is generally wire made out of molybdenum or tungsten. The fill gas in the discharge tube is commonly argon and a very carefully controlled dose of mercury is also added. The discharge tube is fitted into a support frame and the whole assembly is sealed into the outer bulb. The gas fill in the outer bulb is usually nitrogen or argon or a mixture of the two. The pressure of this fill gas is controlled to ensure that the arc tube operating temperature is correct. The outer bulb is made out of a soft soda lime glass for low wattage lamps (up to 125 W). High power lamps use a borosilicate glass outer. There are two common shapes for the outer bulb the ovoid or isothermal bulb, and the reflector bulb. Figure 3.20 show these two shapes.

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Chapter Three: Light sources Figure 3.20 Forms of high pressure mercury lamps The performance of these lamps is not considered to be very good nowadays. Their efficiency is around 40 lumens per watt. Their CIE general colour rendering index is between 40 and 50 and they have a very long life but, because of poor lumen maintenance, it is generally recommended that the lamps are changed after 8,000 to 10,000 hours of use. Because of their poor performance and the fact that better lamp types are available for almost all of the applications these lamps are being phased out. 3.3.5 Metal halide Metal halide lamps were developed as a way of improving the performance of high pressure mercury lamps in terms of their colour appearance and light output. They work by introducing the salts of other metals into the arc tube. As each element has its own characteristic spectral line, by adding a mixture of different elements into the discharge it is possible to create a light source with good colour rendering in a variety of colours. There are a lot of problems with introducing new elements into a discharge. First, the element must be volatile and secondly it should not chemically attack the arc tube. To avoid these problems it has become common practice to introduce metals into the lamp as metal halides. Metal halides are generally more volatile than the metals themselves and the metal halides do not attack the arc tube. The metal halide compound breaks up into the metal and halogen ions at the high temperatures in the centre of the discharge and reforms at the lower temperatures near the wall of the tube. Many different combinations of elements have been used to make metal halide lamps, Figure 3.21 lists some of the more common combinations of elements together with the spectral output they create. 66

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Li ln

400

500

600

Tl

Relative spectral power

Relative spectral power

ln

300

Dy, (Tm, Ho, Tl) T = 5600 K Ra = 86

ln, Ti, Na T = 4500 K Ra = 61

Na

Tl

Dy

300

700

400

500

600

Three colour (indium, thallium, sodium) metal halide lamp. The lithium line is due to impurities in quartz of the tube wall.

Dysprosium lamp with thulium, holmium and thallium additives.

Na

400

Sc Hg Sc Sc

Sc

500

Tm, (Dy, Ho, Tl, Na) T = 4300 K Ra = 87

Relative spectral power

Relative spectral power

Sc, Na T = 3800 K Ra = 56

300

700

Wavelength (nm)

Wavelength (nm)

Sc Hg

Chapter Three: Light sources

Figure 3.21 Relative spectral power distributions of metal halide lamps

Na Tl

Dy

Li

Li

600

700

Wavelength (nm)

300

400

500

600

700

Wavelength (nm)

Scandium lamp with sodium additive.

Thulium lamp with a range of additives.

Relative spectral power

SnI2/SnCl2(Na) T = 3000 K Ra = 74

Na Hg

300

400

500

Li

600

700

Wavelength (nm)

Tin halide lamp with sodium additive. 67

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Chapter Three: Light sources

Because of the differing lamp chemistry there is a wide range of lamps that vary in terms of their efficacy, colour and electrical properties. One of the main problems with metal halide lamps that use quartz discharge tubes is colour stability. As the colour of the light output is a function of the ions present in the discharge tube, any changes to the gas composition due to some metals being absorbed by the quartz tube or changes in temperature in the tube can cause significant colour shifts. These colour shifts are particularly a problem for the lower wattage lamps. This problem has largely been solved by the introduction of a new material for the discharge tube. Ceramic or sintered alumina tubes are much more resistant to chemical attack than quartz tubes and can operate at higher temperatures. Lamps with these tubes are now very popular for low wattage (up to 150 W) metal halide lamps. The construction of a metal halide lamp is similar to that of a high pressure mercury lamp. The key differences are that it is unusual to use an auxiliary electrode in the lamp, lamp ignition being achieved using a high voltage pulse from the control gear. Also, there is no phosphor coating on the outer bulb. There are a wide variety of shapes of lamp. Figure 3.22 shows some of them.

Figure 3.22 Forms of metal halide lamps 68

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There are many points to watch for when selecting metal halide lamps as there are problems associated with some lamp types shattering at the end of life or giving off UV radiation. It is important with these lamps to ensure that the luminaire in which they are used is suitable. 3.3.6 Low pressure sodium Low pressure sodium lamps are similar in many ways to fluorescent lamps as they are both low pressure discharge lamps. All the differences in characteristics stem from the use of sodium in the discharge tube rather than mercury. The key differences are the need to run the lamp hotter to maintain the vapour pressure of sodium, the need to contain the very reactive sodium metal; and the fact that sodium emits its light in the visible rather than the UV frequency range, so there is no need for a phosphor layer.

Chapter Three: Light sources

There is a vast range of metal halide lamps ranging in power between 20 W to over 2 kW. The lamps have a CIE general colour rendering index between 60 and 93 and they have high luminous efficacies, in the range 60 to 98 lumens per watt. For these reasons, this lamp type has many applications where a compact light source with good colour rendering is needed.

There used to be a range of designs for sodium lamps but currently the U-tube lamp is by far the most common type. A typical lamp of this design is shown in Figure 3.23.

Bend Isolation

Discharge tube

Figure 3.23 The construction of a low pressure sodium lamp

Dimple filled with sodium

Outer bulb

Electrode

Getter

Lamp cap

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The main components of a low pressure sodium lamp are as follows. The arc tube; this is made of normal soda lime glass with a coating on the inside of a special sodium resistant aluminoborate glass. Making this ‘ply-glass’ tube is technically difficult as great care is needed to ensure that there are no thermal stresses in the final tube that might lead to cracking during the life of the lamp. Some lamp types have dimples in the side of them to act as reservoirs of sodium. The gas fill of the tube is neon with about 1% of argon at a pressure of approximately 1000 Pa. This mixture is used as it has a much lower breakdown voltage than neon on its own and thus makes starting the lamp much easier. Sodium metal is also put into the tube. The sodium vapour pressure in the tube when it is at its operating temperature of 260 ˚C is about 0.7 Pa. The outer bulb is of soda lime glass, the inside is coated with a layer of indium oxide. This layer reflects the bulk of the infrared radiation from the arc tube and thus keeps it warm. Between the outer bulb and the arc tube the gas pressure is very low, below 0.01 Pa. To maintain the vacuum a barium getter is used. A relatively high voltage is needed to start an arc in the neon fill gas. The arc then slowly warms up the lamp and the discharge tube and the vapour pressure of the sodium starts to rise until the lamp reaches thermal stability after about 15 minutes. One of the curious properties of the sodium atom is the predominance of the energy transitions associated with the two spectral lines at 589 nm and 589.6 nm. This means that virtually all the visible radiation from the lamp is given off in this very narrow band. However, sodium atoms will also re-absorb and re-emit the radiation very readily; this means that nearly all the light emerging from a low pressure sodium lamp has come from close to the arc tube wall. The light from a low pressure sodium lamp is a wavelength close to the peak of the photopic sensitivity curve, and as the lamp is relatively efficient at converting electricity into visible radiation, the lamp is one of the most efficient light sources in terms of lumens per watt. The best of the range can achieve in excess of 180 lumens per watt. The problems with the lamp are large size, long run-up time and monochromatic light that does not render colours. The lamp has been mainly used for street lighting but recently the importance of some colour rendering on roads has been recognised and the lamp is rarely used in new installations. 3.3.7 High pressure sodium The high pressure sodium lamp generates light in a discharge through sodium vapour at high pressure. As the vapour pressure of sodium in a lamp rises the spectrum at first broadens and then it splits in two with a gap appearing at about 586 nm. Figure 3.24 shows the spectra from sodium lamps with different vapour pressures.

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400

500

600

T = 2000 K Ra = 23

Relative spectral power

Relative spectral power

T = 1700 K

700

400

Wavelength (nm)

600

700

T = 2500 K Ra = 80

Relative spectral power

Relative spectral power

T = 2150 K Ra = 40

500

600

Standard high pressure sodium lamp; sodium vapour pressure 10 kPa

Low pressure sodium lamp; sodium vapour pressure 0.7 Pa

400

500

Wavelength (nm)

Chapter Three: Light sources

Figure 3.24 The spectra of sodium lamps with different vapour pressures of sodium

700

400

Wavelength (nm)

500

600

700

Wavelength (nm)

Colour improved high pressure sodium lamp; sodium vapour pressure 40 kPa

White high pressure sodium lamp; sodium vapour pressure 95 kPa

As the vapour pressure rises the colour rendering of the lamp increases. However, this is at the expense of efficacy in terms of lumens per watt. Figure 3.25 shows the construction of a high pressure sodium lamp.

Top support

Discharge tube of sintered alumina Hellical support wire Outer bulb

Figure 3.25 The construction of a high pressure sodium lamp

Electrode Flexible connection to allow for thermal expansion Getter

71

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The main components used in the construction of the lamp are as follows. The arc tube is made of polycrystalline alumina (PCA). This material is ceramic rather than a glass, this makes it very hard to work as it is not possible to soften it and it is hard to cut. PCA is used because it is resistant to chemical attack by hot sodium, it is stable at high temperatures and it is transparent. Because it is not possible to work the PCA the tube is cut to length and fitted with end caps, Figure 3.26 shows some of the designs used for closing the ends of the discharge tube.

Monolithic arc tube

Alumina disc

Niobium support wire

Seal glass

Alumina disc Seal glass

Niobium tube

Niobium wire

b

a

Metallic braze

Titanium braze

Alumina bushing

Seal glass

Niobium tube

Niobium cap

c

Niobium exhaust tube

d

Figure 3.26 Types of arc tube seal in high pressure sodium lamps The use of niobium metal as part of the end cap assembly is common as it expands with temperature at the same rate as the PCA tube and thus does not cause stresses in the lamp as it heats up. The electrodes in the lamp are made from tungsten rods with tungsten wire wound around them, with emitter material made from oxides of metals such as barium, calcium and yttrium. 72

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The fill gas in the tube is usually xenon at a cold pressure of 3 kPa, which corresponds to an operating pressure of about 20 kPa. A higher xenon pressure would improve lamp efficacy but make starting harder as it needs a high voltage to break down. Some types of lamp use high pressure xenon and use an ignition wire held close to the tube to help starting. There are also some lamps that use argon as a fill gas; they are much easier to start but are less efficient in term of lumens per watt. A dose of sodium mercury amalgam is used in most high pressure sodium lamps. Mercury is used because its vapour acts as a buffer gas and helps improve the efficiency of the lamp. However, the mercury contributes very little to the output spectrum of the lamp. Some lamps are now made without mercury in them. The absence of mercury makes the disposal of the lamp at the end of life easier as there are no environmentally damaging substances in the lamp. The metal dose in the lamp is never fully vapourised and so the pressure of the sodium and mercury vapours in the lamp is dependent on the temperature of the coolest part of the discharge tube. This makes the output of the lamp temperature dependent and can also give problems associated with the voltage across the tube rising if the lamp gets too hot. The cold spot on most discharge tubes is in the area behind the electrode. As this area of the tube is blackened through the life of the lamp, the cold spot temperature tends to rise through life. This can give rise to problems in old lamps where the pressure in the discharge tube rises to the point where it is no longer possible for the voltage available from the supply to sustain an arc in the lamp. The discharge tube is mounted into a support frame and sealed into an outer bulb. The outer bulb is generally made of a borosilicate glass and may be in a number of different shapes, Figure 3.27 shows some of the more common shapes.

Tubular outer bulb

Linear double ended in a quartz outer bulb

Ellipsoidal or isothermal coated outer bulb

Reflector bulb

Figure 3.27 Outer bulb shapes for high pressure sodium lamps 73

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The high pressure sodium lamp is an efficient source of light (efficacies up to 142 lumens per watt), it has a long life with reasonable lumen maintenance and whilst the colour rendering on the standard lamp is poor it is acceptable for a number of applications. The white high pressure sodium lamp has a spectrum with minimal output in the yellow. This has the property of making a large number of colours appear more vivid and so this lamp has a number of applications in retail lighting. 3.3.8 Induction Induction lamps are essentially gas discharge lamps that do not have electrodes. Instead the electric field in the lamp is induced by an induction coil that is operating at high frequency. The only types of induction lamps that are currently in production are based on fluorescent lamp technology. Figure 3.28 shows the layout of a cavity type lamp. Phosphor coating Electron/ion plasma

Figure 3.28 Construction of a cavity type induction lamp

Plastic housing Induction coil

Electronics

The lamp consists of a glass bottle with a cavity in it into which the induction coil is placed. The glass vessel has a gas filling similar to a conventional fluorescent lamp and the phosphor coating on the inside of the lamp is also similar. The induction coil in the centre of the lamp is fed from a high frequency generator. An alternative architecture for this type of lamp is to have the induction coil wrapped around a toroidal lamp. Figure 3.29 shows a lamp of this type.

Figure 3.29 An external coil induction lamp

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3.3.9 Light emitting diodes The basic operating principle behind light emitting diodes (LEDs) is covered in section 3.1.3. LEDs are available in a wide variety of sizes, colours and power ratings and development is proceeding at a rapid rate (see the Lighting Industry Federation LED Guide 2005 and the EBV Electronik and Philipslumileds web sites). Whilst LEDs come in a variety of styles, Figure 3.30 illustrates two common forms.

