Tài liệu The SLL Lighting Handbook potx

The Society of Light and Lighting
is part of the Chartered Institution
of Building Services Engineers
The Society of
Light and Lighting
The SLL
Lighting
Handbook
The Society of
Light and Lighting
The SLL Lighting Handbook
The Society of
Light and Lighting
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 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
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The Society of Light and Lighting
is part of the Chartered Institution
of Building Services Engineers
The Society of
Light and Lighting
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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
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
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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
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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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
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
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
Chapter 7: Daylighting
7.1 Benefits of daylight

7.2 Daylight availability
7.3 Daylight as a contribution to room brightness
7.4 Daylight for task illumination
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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
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
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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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
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
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
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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
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
Chapter 13: Lighting for museums and art galleries

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
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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
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
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
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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
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
Chapter 18: Security lighting
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
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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
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
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)
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21.8 Determination of maintenance factor for exterior lighting
21.9 Disposal of lighting equipment
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
Chapter 23: Bibliography
23.1 Standards
23.2 Guidance
23.3 References
Index
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Chapter One: Light
PART 1. FUNDAMENTALS
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.
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.
Wavelength (m)
RADIO
WAVES
MICRO
WAVES
INFRA
RED
ULTRA
VIOLET
X RAYS
GAMMA
RAYS
COSMIC
RAYS
780 nm
380 nm
VISIBLE
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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 V
M
(
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.
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
Relative luminous
efficiency
= Standard photopic observer
= Modified photopic observer
= Standard scotopic observer
= 10 degree field
Wavelength (nm)

300 400 500 600 700 800
1.0
0.8
0.6
0.4
0.2
0
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Chapter One: Light
1.3 The measurement of light — photometry
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:
where:
Φ
= luminous flux (lumens)
= radiant flux in a small wavelength interval ∆
λ
(watts)
= the relative luminous efficiency function for the conditions
K
m
= constant (lumens/watt)
= 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 K
m
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.
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.
Φ
= K
m
Σ
Ψ
λ
V
∆

λ
λ
Ψ
λ
V
λ
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
∆
λ
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Chapter One: Light
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/m
2
.
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 =
where luminance is expressed in candela/m
2
and illuminance is expressed in lumens/m
2
.
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/m
2
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
(illuminance × reflectance)
π
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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
Luminous flux
Luminous
intensity
Illuminance
Luminance
Luminance
coefficient
Reflectance
For a diffuse surface:
Luminance factor
For a non-diffuse
surface, for a specific
direction and
lighting geometry:
Units
lumens (lm)
candela (cd)
lumen/m
2
candela/m
2
candela/lumen
Definition
That quantity of radiant flux which expresses
its capacity to produce visual sensation
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
The luminous flux/unit area at a point on
a surface
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
The ratio of the luminance of a surface to the
illuminance incident on it
The ratio of the luminous flux reflected from a
surface to the luminous flux incident on it
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
luminance = (illuminance × reflectance ) / π
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Chapter One: Light
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/m
2
for illuminance and candela/m
2

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
* 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.
Quantity
Illuminance
Luminance
Luminous
exitance*
Unit
lux
metre candle
phot
footcandle
nit
stilb
apostilb*
blondel*
lambert*
footlambert*
Dimensions
lumen/m
2
lumen/m
2

lumen/cm
2
lumen/ft
2
candela/m
2
candela/cm
2
candela/in
2
candela/ft
2
lumen/m
2
lumen/m
2
lumen/cm
2
lumen/ft
2
Multiplying factor
1.00
1.00
10,000
10.76
1.00
10,000
1,550
10.76
0.32

0.32
3,183
3.43
Situation
Clear sky in summer
in temperate zones
Overcast sky in summer
in temperate zones
Textile inspection
Office work
Heavy engineering
Good road lighting
Moonlight
Illuminance (lm/m
2
)
100,000
16,000
1,500
500
300
20
0.5
Typical surface
Grass
Grass
Light grey cloth
White paper
Steel
Concrete road surface

Asphalt road surface
Luminance (cd/m
2
)
1,910
300
140
120
20
2.0
0.01
Table 1.3 Typical illuminance and luminance values.
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Chapter One: Light
1.4 The measurement of light — colourimetry
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.
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).
2.5
2.0
1.5
1.0
0.5
0
z
y
x
400 450 500 550 600 650 700
Wavelength (nm)
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:
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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).
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Chapter One: Light
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.
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
520 nanometers
480 nanometers

490
560
470
550
460
540
450
530
570
0.2
580
510
24,000
380
500
Y
X
0.1 0.3 0.4 0.5 0.6 0.7 0.80
Equal
energy
10,000
6500
4800
3500
590
600
510
620
630
640

780
2360
1900
1500
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Chapter One: Light
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 self-
luminous 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 – u
n
)
V* = 13 W* (v – v
n
)
W* = 25 Y
0.33
– 17 (where Y has a range from 1 to 100)
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 u
n
, v
n

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.
0.6
0.5
0.4
0.3
0.2
0.1
520
480
560
470
460
540
450
0.2
580
500
0.1 0.3 0.4 0.5 0.60
600
620
640
770 nm
440
400 nm420
V'

U'
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