Chapter Three: Light sources

Induction lamps have many of the same properties as fluorescent lamps. They are, however, slightly less efficient. The big advantage with this type of lamp is long life. This is because there are no electrodes to fail and the inside of lamp does not get coated with material that has been vapourised away from the electrodes. A number of lamps of this type have rated lives of 100,000 hours. These lamps are more expensive than conventional fluorescent lamps so they tend to be used in places where it is difficult to change lamps and thus long life is an important requirement.

The main components of a LED are as follows. The chip of semiconductor material in the centre of the lamp may be made of a wide variety of materials. Differing materials result in a different colour of light being produced Table 3.4 lists some of the more commonly used materials. Table 3.4 Materials used in LEDs and the radiation produced Materials

Radiation

Aluminum gallium arsenide (AlGaAs)

Red and infrared

Aluminum gallium phosphide (AlGaP)

Green

Aluminum gallium indium phosphide (AlGaInP)

Orange-red, orange, yellow, and green

Gallium arsenide phosphide (GaAsP)

Red, orange-red, orange, and yellow

Gallium phosphide (GaP)

Red, yellow and green

Gallium nitride (GaN)

Green, pure green (or emerald green), and blue

Indium gallium nitride (InGaN)

Near ultraviolet, green, bluish-green and blue

Zinc selenide (ZnSe)

Blue

Aluminum nitride (AlN), Aluminum gallium nitride (AlGaN)

Near to far ultraviolet

Diamond (C)

Ultraviolet

The chip is mounted onto one of the lead in wires. In high power LEDs the mounting is designed in such a way as to conduct heat away from the chip. The other lead wire is bonded to the chip generally connecting to a very small area close to the actual semi conductor junction. The whole device is then potted in a plastic resin, usually epoxy. 75

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Light emitted forward

Light emitted forward

LED Chip Epoxy housing

Plastic lens Reflector Silicon encapsulation PC board

Cathode lead

InGan LED chip Cathode lead

Anode lead

Solder connection

Gold wire Heat slug

Reflector cup

Figure 3.30 The construction of low power (left) and high power (right) LEDs LEDs generally have a long life and may last up to 100,000 hours. LEDs generally emit light in a relatively narrow band so that most LEDs produce light that is a saturated colour. It is possible to make white LEDs by using a blue or ultraviolet chip and putting a phosphor coat round it. White can also be achieved by combining a mixture of red, green and blue chips. LEDs have a lot of applications associated with signals and signage. The use of saturated colours in these applications is a real bonus. This coupled with the ease of producing light in a number of small units means that LEDs are replacing a number of other light sources in these areas. It is also possible to make lamps that are a cluster of LEDs of different colours. By controlling the outputs of the different colours it is possible to make a lamp that can produce light in a wide variety of colours. At the time of writing, white LEDs are making fast technical progress but have not proved to have that many applications in the area of general lighting as the lumen packages tend to be small and their efficacy does not compare favourably with other sources such as fluorescent lamps. 3.3.10 Electroluminescent The basic principles of electroluminescent (EL) light sources are discussed in section 3.1.3. Generally the light sources are made up as panels with a construction similar to that shown in Figure 3.31.

Transparent medium Conducting layer S

Phosphor plus phosphor embedding layer Conductive material

Figure 3.31 A section through an electroluminescent panel

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The lower conductor carries one side of the electrical supply into the light source. In older types of panel this conductor may have been a sheet of metal, but in the newer flexible panels it is generally some type of foil. The phosphor layer contains the phosphor used to generate the light together with a medium, usually some form of plastic resin, used to keep the grains of phosphor apart from one another. The top conductor is a made of a transparent material that conducts electricity to the top surface of the phosphor layer. The top layer of the device is a transparent medium. In older devices this layer is usually made of glass, but in more modern units it is likely to be a flexible transparent film.

Chapter Three: Light sources

The EL panel is made up of the following components.

EL panels are not a particularly efficient light source. Typically they have efficacies of a few lumens per watt. The light output of an EL panel is not that great, typically less than 300 lumens per square metre. There are many applications for EL panels as it is relatively easy to cut them to shape and size so they can be used for signage and to backlight displays in electronic equipment.

3.4 Electric light source characteristics There are a number of key properties of lamps that need to be considered when choosing which lamp is right for a particular application. The following sections list these properties. 3.4.1 Luminous flux In any lighting application the amount of light that is needed is a key decision that has to be made. From this it is then possible to work out how many lamps of given rating are needed. There are lamps with lumen outputs less than 1 lumen through to lamps with outputs in excess of 200,000 lumens. In most applications, it is the average maintained illuminance that is important so it is important to consider the lumen maintenance through life at the same time as the initial luminous flux. 3.4.2 Power demand It is important in any lighting scheme to know what the total power demand is going to be so that the electrical infrastructure can be correctly designed. The power consumed by the lamp is important. However with many lamp types it is important also to consider the impact of the control gear as well. In most cases it will be the total circuit watts that is important rather than the lamp wattage. One further complication with some lamp types is that the voltage and current waveforms are not exactly in phase with one another. Thus the volts multiplied by the amps in the circuit may be higher than the watts. The power factor of the circuit is defined by the following equation:

power factor

=

watts volts × amps

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Most high wattage lamp circuits are designed to have a power factor greater than 0.85. The other factor that may affect the sizing of the cables that supply a lighting installation is the current required during the run-up of the lamps. With some types of lamp this can be over double the nominal running current. When using lighting controls the power demand is more difficult to predict as the power consumed may be reduced at times when full output is not required from the lamp. 3.4.3 Luminous efficacy Luminous efficacy is usually expressed in terms of lumens per watt. Many lamp manufacturers produce lumens per watt figures for their lamps. However, for discharge lamps and other lamps requiring some form of control gear, these figures may be misleading as they refer to the power consumed in the lamp only and do not consider the power lost in the control gear. All the values quoted in this chapter for efficacy are based on total circuit watts. Efficacy is a primary concern when selecting a lamp. In general, if a range of lamps suitable for a particular installation then it is the most efficient that should be used. 3.4.4 Lumen maintenance The light output of most lamps decreases as the lamps get older. With some relatively short life lamps this is not a problem as they fail before the light output has fallen significantly. See Section 21.7.1 for further details of the lamp lumen maintenance factor (LLMF). 3.4.5 Life It is normal when considering the life of a lamp to talk about the percentage of lamps that will survive after a certain number of hours of operation. This value is known as the lamp survival factor (LSF). See Section 21.7.2 for further details. Other factors in a particular installation may affect the life of the lamp used. These factors include the switching frequency, the supply voltage, the ambient temperature and presence of vibration. It is often the case that the combined effect of the number of lamp failures coupled with the reduced lumen output of the lamps makes it necessary to replace the lamps in an installation. Sometimes lamp makers quote an economic service life for lamps, this generally is the point where the LSF multiplied by the LLMF falls below 0.7. 3.4.6 Colour properties The colour of the light produced by a lamp is generally described by two parameters; the correlated colour temperature and the CIE general colour rendering index. These two terms are described in Sections 1.4.3 and 1.4.4 respectively For most applications there is a minimum requirement for the colour rendering properties of the lamps used and the correlated colour temperature of the source is generally chosen for the atmosphere that the lighting is designed to produce. 3.4.7 Run-up time When a lamp is switched on it takes a certain amount of time to reach full light output. The usual measure used to assess run-up time is the time that it takes for a lamp to reach 80% of its full output.

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3.4.8 Restrike time When some gas discharge lamps go out due to an interruption in the mains supply it is not possible to restart them until the lamp has cooled down. This may take several minutes. The use of lamps with a long restrike time may cause problems in some installations due to the possibility of a small power outage causing a long blackout. 3.4.9 Other factors There are also many other factors that impact upon the use of lamps in a particular application. These factors include the following.

Chapter Three: Light sources

For a GLS lamp this might be a fraction of a second, while for low pressure sodium this could be as much as 20 minutes. For some applications such as road lighting the run-up time is not important. However, for occasionally used rooms in a home it is very important.

Lamp size: some lamps are too large for certain applications, whilst some small lamps may produce too high a luminance for others. Burning position: not all lamps may be used in all orientations, for some discharge lamps, lamp manufacturers produce diagrams similar to Figure 3.32 to show which burning positions are permitted. The figure shows that the lamp in question must only be used in the horizontal position ± 20˚.

Figure 3.32 A typical restricted burning position symbol Dimming: it is not possible to dim all lamp types and some types may be only dimmed down to given percentage of their output. Dimming for some lamps may require the use of special control gear. Ambient temperature: not all lamps will run at a given temperature. For example some compact fluorescent lamps are not suitable for outdoor use as they will not start if they are too cold. Disposal of lamps: lamps may contain hazardous substances such as lead, sodium and mercury. This may mean with particular lamps particular procedures have to be followed when disposing of the lamps. Under the WEEE Directive of the European Commission it is the responsibility of the lamp manufacturer to provide the means of recycling used lamps. See http://www.recolite.co.uk for more information about the recycling of lamps in the UK 3.4.10 Summary of lamp characteristics Table 3.5 gives a summary of the key characteristics of the main lamp families. 79

80

40–50,000

TH

650–6200

120–8850

T8

T5

100–1500

CFLi (Integral control gear)

2000–58,500

5,200–200,000

1,600–26,000

Quartz tube

Ceramic Tube

Metal halide lamps

MBF/HPL

High pressure mercury

250–9000

CFLni (Non integral control gear)

20–250

85–2050

60–1040

5–30

8–120

6–120

65–97

60–98

33–57

20–50

Yes

Yes

Yes

No

Yes

Yes

20–93 (6)

30–70

Yes

3000–4400

3000–6000

3200–3900

2700

2700–6500

2700–17000

2700–17000

3000–6500

2700–3200

No (2)

Yes

2500–2700

Colour temp (K)

No

Control gear

50–96

50–80

15–25

8–14

Efficacy (lm/W)

(3)

78–93

60–90

2 min

1–8 min

4 min

60 sec

> 80 40–50

15–90 sec

30 sec

30 sec

30 sec

Instant

Instant

Run-up time

85–90

82–95

50–98

50 – 90

100

100

Colour rendering (Ra)

Limited

No

No

(7)

Some types to 20%

Some types to 5%

Easy to 2%

Easy to 2%

Limited to 25%

Easy to 0%

Easy to 0%

Dimming

(5)

(5)

6,000–10,000

2,000 – 7000

8,000–10,000

5,000–15,000

Up to 15,000 (5)

8,000–19,000

8,000–17,000

8,000–12,000

1,500–5,000

1,000

Life (h) (1)

The lamp range is increasing rapidly

There are some higher power lamps available for special applications such as cold stores

Large variety of shapes and sizes of lamp

Comments

10:04

13–70

25–140

4–2000

1–1000

Power range (W)

25/3/09

Compact (CFL)

1000–10,500

T12 (4)

Fluorescent

5–12,000

Output range (lm)

GLS

Incandescent

Lamp name

Chapter Three: Light sources Table 3.5 Summary of lamp characteristics

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Output range (lm)

1,800–32,000

1,800–5,000

White SON

1–5

30–100

47–80

40–44

75–86

Yes

Yes

Yes

Yes

Yes

Yes

Control gear

2,685–6500

2,550–4,000

2,500

2,150

1,900–2,100

N/A

Colour temp (K)

40–85

80

83

65

19–25

N/A

Colour rendering (Ra)

Instant

Easy to 0%

15,000–60,000(9)

60,000+

6,000–9,000

No

2 min

No

10,000–14,000

Limited to 25%

5 min

1 min

10,000–20,000

15,000–20,000

Life (h) (1)

Limited to 25%

No

Dimming

3-7 min

10-20 min

Run-up time

The range of LEDs is increasing rapidly

Good lumen maintenance, but power consumption goes up through life

Comments

1 2 3 4 5 6

Chapter Three: Light sources

Economic lamp life may be limited by lumen depreciation. A lot of TH types are designed to run on low voltages and thus need a transformer or other device to supply the necessary voltage. Some lamps with dichroic reflectors have part of the red end of the spectrum missing and thus do not have a colour rendering index of 100, information from lamp makers on this topic is hard to find. T12 lamps are not generally used in new installations as T5 and T8 types are more efficient. Lamps also available with exceedingly long lamp lives of e.g. 30,000 hours and 60,000 hours. Most T5 lamps are optimised to give maximum light output at 35˚C. The figures in this table are based upon their output at 25˚C. As in most luminaires the lamp runs close to 35˚C then the apparent light output ratio (LOR) of the luminaire appears to be higher than normal. 7 Most manufacturers are working on dimming control gear for this sort of lamp, but most products released onto the market so far have had major problems. 8 The LEDS can be integrated within the LED lamp, LED module or LED luminaire. The values represent the values of the LED alone of current technology Jan 09 and two points should be noted: – thermal, driver and optical losses (potentially 50%), will reduce these lumen output and efficiency values, when built into a luminaire. – the lumen output and efficiency development curves are much steeper than existing other lamp technologies. 9 For lamp life both electrical failures and lumen maintenance should be considered to measurement standards e.g. B10/L70 (10% electrical failures and 70% lumen depreciation at lamp life).

20–220

55–165

45–115

165–430

53–142

70–180

Efficacy (lm/W)

10:04

LEDs(8)

2,600–12,000

12,500–37,000

Delux SON

85–1040

26–200

Power range (W)

25/3/09

Induction

4,300–130,000

Std SON

High pressure sodium

SOX SOX-E

Low pressure sodium

Lamp name

Table 3.5 Summary of lamp characteristics cont...

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3.5 Flames Historically flames were the first form of artificial lighting. They are occasionally still used to create a particular atmosphere, but they are not considered as major sources of artificial light. 3.5.1 Candle It is said that the ancient Egyptians invented the candle. They made candles by soaking reeds in molten tallow (animal fat). However this was not the candle as we know it today as it had no wick as such. It appears that the Romans made the first true candle with a wick, but it still used tallow. The problem with tallow candles is that they produce a lot of smoke and acrid smells. In the Middle Ages beeswax was introduced for making candles. It overcame the problems of tallow candles, but due to its cost only rich people could afford them. The last advance in candle making was in the 19th century when whale oil wax and paraffin wax were introduced. The actual processes involved in a candle burning are very complex; in 1861 Michael Faraday was able to fill a series of six lectures just discussing them. The key points of the process are that the heat of the candle flame melts the wax, which is absorbed in the wick, which transports it to the flame where it is burnt. In the burning process some particles of carbon are produced. These particles glow as they are hot. 3.5.2 Oil The oil lamp has been around for a very long time. Some of the earliest examples are hollowed out stones that were filled with oil and these may be 70,000 years old. There are examples of earthenware lamps made by all the ancient civilisations. In Europe the most common oils used in these lamps were olive and colza. The wick was generally made out of bark, moss or plant fibres. The first major development in modern history was the use of a flat wick in the lamp that started in 1773 and the tubular wick in 1784. This coupled with the glass chimney made the lamps significantly more efficient. Figure 3.32 shows such a lamp.

Figure 3.32 An oil lamp with a tubular wick and a glass chimney

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3.5.3 Gas Gas lighting only became possible during the industrial revolution. During the 1780s several inventors had been working with the flammable gas that is produced when coal is made into coke and they realised that it could be used for lighting. The problem was that it became necessary to set up a whole infrastructure of pipes to supply the gas to where it was needed. In 1813 a company was set up in London to supply gas and by 1815 there were 26 miles of gas pipe installed. The first gas light burners were little more than small openings at the end of a gas pipe. Over a period of time the shape of the burners evolved so that each unit would produce more light. However, a major improvement in performance was achieved in 1887 with the invention of the gas mantle. The gas mantle is a cube of fabric, impregnated with thorium and cerium oxides. When the lamp is lit the fabric burns away leaving a brittle mesh of oxides. The cerium oxide is a thermo-luminescent material, see Section 3.1.8.

Chapter Three: Light sources

In the 1860s with the introduction of paraffin the oil lamp became very popular and was one of the leading sources of artificial light until it was overtaken by gas and electric lighting.

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Chapter 4: Luminaires 4.1 Basic requirements A luminaire is the apparatus containing the light source. A luminaire is designed to: connect the light source to the electricity supply protect the light source from mechanical damage control the distribution of light be efficient withstand the expected conditions of use be safe when used in the recommended manner. To meet these design objectives it is necessary to consider the electrical, mechanical, optical, thermal and acoustic aspects of luminaires. 4.1.1 Electrical Electrical wiring The internal wiring of a luminaire has to be capable of handling the electrical current and the thermal conditions in the luminaire. The cross sectional area of the wire will determine the maximum allowable current. IEC 598 specifies a minimum cross section of 0.5 mm2 although this may be reduced to 0.4 mm2 where space is severely restricted. The wire itself can be solid or stranded. Solid wire is easier to hold in position and to strip, making it simpler to install in a luminaire. However, solid wire is not suitable for luminaires that are subject to vibration or for luminaires that may be frequently adjusted. For such luminaires, stranded wire is better. Both types of wire are covered with insulating material. The choice of insulation material is largely determined by its heat resistance. The wiring of a luminaire has to be capable of withstanding not only the air temperatures inside the luminaire but also the surface temperatures of components that the wiring may contact, such as lamps, control gear and lamp holders. PVC insulation that is heat resistant up to 90 ˚C, 105 ˚C and 115 ˚C is available. Where higher temperatures may be experienced, silicon rubber (170 to 200 ˚C) and PTFE (250 ˚C) insulation may be used. Additional thermal insulation can be achieved by covering the electrical insulation with a glass fibre sleeve. Connection to the electricity supply There are three approaches commonly used to connect a luminaire to the electricity supply; the connection block, automatic connection and through wiring. The most common method is via a connection block within the luminaire. To prevent the connection being accidentally broken, the supply wire should pass through a cable clamp before reaching the connection block.

84

Luminaires mounted on trunking systems are often designed so that connection to the electricity supply occurs when the luminaire is mounted on the trunking. For this to occur the electrical socket carrying the electricity supply is part of the trunking and the plug is contained within the luminaire.

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Through wiring is a system for connecting a series of luminaires in parallel across a supply cable. This reduces the amount of cabling required and speeds up installation. The supply cable should have a cross section of 2.5 mm2 as a minimum, but the wiring from the connection block in each luminaire may have a smaller cross section, typically 0.5 or 0.75 mm2. Earthing Metal parts of Class 1 luminaires (see Section 4.3.2, Table 4.11) that are accessible when the luminaire is installed or open for maintenance or that may become live if the insulation fails should be permanently connected to an earth terminal. The wire used for earthing should be at least 2.5 mm2 in cross section.

Chapter Four: Luminaires

The earth pin of the plug is longer than the live and neutral pins so that when the luminaire is offered up to the track, the earth connection is made before the live and neutral, and when removing the luminaire, the live and neutral connections are broken before the earth.

4.1.2 Mechanical The mechanical integrity of a luminaire depends on the materials used and the quality of its construction. Materials Steel Many interior lighting luminaires are made from ready-painted sheet steel, white being the usual paint colour. Where corrosion is a problem, galvanised sheet steel is used. Where a very durable paint finish is required, enamelling is used. Stainless steel Stainless steel is rarely used for luminaire bodies but it is widely used for many small, unpainted luminaire components that have to remain free from corrosion. Aluminium sheet Aluminium sheet is mainly used for reflectors in luminaires. It can have good reflection properties and the physical strength to form stable reflectors of the desired form. Cast aluminium Cast aluminium is widely used for floodlight housings. Such housings are light in weight and can be used in damp or corrosive atmospheres without any further treatment provided that the correct grade of aluminium has been used. Plastics There are many different forms of plastic used in luminaires, either for complete housings or components. These plastics differ in their transparency, strength, toughness, sensitivity to UV radiation and heat resistance. Glass Three types of glass are used in luminaires; soda lime glass, borosilicate glass, and very high resistance glass. Soda lime glass is used where there are no special heat resistance demands. Where high heat resistance, chemical stability and resistance to heat shock are required, borosilicate glass is used. High resistance glass has the advantage that it can deliver high heat resistance, high thermal shock resistance and great physical strength even in thin sheets. 85

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Ceramics Some components of luminaires that produce very high temperatures are made of ceramics. Construction All luminaires should be designed to withstand the rigours of transport to the site, installation and prolonged use. Generally, exterior luminaires need to be more substantial than those designed for interior use. Some luminaires are designed to resist the ingress of foreign objects, dust and moisture. Such luminaires have a transparent front cover and all points of access to the luminaire have a seal. Front covers are usually made of glass or plastic. Where there is a risk of physical impact, as in a sports hall, glass or acrylic front covers need to be covered with a wire screen. If a polycarbonate front cover is used, no such screen is necessary. As for the seals, these come in various forms from a simple felt seal to convoluted notched rubber seals. The effectiveness of these seals is quantified by the IP classification system (see Section 4.3.2 Table 4.10). 4.1.3 Optical control Optical control of the light output from a light source is achieved by some combination of reflectors, refractors, diffusers, baffles or filters. Reflectors Three types of reflector are used in luminaires; specular, spread and diffuse. Specular reflectors are used when a precise light distribution is required. The shape of the reflector and its position relative to the light source determine the light distribution. The most common shapes for reflectors are circular, parabolic and elliptical. A circular reflector with a point light source at its focus will produce a light distribution of the type shown in Figure 4.1, reflections from some parts of the reflector being almost parallel while those from parts of the reflector away from the axis are divergent. This type of circular reflector is used in cylindrical form for picture lighting using tubular incandescent and fluorescent light sources.

Figure 4.1 The light distribution from a circular reflector with a point light source at its focus 86

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A circular reflector with a point light source at its centre of curvature produces a light distribution of the type shown in Figure 4.2. This type of reflector is widely used in projection systems and spotlights to increase the amount of light delivered to the associated lens system.

Figure 4.2 The light distribution from a circular reflector with a point light source at its centre of curvature A parabolic reflector with a point light source at its focus produces a parallel beam of reflected light (Figure 4.3). Moving the light source in front or behind the point of focus will cause the beam to converge or diverge. The parabolic reflector is widely used in spotlight design either exactly, when the reflector is smooth, or approximately, when the reflector is facetted.

a F b

A

B

Figure 4.3 The light distribution from a parabolic reflector with a point light source at its focus. The beam intensity will be greater at the centre than at the edge — compare cones aFb and AFB. 87

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An elliptical reflector with a point light source at one focus will ensure that the reflected rays all pass through the second focus (Figure 4.4) Elliptical reflectors in trough form are widely used for tubular fluorescent luminaires.

F

F

Figure 4.4 Elliptical reflectors showing the change in light distribution as the point light source is moved relative to the first focus (F) Spread reflectors are deliberately distorted specular reflectors. They can be circular, parabolic or elliptical in cross section and spherical or cylindrical in form. The distortion takes the form of modulating the specular surface of the reflector by hammering (peening) to produce a regular array of dimples, or by etching or brushing the surface. The advantage of this distortion is that it smears out variations in light distribution caused by inaccuracies in the manufacture of the reflector and the size of the light source. Spread reflectors are used where a well-defined but even light distribution is required. Diffuse reflectors are the opposite of specular reflectors. Unlike a specular reflector, the shape of a diffuse reflector has only a small effect on the light distribution. Diffuse reflectors are used where there is a need to redirect light with a very wide beam. Many different materials are used in reflectors. Typical values of reflectance for these materials are given in Table 4.1. Refractors Refractors control light distribution by turning the incident light ray through a desired angle following Snell’s Law. This can be done using either prisms or lenses. For luminaires using large area light sources, such as a fluorescent lamp, multiple prisms are moulded in a transparent material, usually acrylic or polycarbonate plastic. The number, location, angle of incidence and shape of the different types of prism determine the light distribution. For luminaires using a point light source a lens can be used. The position and shape of the lens determines the light distribution.

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Reflector type

Material

Reflectance

Specular

Commercial grade aluminium

0.70

Specular

Aluminium with super purity coating

0.80

Specular

Aluminium with silver coating

0.90

Specular

Glass or plastic with aluminium coating

0.85–0.88

Spread

Peened aluminium

0.70–0.80

Spread

Etched aluminium

0.70–0.85

Spread

Brushed aluminium

0.55–0.58

Spread

Satin chromium

0.50–0.55

Spread

Aluminium painted steel

0.60–0.70

Diffuse

White paint on steel

Up to 0.84

Diffuse

Glossy white plastic

Up to 0.90

Chapter Four: Luminaires

Table 4.1 Typical reflectance values for materials used in reflectors

Diffusers Diffusers are transparent materials that scatter light in all directions. They provide no control of light distribution but do serve to reduce the brightness of the luminaire. Diffusers are commonly made of materials that maximise light scatter and minimise absorption, such as opal glass or plastic. Baffles Baffles can have three functions; to hide the light source from common viewing angles, to reduce the amount of spill light, and to control the light distribution. The extent to which the light source is hidden from view is quantified by two angles, the shielding angle and its complementary, the cut-off angle. The shielding angle is the angle between the horizontal and the direction at which the light source ceases to be visible. Figure 4.5 shows the shielding angle for a simple fluorescent luminaire.

Luminaire using an internal baffle to improve screening

Shielding angle. Beyond this angle the lamp is not visible to the user

Figure 4.5 The shielding angle for a simple fluorescent luminaire 89

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A common example of a baffle being used to hide the light source is the diffusely reflecting louvre. This louvre can take a wide variety of forms, lamellae, eggcrate, concentric rings and honeycomb depending on the shape and size of the luminaire and is usually made of a white diffusely reflecting material. If the purpose is primarily to reduce spill light, the material used for the louvre will be of low reflectance, i.e. black. In addition to louvres, spill light can be controlled by the use of low reflectance baffles, called barn doors, mounted on the luminaire (Figure 4.6).

Figure 4.6 Barn door baffles mounted on a spotlight

If the purpose is to hide the light source and also to control light distribution, the louvre is made from a specularly reflecting material and shaped so as to direct light downwards and hence increase the shielding angle (Figure 4.7). As a general rule, the finer the louvre and hence the more the light source is hidden, the lower will be the light output ratio of the luminaire (see Section 4.1.4).

Shielding angle ca. 45˚

Figure 4.7 A section through and an example of a louvre designed to hide the light source and control the light distribution Filters For display and decorative lighting it is sometimes required to change the colour of light emitted by a luminaire. This can be done by the use of filters, either absorption or interference. 90

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4.1.4 Efficiency The efficiency of a luminaire is quantified by its light output ratio (LOR). This is the ratio of the total light output of a luminaire to the total light output of the light sources used in the luminaire when operating outside the luminaire. LOR is sometimes split into upward and downward components. LOR measures the efficiency of the luminaire in the sense that it quantifies how much of the light emitted by the light source escapes from the luminaire. LOR does not measure the efficiency of a lighting installation. However, LOR is an element in determining a lighting installation’s compliance with Part L of the Building Regulations. Light output ratio is defined as the ratio of luminous flux emitted by the luminaire divided by the flux emitted by the bare lamps in free air. This means that for temperature sensitive lamps the LOR is a function of the increase in temperature of a lamp within the luminaire as well as the optical efficiency of the luminaire.

Chapter Four: Luminaires

Absorption filters are usually made of plastic or glass. They absorb the unwanted wavelengths and thereby raise their temperature. Plastic absorption filters are likely to change their properties if they get too hot. The transmittance of absorption filters is limited. Typical transmittances for different colour filters are blue = 5 percent, red = 20 percent, green = 15 percent, and yellow = 40 percent. Another type of filter is the interference filter. Interference filters are more expensive and more exact than absorption filters and do not absorb the unwanted wavelengths. Rather, they split the light into two beams, one transmitted and one reflected, of two different colours (hence the name dichroic filters).

4.1.5 Thermal All luminaires increase in temperature when in operation. The internal temperature of the luminaire can affect the efficiency of some light sources and the associated control gear. These changes in efficiency contribute to the light output ratio of the luminaire. The external surface temperature of a luminaire may also pose a fire hazard if mounted on a flammable surface (see Section 4.3.2). Of course, the external temperature of the luminaire will increase more when it is surrounded by thermal insulation so care should be taken when considering recessing luminaires into confined or insulated spaces. Air handling luminaires are used to deliver conditioned air to the occupied space or to extract heat from the occupied space and the luminaire. There are three types of air handling system using luminaires; plenum exhaust, single ducted and double ducted. In a plenum exhaust system conditioned air is supplied through air diffusers, while stale air is extracted through slots in the luminaires into the plenum that acts as a return duct (Figure 4.8a). The plenum exhaust system is only used where the maximum number of air changes per hour is less than six. In a single ducted system, conditioned air is delivered along the plenum and then through air diffusers, and stale air is extracted through the luminaires into a duct (Figure 4.8b). The single ducted system is used for spaces with low ceiling heights. In a double ducted system conditioned air is delivered through air diffusers supplied by a duct, and stale air is extracted through the luminaires and an attached duct (Figure 4.8c). These three systems differ in cost and efficiency. The double ducted system is the most expensive and most efficient.

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Chapter Four: Luminaires (a)

(b)

(c)

Figure 4.8 Sections through and drawings of (a) the plenum exhaust, (b) the single ducted and (c) the double ducted air handling systems 92

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4.1.6 Acoustics Luminaires may amplify the sound produced by components in the luminaire, e.g. the control gear, or produced elsewhere but transmitted to the luminaire through the building structure. Either way, the result is noise. Some spaces, such as concert halls have strict criteria about background noise, usually expressed as a noise rating (NR). A noise rating consists of numbered curve showing the maximum sound pressure level allowed in each frequency band (Figure 4.9). Table 4.2 gives recommended NR values for different applications. Where noise is likely to be a problem, care should be taken to use well constructed luminaires and to mount them so they are free from vibration. It is also desirable to use high frequency control gear but if this is not possible, remote positioning of control gear may be necessary. 70

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The design of air conditioning incorporating luminaires is complex and requires knowledge of heat load, air change rates, pressure drop and supply air temperatures. Failure to consider these factors carefully could cause the luminaires to fail to fulfil their function.

NR 70

60 60

50 40 40 30 30

Noise rating (NR)

Sound pressure level (dB)

50

20 20 10 10 0

0

–10 62.5

125

250

500

1000 2000 4000

Frequency (Hz)

Figure 4.9 A set of noise rating curves, plotted as sound pressure level at different frequencies Application

Noise rating

Studios

10

Concert halls

20

Conference rooms

25

Lecture rooms

25

Auditoria

25

Hospitals

30

Private offices

30

Table 4.2 Recommended noise rating values for different applications

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4.1.7 Environmental Luminaires may contain a variety of materials and some of these could be hazardous to the environment when the luminaire is disposed of at the end of life. To stop environmental pollution there are two sets of European regulations, WEEE and RoSH. WEEE or more fully the Waste Electrical and Electronic Equipment Directive requires that all luminaires are recycled at the end of life and are not just thrown away. To ensure that this occurs, luminaire suppliers are required to make provision for the collection and recycling of old luminaires; see http://www.lumicom.co.uk for more information. RoHS, the Restriction of Hazardous Substances Directive, controls the use of certain materials used in luminaires. These materials such as lead, mercury, cadmium and polybrominated biphenyls are all toxic and their use in luminaires is limited.

4.2 Luminaire types The lighting industry produces many thousands of different luminaires. Given below are brief outlines of the main types of luminaire used in interior and exterior lighting. Details of any specific luminaire are best obtained from the manufacturers. 4.2.1 Interior lighting Direct luminaires Direct luminaires are luminaires in which the light distribution is predominantly downward (see Table 4.7). Such luminaires are typically recessed into or surface mounted on the ceiling. They are widely used in offices where the ceiling height is restricted. The usual light source is a fluorescent lamp, either linear or folded. Many different forms of optical control are available, from diffusers through prismatic refractors to parabolic reflectors and louvres. Consequently, direct luminaires are available with a wide range of luminous intensity distributions. Direct luminaires are available for operation in dirty, corrosive or hazardous conditions. Direct luminaires are available with dimming or switching facilities linked to manual, occupancy sensor and photocell control. The most common problems with lighting installations using direct luminaires is the creation of a dark ceiling and poor illuminance uniformity in obstructed spaces. This problem can be overcome by choosing direct luminaires with a little upward light output or by having high reflection factors in the space. Figure 4.10 shows a direct luminaire.

Figure 4.10 An example of a direct luminaire 94

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Indirect luminaires Indirect luminaires are luminaires in which the light distribution is predominantly upward (see Table 4.7). Such luminaires can be suspended below the ceiling, wall mounted or free standing. They require a clean, white ceiling for efficient operation. Indirect luminaires are most practical where the ceiling height is over 2.75 m. The usual light source in suspended indirect luminaires is a linear fluorescent lamp. Wall mounted and free-standing indirect luminaires tend to use a high intensity discharge lamp. Optical control is confined to ensuring that the light output from the luminaire is widely spread across the ceiling so that no hot spots of high luminance are apparent. While indirect luminaires have a high light output ratio, lighting installations using indirect luminaires are usually less energy efficient than those using direct luminaires because of the losses caused by having to use the ceiling as a secondary reflector. This is compensated by the bright appearance of the space, the high level of illuminance uniformity and the absence of discomfort glare. Figure 4.11 shows a selection of indirect luminaires.

Figure 4.11 Examples of indirect luminaires Direct/indirect luminaires Direct/indirect luminaires are luminaires in which the light distribution is evenly divided between the upward and downward directions. In many ways, direct/indirect luminaires provide the best of both worlds. The energy efficiency of a lighting installation using direct/ indirect luminaires will be higher than that of one using indirect luminaires but the problems of dark ceilings and poor illuminance uniformity are reduced by the indirect component. Direct /indirect luminaires are suspended below the ceiling. They are difficult to use where the ceiling height is below about 2.75 m. The usual light source in direct/indirect luminaires is a linear fluorescent lamp. Optical control is different for the two directions of light output, being much tighter for the downward component than the upward. Direct/indirect luminaires are available with individual dimming of the direct component. Figure 4.12 shows a selection of direct/indirect luminaires.

Figure 4.12 Examples of direct/indirect luminaires 95

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Downlights Downlights are a form of direct luminaire characterised by a small light emitting aperture. Downlights are usually recessed into the ceiling so they direct all of their light output downward. They are widely used in shops, hotels and other places where a lighting installation with a discreet appearance is desired. Many different light sources can be used in downlights, the most common being incandescent, tungsten halogen, compact fluorescent and metal halide. Through the use of reflectors, louvres, lenses and refractors many different beam spreads and beam sizes are possible (see Section 4.3.2). Some downlights allow for adjustable aiming which is useful when the intention is accent lighting. A number of downlights are fitted with decorative elements directly beneath the downlight aperture to give an impression of brightness to the luminaire. The most common problems with lighting installations using an array of downlights to create uniform illumination are poor illuminance uniformity caused by overspacing and dark ceilings. Care is necessary to avoid a fire hazard when recessing downlights into an insulated ceiling. Figure 4.13 shows a selection of downlights.

Figure 4.13 Examples of downlights Spotlights Spotlights are narrow beam luminaires with beam spreads in the range 5 to 30 degrees. They are usually mounted on either a base plate or lighting track. When track mounted, spotlights can be obtained for operation at mains voltage, low voltage or extra low voltage, the latter requiring the installation of a step-down transformer. Spotlights are widely used in shops, hotels and museums for accent lighting. Spotlights are available that use incandescent, tungsten halogen, metal halide and extra high pressure sodium light sources of small physical size. Some incandescent and tungsten halogen light sources can be used as spotlights themselves because they have reflectors giving the desired beam spread built in. Other light sources have to use reflectors to attain optical control. Filters mounted in front of the spotlight can be used to change the light colour. Irises and baffles mounted in front of the spotlight can be used to modify the beam shape. Care is necessary when using spotlights to avoid glare to passers by. Figure 4.14 shows a selection of spotlights.

Figure 4.14 Examples of spotlights 96

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Wall washers Wall washers are asymmetric luminaires with beam spreads up to 130 degrees. They are usually mounted on either a base plate or lighting track. As their name implies, wall washers are used where it is required to wash a large area with light, evenly. Point and extended light sources can be used in wall washers, typically tungsten halogen and fluorescents. Wall washers require careful aiming to achieve their best effect. Figure 4.15 shows an example of a wall washer.

Figure 4.15 Example of a wall washer Task lights Task lights are a necessary part of a task/ambient lighting system. They provide local lighting of a specific area by bringing the light source closer to the task. The value of task lights is that they enable the user to have some control of the amount and distribution of light on the task by switching or dimming the light source and by changing the position of the luminaire relative to the task. Typically, the light sources used in task lights are incandescent, tungsten halogen or compact fluorescent. The degree of adjustment available can vary widely as can the amount of desk space taken. When selecting task lights attention should be given to the coverage area for common positions and the likelihood of glare to the user. Figure 4.16 shows a selection of task lights.

Figure 4.16 Examples of task lights 97

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4.2.2 Exterior lighting Road lighting luminaires Road lighting luminaires used for lighting traffic routes are designed to deliver light to a road so that the surface is seen to be of uniform luminance and objects on the road can be seen in silhouette. The light distribution is therefore dependent on the position of the luminaire relative to the road. Most road lighting luminaires are mounted on columns placed at regular intervals at the side of the road or between crash barriers in the median. A few installations use a catenary system in which the luminaires are suspended over the median in a continuous series. For conflict areas and subsidiary roads (see Chapter 16) the luminaires are designed with a wide light distribution so as to give a uniform illuminance across the road. The light sources used in road lighting luminaires are typically low pressure sodium, high pressure sodium or metal halide. Road lighting luminaires are often provided with adjustable lamp holders and/or reflectors so as to allow the light distribution to be optimised for the light source and road layout. Two broad classes of road lighting luminaire are semi-cutoff and full cutoff (see Section 4.3.2, Table 4.9) these classes reflecting a different balance between luminaire efficiency and the control of glare. Road lighting luminaires need protection against dust and moisture and so are classified according to the IP system (see Section 4.3.2, Table 4.10). They are almost always fitted with a photoelectric control package. Figure 4.17 shows a selection of road lighting luminaires.

Figure 4.17 Examples of road lighting luminaires Post tops Post top luminaires are a form of road lighting luminaire but unlike the road lighting luminaires described above, which are intended for the lighting of high speed traffic routes, post top luminaires are intended for urban areas, where pedestrians are considered as important as drivers and the decorative aspect of the luminaire is as important as the functional. Post top luminaires are available with either rotationally symmetric or road lighting light distributions, so that the same luminaire can be used to light both roads and open pedestrian areas in a city. Post top luminaires take many different forms, some mimicking traditional styles for historic areas, while others represent the latest design trends. Because of their use in urban areas, low pressure sodium light sources are not used in post top luminaires, the most common light sources being high pressure sodium, metal halide, compact fluorescent and induction lamps. Post top luminaires need protection against dust and moisture and so are classified according to the IP system (see Section 4.3.2, Table 4.10). Because of their relatively low mounting heights, post top lanterns are often constructed of materials that resist attacks by vandals. They are almost always fitted with a photoelectric control package. The most common problem with post top luminaires is glare. This problem can be avoided if there is no direct view of the light source. Figure 4.18 shows a selection of post top luminaires. 98

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Figure 4.18 Examples of post tops Secondary reflectors Secondary reflector luminaires are designed for use in pedestrianised places such as city squares and parks. In this luminaire, light is directed up from the light source in or on the column and then distributed from a large surface at the top of the column. By changing the area and tilt of the reflecting surface, the light distribution can be altered. Secondary reflector luminaires are inevitably inefficient compared to post top luminaires, but they do not cause glare, are not easily damaged by vandals and can provide a pleasing ambience. Figure 4.19 shows two secondary reflector luminaires.

Figure 4.19 Examples of secondary reflector luminaires Floodlights Floodlights can be used to wash a large surface with light or to pick out a specific feature of a building. Floodlights vary enormously in their size, power and light distribution. The smallest floodlights consist of little more than a 150 W linear tungsten halogen lamp with a spread reflector. The largest consist of a high intensity discharge lamp with power in the kilowatt range and a carefully shaped reflector. The light distribution of a floodlight can be rotationally symmetric, symmetrical about one axis or asymmetrical about one axis. This distribution is usually classified as narrow, medium or wide beam (see Section 4.3.2, Table 4.8). The light sources used in floodlights include incandescent, tungsten halogen, high pressure sodium and metal halide. Floodlights need protection against dust and moisture and so are classified according to the IP system (see Section 4.3.2, Table 4.10) and are often soundly constructed of materials that resist attacks by vandals. Filters mounted in front of the floodlight can be used to change the light colour. Barn door baffles mounted on the floodlight can be used to modify the beam shape. Care is necessary when using floodlights to avoid glare to passers by. Figure 4.20 shows a selection of floodlights. 99

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Figure 4.20 Examples of floodlights Wallpacks As their name suggests, wall packs are designed to be mounted on walls so as to provide a low level of illumination in the nearby area. They are widely used for security and amenity lighting. The light distribution is usually wide and is achieved by a combination of reflecting and refracting elements. The light sources used in wall packs are usually low wattage low pressure sodium, high pressure sodium and compact fluorescent. Wallpacks need protection against dust and moisture and so are classified according to the IP system (see Section 4.3.2, Table 4.10). Because of their relatively low mounting heights, wallpacks should be solidly constructed of materials that resist attacks by vandals. The most common problem experienced with wallpacks is glare. This problem is much reduced if there is no direct view of the light source. Figure 4.21 shows a selection of wallpacks.

Figure 4.21 Three examples of wallpacks

4.3 Certification and classification 4.3.1 Certification The principal EU Directives for electrical products are the Electro-Magnetic Compatibility (EMC) Directive and the Low Voltage (LV) Directive, summarised for lighting products in Table 4.3. The LV Directive and the EMC Directive both require products put on the EU market to be safe: Compatibility being designated by the CE mark. Products complying with specified Euronorm (EN) safety standards are presumed to comply. EN standards are based upon existing international standards, e.g. an IEC standard. For a list of current EN standards relevant to lighting products see Tables 4.4 and 4.5 (EMC and Safety), and Table 4.6 (Performance). In most instances, there is an equivalent British Standard (BS), known as a BS EN. (For established products a compatible BS may still be used, but preference should be given to the EN.) Electrical EN standards are issued by the EU sponsored organisation, CENELEC. They are type tests, and manufacturers are required to associate them with controls for conformity of production. 100

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Table 4.3 EU Directives and lighting products EMC Directive

LV Directive

From 1 January 1996

From 1 January 1997

Applies to: (see Table 4.4)

Applies to: Luminaires Lighting components Lamps

EN standards Table 4.4

EN safety standards Table 4.5

Chapter Four: Luminaires

Responsibility for compliance of a product with the Directives and with the specified EN standards rests on the person putting the product on the EU market, usually the manufacturer.

UK enforcing authorities Trading Standards Officers HM Revenue and Customs

The EMC and LV Directives, in conjunction with the CE Marking Directive, require complying products to be accompanied by the CE-mark. CE represents Conformity European. The CE-mark should preferably be on both product and packaging. Responsibility for marking rests on the person putting the product on the EU market. It is important to note that CE-marks on components do not imply that a luminaire complies. The luminaire as a whole must comply and carry the CE-mark. Further, if a luminaire is modified for use in the EU (e.g. with emergency lighting) the modifier takes over responsibility and must make a new CE mark. A lighting product outside the LV Directive (e.g. an ELV product) comes under the General Products Safety Directive. The ENEC mark indicates independent confirmation that the product complies with all relevant EN safety standards and, where available, EN performance standards. (Note: the ENEC mark is not applicable to lamps or emergency luminaires). The ENEC mark is not obligatory. Testing and approval are carried out by national Certification Bodies, e.g. in the UK by BSI. The XX in the diagram is replaced by a number from 01 to 17, e.g. 12 for the UK. The ENEC mark of each of the Certification Bodies is valid throughout the EU. Again, it is important to note that ENEC marks on components do not imply that a luminaire has an ENEC mark. Further, if a luminaire is modified the modifier must remove the ENEC mark.

XX

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Table 4.4 EN standards and lighting products (CE-mark and EMC Directive) Product

EN

BS EN

Disturbance in supply system

EN 60555-2 EN 61000-3-2

BS EN 61000-3-2 M M M

Luminaires with control gear Control gear Lamps with integral control gear

Radio frequency interference (up to 30 MHz)

EN 55015

BS EN 55016

M M M

Luminaires with control gear Control gear Lamps with integral control gear

Immunity

EN 50081 & 2 EN 61547

Luminaires with electronic control gear Control gear with electronics Lamps with integral electronics (electro-magnetic control gear deemed to comply)

Notes for Tables 4.4, 4.5 and 4.6 M = CE-mark obligatory (LV Directive) S = ENEC mark optional (safety standard only available) SP = ENEC mark optional (to safety standard and performance standard) V = Older standard, still valid n/a = Not applicable Associated standards: BS EN 40 Lighting columns; BS EN 60730–2–3 Thermal protectors for ballasts. The EN standards are based on IEC standards, and their numbers are the IEC numbers plus 60,000; for example EN 60570 = IEC 570. BS EN standards have the EN number. BS EN 60598–2 is linked to BS EN 60598–1

102

CE-mark

M M M

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Product

EN

BS EN

Compatible BS

CE ENEC mark mark

Luminaires EN 60570

BS EN 60570

–

M

S

Street lighting

Track systems

EN 60598–2–3

BS EN 60598–2–3

–

M

S

Emergency

EN 60598–2–22

–

BS 4533–102–22

M

n/a

–

M

S

BS EN 60920

–

M

SP

Others (see X)

EN 60598–2–X BS EN 60598–2–X

Ballasts For fluorescent – Safety

EN 60920

For discharge – Safety

EN 60922

BS EN 60922

–

M

SP

DC electronic – Safety

EN 60924

BE EN 60924

–

M

SP

AC electronic – Safety

EN 60928

BS EN 60928

–

M

SP

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Table 4.5 EN Safety standards for lighting products (CE mark and LV Directive)

Transformers EN 61046

BS EN 61046

–

M

SP

Isolating

EN 60742

BS EN 60742

–

M

S

Neon

EN 61050

–

–

M

S

EN 60926

BS EN 60926

–

M

SP

EN 60155

–

BS 3772

M

S

EN 61048

BS EN 61048

–

M

SP

Electronic stepdown – Safety

Starters/Ignitors Electronic starters – Safety Glow starters Capacitors For lamp circuits – Safety Lampholders Edison screw

EN 60238

BS EN 60238

–

M

S

Fluorescent lamp & starter holders

EN 60400

BS EN 60400

–

M

S

Bayonet

EN 61184

BS EN 61184

–

M

S

Lamp caps and holders

EN 60838

BS EN 60838

–

M

S

Lamp caps and holders (V)

EN 60061

BS EN 60061

–

M

S

Lamps GLS

EN 60432–1

BS EN 60432–1

–

M

n/a

Tungsten halogen – Domestic

EN 60432–2

BS EN 60432–2

–

M

n/a

Double capped fluorescent

EN 61195

BS EN 61195

–

M

n/a

Single capped fluorescent

EN 61199

BS EN 61199

–

M

n/a

CFL Integral – Safety

EN 60968

–

BS 7173

M

n/a

High pressure sodium

EN 60662

BS EN 60662

–

M

n/a

Low pressure sodium

EN 60192

BS EN 60192

–

M

n/a

High pressure mercury

EN 60188

–

BS 3677

M

n/a

Metal halide

EN 61167

BS EN 61167

–

M

n/a

Double capped fluorescent (V)

EN 60081

BS EN 60081

–

M

n/a

Single-capped fluorescent (V)

EN 60901

BS EN 60901

–

M

n/a

Note: ‘X’ identifies luminaire types as follows: 1 General purpose, 2 Recessed, 4 Portable, 5 Floodlights, 6 With transformer, 7 Portable – garden, 8 Handlamps, 9 Photo – amateur, 17 Stage and studio, 18 Swimming pools, 19 Air-handling and 20 Lighting chains

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Table 4.6 EN Performance standards and lighting products Product

EN

BS EN

Compat. BS

EN EC mark

For fluorescent – Performance

EN 60921

BS EN 60921

–

SP

For discharge – Performance

EN 60923

BS EN 60923

–

SP

DC electronic fluorescent – Performance

EN 60925

BS EN 60925

–

SP

AC electronic fluorescent – Performance

EN 60929

BS EN 60929

–

SP

EN 61047

BS EN 61047

–

SP

EN 60927

BS EN 60927

–

SP

EN 61049

BS EN 61049

–

SP

GLS

EN 60064

BS EN 60064

BS 161

n/a

Tungsten halogen (non-vehicle) (V)

EN 60357

BS EN 60357

BS 1075

n/a

CFL integral – Performance

EN 60969

BS EN 60969

–

n/a

Luminaires No performance standard at present Photometry – see EN 13032–1 Ballasts

Transformers Electronic stepdown – Performance Starters/ignitors Electronic starters – Performance Capacitors For lamp circuits – Performance Lamps

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Table 4.7 CIE classification of general indoor luminaires Luminaire class

Percentage of total luminous flux emitted above the horizontal

Percentage of total luminous flux emitted below the horizontal

Direct

0–10

90–100

Semi-direct

10–40

60–90

General (diffuse)

40–60

40–60

Semi-indirect

60–90

10–40

Indirect

90–100

0–10

Chapter Four: Luminaires

4.3.2 Classification General lighting for interiors — luminous flux distribution Luminaires for general indoor lighting are classified by the CIE according to the percentage of the total luminous flux emitted above and below a horizontal plane through the luminaire (Table 4.7).

Spotlights — luminous intensity distribution Spotlights are characterised by their tight beam control. Most have a rotationally symmetric luminous intensity distribution. The most common way of classifying spotlights is by their beam spread. The beam spread of a spotlight is the angle over which the luminous intensity is 50 percent or more of the maximum luminous intensity in the beam. It is important to note that beam spread, expressed in this way, is not a good indication of the appearance of the beam. A better classification of the appearance of the beam is the beam size. The beam size is derived from the distribution of illuminance across a uniformly reflecting surface at a given distance from the spotlight. This distribution is differentiated to obtain the illuminance gradient. The locations of the peaks in the illuminance gradient distribution define the edges of the beam. The beam size is given as the angle subtended at the spotlight by the distance between the two edges and is expressed in degrees. The magnitude of the peaks in the illuminance gradient profile indicate the sharpness of the edge of the beam; the higher the peaks, the sharper is the edge of the beam. Floodlights — luminous intensity distribution Floodlights are classified according to their beam spread. The beam spread is the angle over which the luminous intensity drops to a stated percentage of the maximum, usually 50 percent or 10 percent. For a floodlight having a rotationally symmetric luminous intensity distribution, only one figure is necessary to specify the beam spread. For a floodlight with an asymmetrical luminous intensity distribution, as is usual with rectangular floodlights, two beam spreads are needed, one for the vertical plane and one for the horizontal plane. If the luminous intensity distribution in either of these planes is itself asymmetrical relative to the beam axis, two angles are given for that plane and one for the other plane. A simple classification of beam spreads is sometimes used (Table 4.8). Luminaire classification

Beam spread at 50 percent of maximum luminous intensity

Narrow beam

< 20°

Medium beam

20° to 40°

Wide beam

> 40°

Table 4.8 Floodlight beam spread classification

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Chapter Four: Luminaires

Road lighting luminaires — luminous intensity distribution Road lighting luminaires have traditionally been classified as full cutoff or semi-cutoff, according to their luminous intensity distribution. BS EN 13201: Part 2: 2003 has introduced a finer classification designed to give better control of disability glare and obtrusive light. This classification uses the maximum luminous intensity per 1000 lamp lumens at different angles from the downward vertical in any direction as a criterion. Table 4.9 shows the limits for each of the six classes (G levels) and their relationship to the traditional semi-cutoff and full cutoff terms. Table 4.9 BS EN 13201: Part 2: 2003 road lighting luminaire classification G level Cutoff classification

Semi-cutoff

Full cutoff

Maximum luminous intensity/1000 lamp lumens, at 70° from downward vertical

Maximum luminous intensity/1000 lamp lumens, at 80° from downward vertical

Maximum luminous intensity/1000 lamp lumens, at 90° from downward vertical

Other requirements

G1

-

200

50

None

G2

-

150

30

None

G3

-

100

20

None

G4

500

100

10

0 at greater than 95°

G5

350

100

10

0 at greater than 95°

G6

350

100

0

0 at greater than 95°

Operating conditions The International Protection (IP) system classifies luminaires according to the degree of protection provided against the ingress of foreign bodies, dust and moisture. The degree of protection is indicated by the letters IP followed by two numbers. The first number indicates the degree of protection against the ingress of foreign bodies and dust. The second indicates the protection against the ingress of moisture. Table 4.10 shows the degree of protection indicated by each number. Using Table 4.10 it can be seen that a luminaire classified as IP55 is dust protected and able to withstand water jets.

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First number

Degree of protection

Second number

Degree of protection

0

Not protected

0

Not protected

1

Protected against solid objects greater than 50 mm

1

Protected against dripping water

2

Protected against solid objects greater than 12 mm

2

Protected against dripping water when tilted up to 15 degrees

3

Protected against solid objects greater than 2.5 mm

3

Protected against spraying water

4

Protected against solid objects greater than 1.0 mm

4

Protected against splashing

5

Dust-protected

5

Protected against water jets

6

Dust-tight

6

Protected against heavy seas

7

Protected against the effects of immersion

8

Protected against submersion to a specified depth

Chapter Four: Luminaires

Table 4.10 IP classification of luminaires according to the degree of protection against foreign bodies, dust and moisture

Electrical protection Luminaires are also classified according to the protection they provide against electric shock. Table 4.11 shows the luminaire classes in the IEC classification.

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Chapter Four: Luminaires

Table 4.11 The classification of luminaires according to the degree of electrical protection Luminaire class

Degree of electrical protection

0

A luminaire having functional insulation, but no double insulation or reinforced insulation throughout, and without provision for earthing. This type of luminaire is not permitted in the UK.

1

A luminaire having at least functional insulation throughout and provided with an earthing terminal or earthing contact, and, for luminaires designed for connection by means of a flexible cable or cord, provided with either an appliance inlet with earthing contact, or a non-detachable flexible cable or cord with earthing contact and a plug with earthing contact

2

A luminaire with double insulation and/or reinforced insulation throughout and without provision for earthing

3

A luminaire designed for connection to extra-low voltage circuits and which has no circuits, either internal or external which operate at a voltage greater than extra-low safety voltage

Flammability The temperature of a luminaire may limit the surfaces on which it can be mounted. If the surface is non-combustible, then any luminaire may be mounted on it. But when the surface is either normally flammable or readily flammable, restrictions may apply. A normally flammable surface is one having an ignition temperature of at least 200 ˚C and that will not deform or weaken at this temperature. A readily flammable surface is one that cannot be classified as normally flammable or non-combustible. Readily flammable materials are not suitable for direct mounting of luminaires. The IEC recommends a two part classification system. For luminaires suitable for direct mounting only on non-combustible surfaces, a warning notice may be required. For luminaires suitable for direct mounting on normally flammable surfaces a symbol consisting of a letter F inside an inverted triangle is required.

F 108

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5.1 Control gear A wide range of lamps require control gear of some kind to ensure correct running and, in some cases, starting of the lamp. With discharge lamps it is the job of the control gear to limit the current through the lamp whereas with some incandescent lamps the gear is there to reduce the voltage. Some low voltage tungsten lamps need units to supply them with the correct voltage and LEDs need electronics to limit the current going through them.

Chapter Five: Electrics

Chapter 5: Electrics

5.1.1 Ballasts for discharge light sources General principles Control gear for discharge lamps has to perform a number of functions: limit and stabilises lamp current: due to the negative resistance characteristic of gas discharge lamps (see Section 3.1.2) it is necessary to control the current in the lamp circuit ensure that the lamp continues to operate despite the mains voltage falling to zero at the end of each half cycle provide the correct condition for the ignition of the lamp: this generally requires the gear to provide a high voltage and in the case of fluorescent lamps requires a heating current to be passed through the electrodes. As well as these basic functions control gear may also have the following requirements placed on it: ensure a high power factor limit the harmonic distortion in the mains current limit any electromagnetic interference (EMI) produced by the lamp and ballast limit the short-circuit and run up currents to protect the lamp electrodes and to help the supply wiring system keep the lamp current and voltage within the specified limits for the lamp during mains voltage fluctuations. With electromagnetic control gear several separate control components may be needed — these may include ballasts, starters, igniters, capacitors and filter-coils. When electronic control gear is used it is common to integrate all the components into one package. The details of the various circuits used are discussed in the following sections. Electromagnetic control gear for fluorescent light sources Choke coils used to be the most common type of current limiting device used with linear and compact fluorescent lamps. The most common circuit is the switch start, see Figure 5.1.

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Chapter Five: Electrics

B La

La = Lamp B = Ballast S = Switch

S

0

Figure 5.1 Schematic diagram of a fluorescent lamp operated using a choke ballast and a switch start The choke ballast is made from a large number of windings of copper on a laminated iron core. It works on the self-inductance principle and is designed so that impedance of the choke limits the current through the circuit to the correct value for a given lamp and supply voltages. A range of ballasts is available for different lamps and different voltages. Also the ballast design has to be changed if it is to operate at a different mains supply frequency. To start the lamp it is common to use a glow starter. The glow starter switch consists of one or two bi-metallic strips enclosed in a glass tube containing a noble gas. The glow starter is connected across the lamp so it is possible for a current to pass through the ballast, through the electrode at one end of the lamp, through the electrode at the other end of the lamp and back to neutral. When the mains voltage is first applied to the lamp circuit, the total mains voltage appears across the electrodes of the starter and this initiates a glow discharge. This discharge heats the bi-metallic elements within the starter and as the electrodes heat up they bend towards each other until eventually they touch. While the electrodes are touching the current passing through the lamp electrodes pre-heats them. While the electrodes in the starter are touching there is no glow discharge and so the electrodes cool and separate. At the moment that the electrodes come apart the current through the ballast is interrupted causing a voltage peak across the lamp. Note: the glow starter does not always create the conditions for the lamp to start and sometimes the starting cycle has to be repeated a number of times. Figures 5.2 to 5.4 illustrate the starting process. Figure 5.2 The heat from the discharge in the starter causes the bi-metallic electrodes to bend together

Figure 5.3 The bi-metallic electrodes touch and a current flows through the circuit preheating the electrodes of the lamp

Figure 5.4 The electrodes cool and separate, causing a voltage peak which ignites the lamp

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Electromagnetic control gear for HID light sources There are a number of different types of circuits used for high intensity discharge (HID) lamps; they vary according to the type of lamp and its requirements for starting. The most common type of ballast used is a choke or inductive ballast in series with the lamp. The choke, which is a coil of copper wire wound on a laminated iron core, limits the current through the lamp. Figure 5.5 shows a typical circuit using a choke.

+

B

–

+

Chapter Five: Electrics

In addition to the ballast and the starter most fluorescent lamps circuits have a capacitor connected across the supply terminals to ensure a high power factor for the circuit.

– La

+ La = Lamp B = Ballast

0

–

Figure 5.5 Schematic diagram of a HID lamp circuit using a choke This type of circuit is used for all high intensity discharge lamps apart from the low pressure sodium lamp. The low pressure sodium lamp has a long run-up during which time the voltage across the lamp needs to be greater than normal mains voltage; this has given rise to a number of circuits for running the lamp that provide the necessary voltage. The most common of these circuits is the autoleak transformer (Figure 5.6). La = Lamp B = Ballast C = Capacitor

L

B

La

C

N

Figure 5.6 Schematic diagram of a low pressure sodium lamp circuit using an autoleak transformer 111

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Chapter Five: Electrics

The autoleak transformer works like an autotransformer increasing the supply voltage, but by careful design of the secondary winding it can also act as a choke to control the current through the lamp. Most high pressure sodium lamps and metal halide lamps require a high voltage pulse to start the arc in the lamp. This is usually provided by an electronic ignitor. There are several types of ignitor circuits, the two most common are the semi-parallel and the superimposed pulse type (Figures 5.7 and 5.8). B

B L

L

La

La

C

N

N La = Lamp B = Ballast C = Capacitor

Figure 5.7 A semi-parallel ignition system

La = Lamp B = Ballast

Figure 5.8 A superimposed ignition system

The semi-parallel ignitor relies on the tapped ballast coil to generate the ignition pulse whereas the superimposed type ignitor has its own coil to generate the pulse. The semi-parallel has many advantages in that it consumes no power when the lamp is running, it is cheaper and lighter but, as it relies on the ballast, it may only be used with the ballast for which it has been specifically designed. Ignitors sometimes have other features built in such as self-stopping ignitors that will not continually try to restrike a lamp that has come to the end of its life. There are also some that are designed to produce extra high voltages that can restrike hot lamps. Electronic control gear for fluorescent light sources Operating fluorescent lamps at high frequency has a number of advantages (see Section 3.3.3) and most modern control gear is now of this type. Most electronic ballasts for fluorescent lamps are integrated into a single package that performs a number of functions. These functions are: a low pass filter: this limits the amount of harmonic distortion caused by the ballast, controls the amount of radio frequency interference, protects the ballast against high voltage mains peaks and limits the inrush current the rectifier: this converts the AC power from the mains supply into DC a buffer capacitor: this stores the charge from each mains cycle thus providing a steady voltage to the circuits that provide the power to the lamps 112

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the output of the power oscillator is fed through a small HF coil that acts as a stabilisation coil to the lamp. Figure 5.9 shows the main components in typical HF fluorescent lamp ballast

Chapter Five: Electrics

the HF power oscillator takes the steady DC voltage from the buffer capacitor and using semi conductor switches controlled by the ballast controller creates a high frequency square wave

C S1 L L Control 1 electronics

2 L

N + –

S2

C low pass filter

rectifier

buffer capacitor

HF power oscillator

Lamp stabilisation

Figure 5.9 A circuit diagram of an electronic ballast for two fluorescent lamps In some ballasts the electronics that control the power oscillator can vary the frequency at which the power oscillator runs; as the frequency increases the current passing through the coils decreases and thus it is possible to dim the lamps. Some types of ballast have a 0 to 10 volt input that is used to regulate the output while some have digital interfaces. See Section 5.2 for further information on controls. Electronic gear for HID light sources Making electronic control gear for HID light sources is a complex process. There are many different lamp types each with different electrical requirements and a limited range of frequencies in which they can be operated. Also many lamp types do not show a significant gain in efficiency when operated on high frequencies. For these reasons electronic control gear has been developed more slowly for HID lamps than for fluorescent lamps. However, it is possible to gain a number of benefits from electronic gear for HID lamps. These include: increased lamp life elimination of visible flicker better system efficacy less sensitivity to mains voltage or temperature fluctuations the possibility of dimming with some lamp types. 113

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Chapter Five: Electrics

Not all these benefits are possible for all lamp types and all control gear combinations. However, the availability and quality of electronic gear available for HID lamps is rapidly increasing. 5.1.2 Transformers for low voltage light sources Many tungsten halogen lamps are designed to run on low voltages the most common of which is 12 volts. Thus they need a device to reduce the supply voltage. The traditional way to do this was by using a transformer. Figure 5.10 shows the various currents and voltages in a transformer and gives the approximate relationship between the voltages, currents and the number of turns in the primary and secondary coils. Ip

Vp

Np : Ns

Is

Ep

Es

Vp Vs

=

Is Ip

=

Vs

Np Ns

Figure 5.10 A circuit diagram for a transformer As well as reducing the voltage the transformer also isolates the lamp supply from the mains. This means that even under a fault condition the voltage in the secondary circuit will not rise significantly above the nominal output voltage and so it will always be safe to touch the conductors on the low voltage side. Most modern transformers for halogen lamps involve electronics. They usually contain high frequency oscillators to permit the use of smaller transformers that have smaller power losses. With the introduction of electronics it is possible to introduce additional features such as constant voltage output and soft starting of the lamps. 5.1.3 Drivers for LEDs LEDs need to be run at a controlled current to ensure proper operation. To provide this drivers are used. Most drivers take mains power and provide a constant current output.However, it is possible to control some drivers so that output current is varied so that the LED may be dimmed. In more complex systems it is possible to dim three separate channels separately, so that when red, green and blue LEDs are used together it is possible to make colour changes. Most LED drivers can maintain their constant current output over a range of voltages so it is often possible to connect a number of LEDs in series on one driver. 114

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5.2.1 Options for control There are a number of factors that need to be considered in any control system; these are the inputs to system, how the system controls the lighting equipment and what is the control process that decides how a particular set of inputs will impact on the lighting. Thus for a control system to work it must have: input devices: such as switches, presence detectors, timers and photocells

Chapter Five: Electrics

5.2 Lighting controls

control processes: these may consist of a simple wiring network through to a computer based control system controlled luminaires: the system may control luminaires in a number of ways, from simply switching them on and off to dimming the lamp and in more complex systems causing movement and colour changes. 5.2.2 Input devices Manual inputs These vary from simple switches used to turn the lights on though dimmer switches and remote control units that interface to a control system to lighting control desks that are used in theatres. The point of these units is to allow people to control the lighting and care is always needed in the application of such devices to ensure that users of the system can readily understand the function of any such control. Presence detectors Most presence detectors are based on passive infrared (PIR) detectors, however some devices are based on microwave or ultrasonic technology. PIR devices monitor changes in the amount of infrared radiation that they are receiving. The movement of people in a space will be detected by them and this can be signalled to a control system. Thus, if a device detects the presence of a person this can be used to signal the control system to switch the lights on, but if the device has not detected anybody for some time this can be used to signal that there is nobody there and that the lights can be turned off. Timers Most computerised control systems have timers built in so that they can turn the lighting on or off at particular times. However, there are also a large number of time switches available that can turn lamps on an off at given times. There are also timers used in street lighting that change the time that they switch at throughout the year so that the lamps are switched at dawn and dusk. Photocells There are many different types of photocell used to control lighting. The simplest to use are those which switch on at one illuminance value and switch off at another; these are commonly used to turn exterior lights on at dusk and off at dawn. Some photocells communicate the illuminance value to the central control system, which uses the information to adjust the lighting in some way. Some photocells are mounted on ceilings with shields around them so that they only receive light reflected from the working plane, this makes them act like luminance meters and provided the reflectance of the working plane remains constant they can be set up to follow the illuminance of that plane. 115

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Chapter Five: Electrics

5.2.3 Control processes and systems In the case of simple control systems these are generally configured as some form of automated switching in the power supply to a luminaire or group of luminaires. However, more complex systems are generally configured as a network of devices including luminaires, sensors and control inputs. In most systems the devices are physically connected using some form of cabled network but, in principle, devices can be controlled using wireless or infrared communication. There are several systems in common use for lighting systems and care needs to be taken to specify the correct type for each component in the system. Two of the most common systems available are DALI and DMX 512. The basic specification for DALI systems is contained in BS EN 60929: 2006: AC-supplied electronic ballasts for tubular fluorescent lamps — Performance requirements. The DALI system is largely used for lighting systems in buildings but has been extended so that it can be used more widely. It controls luminaires via the ballast used to control the lamps. The system is designed to run up to 64 luminaires on one circuit but there are devices that can control a series of different DALI clusters thus making it possible to control all the lights in a large building. DMX 512 was designed to control lights and other equipment in the entertainment industry. The system provides 512 channels of control to a series of devices. In a typical spotlight that has its aiming controlled, three channels may be used, one to dim the luminaire and one for each axis of rotation. The system has traditionally been used in theatres but is increasingly being used in architectural feature lighting where the lighting equipment is more complex. The basic operating properties of the system are described in ANSI E1.11: USITT DMX512-A: Asynchronous serial digital data transmission standard for controlling lighting equipment and accessories.

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6.1 Objectives and constraints Lighting design can have many different objectives. Ideally, these objectives are determined by the client and the designer in collaboration and cover both outcomes and costs (Figure 6.1). The most common objective for a lighting installation is to allow the users of a space to carry out their work quickly and accurately, without discomfort. However, this is a rather limited view of what a lighting installation can achieve. For traffic routes, the objective of lighting is to facilitate the safe and rapid movement of vehicles after dark. For urban areas where people and traffic may come into conflict, safety is the primary concern although the appearance of people and buildings is also important. In areas where crime is rampant, lighting can be used to enhance security. Sport facilities are lit at night to encourage their use. Businesses use lighting to promote their brand and attract customers. Most lighting installations have to serve multiple functions. When designing lighting it is always desirable to identify all the functions that the lighting is expected to fulfill.

Chapter Six: Lighting design

PART 3: APPLICATIONS Chapter 6: Lighting design

As for constraints, an important aspect of lighting design is the need to minimise the amount of electricity consumed, for both financial and environmental reasons. It is also necessary to consider the sustainability of the lighting equipment. This means using materials that can be easily replaced and considering to what extent the equipment can be recycled at the end of its life. The financial costs, particularly the capital cost, are always an important constraint. No one wants to pay more for something than is absolutely necessary so the designer needs to be able to justify the proposal in terms of value for money. Architectural integration

Visual amenity

Visual function

Lighting design

Costs (capital and operating)

Energy efficiency

Installation maintenance

Figure 6.1 Objectives, outcomes and costs

6.2 A holistic strategy for lighting A holistic strategy for lighting design is necessary because without it important benefits will be lost and money and human resources will be wasted. The starting point is an in-depth conversation with the client and other members of the design team to formulate a design brief. At such a discussion, it will be necessary to address such fundamental questions as what do you want to see and what do you not want to see, what is the function of the space, what is the proposed architectural style and what is the budget?

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Chapter Six: Lighting design

More formally, nine distinct aspects of lighting need to be considered. They are: legal requirements

maintenance

visual function

costs

visual amenity

photopic or mesopic vision

architectural integration

light trespass and sky glow.

energy efficiency and sustainability All these aspects will contribute to the success of a design, but they may not all carry equal weight depending on the particular application and situation. Also there is no particular order in which they should be considered. The important issue is that all the elements are considered, perhaps more than once, for a satisfactory solution to emerge. 6.2.1 Legal requirements There are a number of legal requirements that apply to all lighting installations. Some are general, e.g. the Construction (Design and Management) Regulations. Some are specific about the type and form the lighting that should be provided, e.g. emergency lighting in buildings (see Chapter 8). Others influence lighting design by the limits they place on the type or amount of equipment that can be used, e.g. Building Regulations. Details of the requirements of the Construction (Design and Management) Regulations can be obtained from the Health and Safety Executive publications. Details of the significance of Part L of the Building Regulations can be found in SLL Factfile 9: Lighting and the 2006 Building Regulations. It is essential that the designer and the client are aware of the relevant legal requirements. 6.2.2 Visual function This aspect is related to the lighting required for doing tasks without discomfort. Chapter 2 has shown how the illuminance incident on the task will affect the level of visual performance achievable. Recommended illuminances for different tasks are given in the SLL Code for lighting, various SLL Lighting Guides as well as in Part 3 of this Handbook. These values apply to the task area and do not necessarily need to apply to the whole working plane. The traditional way of lighting an interior work place has been a regular array of luminaires. For this approach, a minimum task illuminance uniformity (minimum/average task illuminance ≥ 0.7) is recommended. This approach has the benefit that the tasks can be carried out on the horizontal plane anywhere in the work place. In some cases the task will have a colour recognition element. In such cases it will be necessary to use lamps with a high general colour rendering index (CRI). For such tasks it will be appropriate to use lamps with a CRI ≥ 80 but for tasks with a requirement for very good colour discrimination, lamps with a CRI ≥ 90 will be necessary. The human visual system can adapt to a wide range of luminances but it can only cope with a limited luminance range at any single adaptation state. When this range is exceeded, glare will occur. If a field of view contains bright elements that cause glare it is likely that they will affect performance or at least cause stress and fatigue which in turn will cause problems.

118

To avoid this will mean using luminaires and windows that have limited luminances within the normal fields of view relative to the adaptation level. Glare limits for different tasks are given in the SLL Code for lighting, various SLL Lighting Guides as well as in Part 3 of this Handbook.

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Leisure

Figure 6.2 Map showing the possible locations of three application areas on a schematic diagram linking subjective impressions of visual interest and visual lightness

Commercial

Industrial

Low

Visual interest (Degree of light non-uniformity)

High

Chapter Six: Lighting design

6.2.3 Visual amenity There is no doubt that lighting can add visual amenity to a space which can give pleasure to the occupants but whether this provides a more tangible performance benefit is uncertain (Boyce, 2003). Studies have shown that people respond to the lit appearance of a room on two independent dimensions: visual lightness and visual interest (Hawkes et al, 1979, Loe et al, 1994, 2000). Visual lightness describes the overall lightness of the space, which is related to the average luminance of vertical surfaces. Visual interest refers to the non-uniformity of the illumination pattern or the degree of ‘light and shade’. People prefer some modulation in the light pattern rather than an even pattern of illumination, the magnitude of the modulation depending on the application. There is some evidence that visual lightness and visual interest are inversely correlated (Figure 6.2).

Low

High

Visual lightness (brightness)

Although variation in the light pattern is desirable, it has to be seen as meaningful in terms of the application and the architecture. To provide random patches of light in an uncoordinated way for no reason other than to provide light variation would be a poor design solution. Acceptable examples could be highlighting displays within a retail outlet, or a floral display in a hotel lobby. There are two further areas of visual amenity that need to be considered and these are in the colour rendering and colour appearance of the lighting. The required colour rendering will depend on the functions the lighting is designed to fulfill. Where fine colour discrimination is required, light sources with a CIE general colour rendering index of at least 80 should be used. Where a natural appearance is required for people and objects, light sources with a CIE general colour rendering index of at least 60 and preferably higher should be used. Where such functions are not important poorer colour rendering light sources can be used. As for colour appearance, a light source with a correlated colour temperature (CCT) ≤ 3000 K will appear warm and if it has a CCT ≥ 5300 K it will appear cool (see Section 1.4.3). Where on this scale from warm to cool the colour appearance should be will depend on the nature of the space. In quasi-domestic situations, such as hotels, a warm colour appearance will be required but in commercial interiors a CCT of around 4000 K is appropriate as it blends reasonably well with daylight. The designer should be wary of the names applied to light sources as these can be misleading and differ between manufacturers. The best way to choose colour appearance is through practical trials. 119

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Chapter Six: Lighting design

There is still much to learn about design for visual amenity but it would be negligent to ignore it. The best way to develop an understanding of visual amenity is through personal observations and trial installations. 6.2.4 Lighting and architectural integration All elements of a lighting installation contribute to the architecture or the interior design of a building. Understanding the space will be important when deciding what sort of lighting is to be employed. The dimensions, finishes, texture and colour of the materials forming the space and the appearance of the luminaires, lit and unlit, should be considered if the desired atmosphere is to be achieved. A good place to start is with the daylighting since the windows and roof lights are a fundamental element of the fabric of the building. This means considering the amount and pattern of daylight required for the particular application, and hence the size and positions of windows and roof lights. But windows cannot be designed on the basis of the daylighting alone and other visual, thermal, acoustic and privacy issues need to be addressed. Only a few lighting designers get involved with daylighting design. This is a pity because only a few architects have the skills to design an effective and efficient window system, which means that many opportunities are lost. More information on daylighting design can be obtained from the SLL Lighting Guide 10: Daylighting and window design and Chapter 7 of this Handbook. Once the daylighting has been determined then the electric lighting can be planned. To integrate electric lighting with the architecture means considering not only its operation with respect to the daylighting, but the appearance of the luminaires and controls and the way they are incorporated into the fabric of the building, as well as the lighting effect produced. Just as the light pattern needs to be meaningful with respect to the building use, the lighting scheme needs to be meaningful with respect to the architecture. 6.2.5 Energy efficiency and sustainability It is the responsibility of the lighting profession to use energy as efficiently as possible but at the same time to provide lit environments that enable people to operate effectively and comfortably. The current estimate for the UK is that approximately 19% of the electricity generated is consumed by lighting. This amounts to around 64 TW⋅h/annum. Energy use involves two components: the power demand of the equipment and its hours of use. The lighting industry has worked hard to develop equipment that has reduced the demand for electricity for lighting by producing more efficient light sources and their related control circuits, as well as more efficient luminaires. Then there are design options to be considered, such as the use of task/ambient lighting rather than a blanket provision of light by a regular array of ceiling mounted luminaires. The savings for the task/ambient approach have been estimated to be up to 50% (Loe, 2003). Good energy efficient lighting design is not just about equipment; it is also about the use of lighting. There are many examples where lighting is left on when it is not required. This may be because there is inadequate lighting through daylighting or because people are not present and therefore the lighting is unnecessary. This aspect of lighting design needs a dramatic change in attitude to improve the energy efficiency of all lighting installations. This requires changes to how the lighting is controlled both manually and automatically as well as how lighting is provided in terms of the distribution of light, particularly with respect to the daylighting. It is also necessary for the lighting industry and its customers to use equipment that is sustainable.

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6.2.6 Maintenance It must be recognised that both daylight and electric light within a building will depreciate with time. To minimise the effect of this a maintenance programme will need to be designed and implemented. The maintenance programme will also affect the lighting design and the designer will need to state the maintenance programme on which the design has been based, otherwise there could be problems when a client is comparing different design proposals. It will also be important for the client to be provided with a maintenance schedule so that they know what will need to be done. Chapter 21 discusses the various factors that need to be considered when developing a maintenance program.

Chapter Six: Lighting design

This means that the materials used are wherever possible from renewable sources and that at the end of life the redundant equipment can be disposed of safely with most of the base materials being recycled.

6.2.7 Lighting costs Costs are always a major concern for any project and it is important to consider these before any work is undertaken. Both the capital cost and the running, or operational, costs must be considered at the outset. If the two cost elements are not considered together in terms of life cycle costing, then a solution which has a low capital cost but a high operational cost could be more costly overall than an installation with a more expensive capital cost but a low operating cost. A conflict of interests may arise if the two cost elements are paid for from different budgets or organisations. This can happen with local authority projects. Here the designer needs to present a balanced view of the options to enable the clients to decide on the best approach. The capital costs include the cost of the design process, the equipment and the installation process, both physical and electrical. It also includes the commissioning and testing of the installation. Allowance must also be made for any builders’ work that forms part of the lighting installation. Any other costs that are particular to the lighting design need to be included. It is important that the capital cost is agreed at an early stage if a lot of time is not to be wasted. The capital cost should be challenged if the client’s expectations seem to be unrealistic. The operational costs include the cost of the electricity consumed, which comprises items such as standing charges, maximum demand charges and electricity unit costs. They will also include the cost of maintenance, which includes cleaning and relamping throughout the life of the installation. In some cases charges may have to be budgeted for the disposal of redundant equipment although this may be borne by the supplier. 6.2.8 Photopic or mesopic vision The photometric quantities used to characterise lighting are all based on photopic vision (see Section 1.2). This makes sense for interior lighting where the luminances are usually high enough to ensure the visual system is operating in the photopic state but there may be problems for exterior lighting. This is because for adaptation luminances below about 3 cd/m2 peripheral vision is operating in the mesopic state (see Section 2.2.2) and exterior lighting often produces luminances below this level. This is a problem because the spectral sensitivity of the peripheral retina changes continually during mesopic vision depending on the adaptation luminance, the peak sensitivity moving from the 555 nm to 507 nm as the adaptation luminance decreases to the scotopic state. There is no CIE mesopic observer so no system of mesopic photometry. In this situation, the simplest approach to ensuring good mesopic vision in exterior lighting is to use a light source with a scotopic/photopic (S/P) ratio greater than 1.5. Such light sources provide stimulation to both the cone and rod photoreceptors of the retina. 121

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Chapter Six: Lighting design

6.2.9 Light trespass and skyglow Light can be considered a form of pollution. This is implied by the inclusion of light as a statutory nuisance in the Clean Neighbourhoods and Environment Act: 2005. Exterior lighting is the major source of light pollution. Complaints about light pollution from exterior lighting can be divided into two categories, light trespass and skyglow. Light trespass is local in that it is associated with complaints from individuals in a specific location. The classic case of light trespass is a complaint about light from a road lighting luminaire entering a bedroom window and keeping the occupant awake. Light trespass can be avoided by the careful selection, positioning, aiming and shielding of luminaires and by operating a curfew system where lighting is only available during specified times. The Institution of Lighting Engineers (ILE) has produced general guidance on the vertical illuminance that should be allowed to fall on windows, the maximum luminous intensity of any obtrusive light source and a maximum building luminance for floodlighting. These limits are different for different environmental zones. The idea behind environmental zones is that some locations are more sensitive to light pollution than others. Table 6.1 shows the four environmental zones identified by the CIE. The limits recommended by the ILE for limiting light trespass are given in Table 6.2. Table 6.1 The environmental zoning system of the CIE Environmental zone

Zone description and examples of sub-zones

E1

Areas with intrinsically dark landscapes: National Parks, areas of outstanding natural beauty (where roads are usually unlit)

E2

Areas of ‘low district brightness’: outer urban and rural residential areas (where roads are lit to residential road standard) Areas of ‘middle district brightness’: generally urban residential areas (where roads are lit to traffic route standard)

E3

Areas of ‘high district brightness’: generally, urban areas having mixed recreational and commercial land use with high night-time activity

E4

Table 6.2 Maximum vertical illuminance on windows, maximum luminous intensity for obtrusive luminaires and maximum building luminance produced by floodlighting, for four environmental zones Environmental zones

122

Maximum vertical illuminance on windows (lx)

Maximum luminous intensity (cd)

Maximum building luminance (cd/m2)

Before curfew

After curfew

Before curfew

After curfew

E1

2

1

0

0

0

E2

5

1

50

0.5

5

E3

10

5

100

1.0

10

E4

25

10

100

2.5

25

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Sky glow is more diffuse than light trespass in that it can affect people over great distances. Sky glow is caused by the multiple scattering of light in the atmosphere, resulting in a diffuse distribution of luminance. The problem this causes is that it reduces the luminance contrast of all the features of the night sky thereby reducing the number of stars and other astronomical phenomena that can be seen. Sky glow has two components, one natural and one due to human activity. Natural sky glow is light from the moon, planets and stars that is scattered by interplanetary dust, and by air molecules, dust particles, water vapour and aerosols in the Earth’s atmosphere, and light produced by a chemical reaction of the upper atmosphere with ultra-violet radiation from the sun. The luminance of the natural sky glow at zenith is of the order of 0.0002 cd/m2. The contribution of human activity is produced by light traversing the atmosphere and being scattered by dust and aerosols in the atmosphere. The magnitude of the contribution of city lights to sky glow at a specific remote location can be crudely estimated by Walker’s Law. This can be stated as

Chapter Six: Lighting design

The values in Table 6.2 are for general guidance only and may need to be adjusted for specific circumstances. For example, the criteria given under zone E1 would not preclude the installation of lighting to meet health and safety requirements. As for the maximum building luminance, this is given to avoid overlighting but should be adjusted according to the general district brightness. An alternative approach based on limiting light crossing a property’s boundary is the outdoor site-lighting performance (OSP) method (Brons et al, 2008). This method has the advantage that it deals with the site the designer is responsible for and does not require detailed knowledge of areas outside the site.

I = 0.01 P d–2.5 where: I = the proportional increase in sky luminance relative to the natural sky luminance, for viewing 45° above the horizon in the direction of the city (e.g. I = 0.1 = 10 percent increase) P = the population of the city d = distance to the city (km) This empirical formula assumes a certain use of light per head of population. Experience suggests the predictions are reasonable for cities where the number of lumens per person is between 500 and 1000 lumens. Sky glow can be reduced by limiting the amount of light used for exterior lighting, by using full-cutoff luminaires that have no upward component (see Table 4.9) and by adopting a curfew in which the exterior lighting is either extinguished or reduced to a lower level when there are few people using it. For each environmental zone the maximum installed upward light output ratio of the luminaires used should be limited as shown in Table 6.3. Again, this is general guidance only and may need to be overturned in specific circumstances. The OSP method (Brons et al, 2008) again provides an alternative and more comprehensive approach in that it takes the whole installation and covers reflected light as well as direct upward light.

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Chapter Six: Lighting design

Table 6.3 Maximum installed upward light output ratio; luminous flux emitted above the horizontal plane as a percentage of the total luminous flux emitted by the luminaire Environmental zone

Maximum upward light output ratio (%)

E1

0

E2

5

E3

15

E4

25

6.3 Basic design decisions 6.3.1 Use of daylight One of the first decisions to be made when approaching lighting design for interiors is what role will daylight play. The role will depend on the building use but the decision should be recorded early on and be part of the brief. The roles may be any one or any combination of the following: to provide a view out to provide enough light to work by to save energy to provide lighting for particular tasks requiring very good colour rendering to enhance the appearance of the space by providing meaningful variation in the lighting. Depending on the primary role or roles of daylight and hence the amount and distribution of daylight in the space, the electric lighting will need to be designed as a stand-alone system or as an integrated system. 6.3.2 Choice of electric lighting system The selection of the luminaire, light source and control system to be used is an important one if electricity is not to be wasted and an efficient lighting installation achieved. The first choice to be made will be to determine the technique to be employed. For interiors, the techniques, in order of decreasing energy consumption, can be simply categorised as: a general system: providing a uniform illuminance over the whole working plane area a localised system: using luminaires located adjacent to the workstation to provide the task illuminance, whilst the overall ambient lighting is provided by the spill light from the luminaires

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For exteriors, a general system is the usual choice where work is carried out but much greater degrees of non-uniformity are acceptable where the function of the lighting is essentially decorative. The second decision to be made will be the choice of light source and luminaire. The characteristics of available light sources and luminaire types are set out in Chapters 3 and 4 respectively. It is important to appreciate that light sources differ in their luminous efficacy, life, colour properties, run-up and restrike times and in their ability to be dimmed. Luminaires differ in the distribution of light and the efficiency with which they emit the light produced by the light source.

Chapter Six: Lighting design

a local lighting system, sometimes called task/ambient: this uses two separate systems that are inter-dependant; a general lighting installation to provide an ambient lighting condition and a desk or partition mounted task light under the control of the occupant and topping up the ambient lighting illuminance to achieve the required task illuminance.

The third choice to be made is the type of control system. Switching luminaires used to be the only viable approach to take, but now, with high frequency electronic dimmable ballasts dramatically reducing in price, dimming is a realistic option. For interiors, dimming can be used to save energy even when daylight is absent. This is due to the fact that all lighting is designed for average maintained illuminance, which provides more light to start with than is required. When daylight is present, dimming can be used to balance the electric lighting to the daylight. For exteriors, switching and dimming can be used to match the lighting to the patterns of use, e.g. a supermarket car park does not need to be completely lit at 3 a.m. Any users at that hour will park near the doors. There are basically two forms of lighting control systems: analogue and digital (see Section 5.2). Analogue systems typically use a 1–10 volt protocol providing continuously variable dimming. The digital systems most widely used are DALI and DMX512 (see Section 5.2.3). Both also provide continuously variable dimming. The advantages of digital over analogue control are many, one of the most important being the facility to monitor an installation through a two-way communication capability. This transfer of information makes preventative maintenance and energy monitoring possible. Control systems can provide the facility for individual or group addressing, zoning and scene setting. The recording of energy consumption is also highly desirable if the installation is to provide the information for building monitoring required by Part L of the Building Regulations. Some control systems allow remote monitoring via the internet. This can be of great benefit to companies with large building estates, such as the NHS, education authorities, banks, national retail chains etc. By monitoring centrally in a region or area, preventative maintenance can be undertaken such as the anticipation of bulk lamp replacement from the hours-run data. 6.3.3 Integration Integration of a lighting installation takes four forms: integration within the space, architecture, interior design integration with other services integration with daylight integration with the surroundings. 125

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Chapter Six: Lighting design

Integration within the space A lighting installation can be visible and express the interior design or it can disappear into the background with only its effect being seen. Both approaches rely heavily on attention to detail, Specifically, attention to the appearance of the luminaire, lit and unlit, is necessary for a design that is intended to express the interior design, while attention to the builder’s work details is required if the intention is to hide the luminaires. The other aspects of the space that can interact with the lighting are the reflectances and colours of the interior décor. Large areas of low reflectance reduce the amount of inter-reflected light. If inter-reflected light is planned to make a significant contribution to the amount of light delivered, large areas of high reflectance surfaces are needed. As for surface colour, the extent to which they interact with the lighting depends on the saturation of the colour and the area it covers. Large areas of saturated colour can distort the colour of the light delivered. However, spaces without colour can be very uninteresting. The use of saturated colours over small areas provides some interest without distorting the lighting. Integration with other services Most services like sprinklers, loudspeakers, fire detectors and supply and extract grilles/diffusers have an optimum spacing to cover the area under consideration and lighting to some extent is the same. Creating a ceiling plane that is restful and harmonious to the eye requires compromise. Integration with air conditioning systems needs particular care. Luminaires on the ceiling can short circuit or deflect airflows thereby creating overheating or draughts. Also, the light output of fluorescent lamps is affected by the ambient temperature. This means extracting hot air through luminaires and integrating luminaires into chilled beams needs careful consideration. Integration with daylight To integrate electric lighting with daylight requires zoning of the electric lighting according to the distribution of daylight and a choice between switching and dimming control. For buildings with perimeter glazing a rough guide to zoning is to divide the floor plate into 3 m wide strips starting at the perimeter. For buildings with a regular array of rooflights, zoning may not be necessary. As for switching or dimming, field studies of switching behaviour have shown that, with manual switching, electric lighting is usually either all on or all off. Switching is almost entirely confined to the beginning and end of a period of occupation; people may switch lighting on when entering a room but seldom turn it off until they all leave. The year-round probability that an occupant will switch lights on when entering a room depends on the time of day, orientation of the windows and the minimum orientation-weighted daylight factor on the working area (see Chapter 7). Figure 6.3 can be used to estimate the likely energy consumption of an electric lighting installation when manual switching is used to adjust it to daylight. If the minimum orientationweighted daylight factor in the room is 1 percent and work starts at 0800 hours, Figure 6.3 shows a 60 percent probability of switching on on entering. If the room is continually occupied, we may conclude that 60 percent is the probability that the lighting will be on at any moment during the working day. Thus for a lighting installation with a load of 3 kW, and a working year consisting of 260 days, each of 8 hours, the total annual energy consumption would be 260 × (8 × 0.60) × 3 = 3744 kW⋅h

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Probability of switch on (%)

90 80 70 0.5 percent

60

1 percent

50

2 percent

40

5 percent

30 20 10

Chapter Six: Lighting design

100

0 4

6

8

10

12

14

16

18

20

22

Time

Figure 6.3 The probability of electric lighting being switched on at different times of day for locations with different orientation-weighted daylight factors (from Crisp and Henderson, 1982) If luminaires are logically zoned with respect to daylight with convenient pull-cord switches for the occupants to use, each zone can be treated as a separate room. The probability of switching will differ from zone to zone, depending on the minimum orientation-weighted daylight factor in each zone. Figure 6.3 will still be applicable but the minimum orientation-weighted daylight factor, and consequent energy savings, must be estimated separately for each zone. If switching is to be relatively unnoticeable to the occupants, the proportion of the electric lighting switched should not be more than 20 percent of the total task illuminance. Automatic photo-electric controls can be used to switch electric lighting in response to daylight. Figure 6.4 shows the percentage of a normal working year during which the luminaires would be off, as a function of the orientation-weighted daylight factor and of the illuminance at which the luminaires are switched; the ‘design’ illuminance. These curves assume that ‘on’ and ‘off ’ switching will occur at the same illuminance. Where this is not the case, where the luminaires are switched off at an illuminance appreciably greater than that at which they are switched on, the mean of the two illuminances should be taken as the ‘design’ illuminance.

100 90

Percentage off

100 lx 80

300 lx

70

500 lx 1000 lx

60 50 40 30 20 10 0 0

1

2

3

4

5

Figure 6.4 The percentage of the working year that electric lighting will be switched off plotted against orientationweighted daylight factor for different ‘design’ illuminances, assuming an on/off photoelectric switching system (from Hunt, 1979)

6

Orientation weighted daylight factor

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Chapter Six: Lighting design

Automatic photoelectric controls can also be used to dim the electric lighting in response to daylight. Figure 6.5 shows the percentage of a normal working year during which the luminaires would have to be switched off in order to ensure the energy saving obtainable by continuous photo-electric dimming are achieved. It applies to dimmer systems that can control down to 10 percent light output or less. Recommendations for daylighting and supplementary electric lighting are given in BS 8206: Part 2. Integration with the surroundings For exterior lighting, the lighting of the surrounding area has an impact on the perception of the brightness of the installation. The same installation in rural and urban settings will look very bright in the former and dim in the latter. This means that the maintained illuminance selected needs to be matched to the illuminances of the surroundings if the expected appearance is to be achieved. 100 90 100 lx

Percentage off

80

300 lx 500 lx

70

1000 lx 60 50 40 30 20 10 0 0

1

2

3

4

5

6

Orientation weighted daylight factor

Figure 6.5 The percentage of the working year that electric lighting will be switched off plotted against orientation-weighted daylight factor for different ‘design’ illuminances, assuming a top-up photoelectric dimming system (from Hunt, 1979) 6.3.4 Equal and approved One problem that frequently afflicts lighting designers is the substitution of a cheaper luminaire for the one specified in the original design. Such substitutions are usually made in an effort to save money. Sometimes, substitutions are justified, sometimes they are not. The key to determining if a substitution is justified is a review carried out by the original designer to determine if the substitute luminaire is the equal of the originally specified luminaire and approved to the relevant standards, i.e. if it is equal and approved. The factors to be considered in the review are the photometric characteristics, the construction and the aesthetics of the substitute luminaire. In addition, attention should be paid to the electrical characteristics, conformity to the relevant standards and the impact on maintenance. Further details of these elements of the review can be found in the joint statement issued by the Society of Light and Lighting, the Electrical Contractors Association, the Institution of Lighting Engineers and the Lighting Industry Federation in 2004 (Joint statement, 2004). 128

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7.1 Benefits of daylight In the face of increasing energy costs and concern about global warming, there is considerable interest in using daylight as the major light source in buildings. Unfortunately, there is little point in doing this if daylighting causes problems to the occupants of buildings. The possibility of daylight causing problems to occupants may seem unlikely given the well established desire of people to have natural light wherever possible, whenever available. However, a short walk around any city will reveal numerous well-glazed office buildings where the blinds on many windows are permanently closed (Figure 7.1). Such behaviour demonstrates the existence of a failed daylighting design for at least some people within the building. Nonetheless, unless there is a good reason why there should be no daylight in the building, daylighting should always be encouraged.

Chapter Seven: Daylighting

Chapter 7: Daylighting

Figure 7.1 A modern office building with extensive glazing and extensive use of blinds To make the best use of daylight it is first necessary to recognise that daylight can have both positive and negative effects on people. The conclusions of an extensive literature review on daylight (Boyce et al, 2003a)