handbook of human factors and ergonomics (salvendy/handbook of human factors 4e) || illumination

CHAPTER 24ILLUMINATION

Peter R. BoyceConsultantCanterbury, United Kingdom

1 INTRODUCTION 673

2 MEASUREMENT OF ILLUMINATION 674

2.1 Photometric Quantities 674

2.2 Colorimetric Quantities 674

2.3 Instrumentation 676

3 PRODUCTION OF ILLUMINATION 677

3.1 Daylight, Sunlight, and Skylight 677

3.2 Electric Light Sources 678

3.3 Control of Light Distribution 678

3.4 Control of Light Output 678

4 FUNCTIONAL CHARACTERISTICSOF HUMAN VISUAL SYSTEM 679

4.1 Visual System Structure 679

4.2 Wavelength Sensitivity 680

4.3 Adaptation 681

4.4 Color Vision 682

4.5 Receptive Field Size and Eccentricity 682

4.6 Meaningful Stimulus Parameters 682

5 EFFECTS ON THRESHOLD VISUALPERFORMANCE 683

5.1 Visual Acuity 683

5.2 Contrast Sensitivity Function 684

5.3 Temporal Sensitivity Function 684

5.4 Color Discrimination 685

5.5 Interactions 686

5.6 Approaches to Improving ThresholdVisual Performance 686

6 EFFECTS ON SUPRATHRESHOLDVISUAL PERFORMANCE 686

6.1 Relative Visual Performance Modelfor On-Axis Detection 686

6.2 Visual Search 687

6.3 Visual Performance, Task Performance,and Productivity 688

6.4 Approaches to Improving SuprathresholdVisual Performance 689

7 EFFECTS ON COMFORT 689

7.1 Symptoms and Causes of VisualDiscomfort 689

7.2 Lighting Conditions That Can CauseDiscomfort 690

7.3 Comfort, Performance, and Expectations 691

7.4 Approaches to Improving Visual Comfort 691

8 INDIVIDUAL DIFFERENCES 692

8.1 Changes with Age 692

8.2 Helping People with Low Vision 693

8.3 Consequences of Defective Color Vision 693

9 OTHER EFFECTS OF LIGHT ENTERINGTHE EYE 694

9.1 Human Circadian System 694

9.2 Positive and Negative Affect 694

10 TISSUE DAMAGE 695

10.1 Mechanisms for Damage to Eyeand Skin 695

10.2 Acute and Chronic Damage to Eyeand Skin 695

10.3 Damage Potential of DifferentLight Sources 696

10.4 Approaches to Limiting Damage 696

11 EPILOGUE 696

REFERENCES 696

1 INTRODUCTION

Illumination is the act of placing light on an object. Byproviding illumination, stimuli for the human visualsystem are produced and the sense of sight is allowedto function. With light we can see, without light we

cannot see. This chapter is devoted to describing howto measure and produce illumination, the effects ofdifferent lighting conditions on visual performance andvisual comfort, the photobiological and psychologicaleffects of illumination, and the risks inherent inexposure to light.

673Handbook of Human Factors and Ergonomics, Fourth Edition Gavriel SalvendyCopyright © 2012 John Wiley & Sons, Inc.

674 EQUIPMENT, WORKPLACE, AND ENVIRONMENTAL DESIGN

2 MEASUREMENT OF ILLUMINATION

2.1 Photometric Quantities

Light is a part of the electromagnetic spectrum, lyingbetween the wavelength limits 380–780 nm. What sep-arates this wavelength region from the rest is thatradiation in this region is absorbed by the photoreceptorsof the human visual system, which initiates the processof seeing.

The most fundamental measure of the electromag-netic radiation emitted by a source is its radiant flux.This is a measure of the rate of flow of energy emittedand is measured in watts. The most fundamental quantityused to measure light is luminous flux. Luminous fluxis radiant flux multiplied by the relative spectral sensi-tivity of the human visual system over the wavelengthrange 380–780 nm.

The relative spectral sensitivity of the humanvisual system is based on the perception of brightnessassociated with each wavelength. In fact, there are twodifferent relative spectral sensitivities, sanctified byinternational agreement arranged through the Commis-sion Internationale de l’Eclairage (CIE, 1983, 1990).There are two relative spectral sensitivities because thehuman visual system has two classes of photoreceptor:cones, which operate primarily when light is plentiful,and rods, which operate when light is very limited.These two photoreceptor types have different spectralsensitivities: the day photoreceptor, the cones, charac-terized by the CIE standard photopic observer, and thenight photoreceptor, the rods, characterized by the CIEstandard scotopic observer (Figure 1).

Luminous flux is used to quantify the total light out-put of a light source in all directions. While this is im-portant, for lighting practice it is also important to beable to quantify the luminous flux emitted in a givendirection. The measure that quantifies this concept isluminous intensity. Luminous intensity is the luminousflux emitted per unit solid angle in a specified direction.

1.0

Rel

ativ

e lu

min

ous

effic

ienc

y

0.5

0.0400 450 500 600 700550

Wavelength (nm)

650

bc a

Figure 1 Relative luminous efficiency functions for (a) theCIE standard photopic observer and (b) the CIE standardscotopic observer. The CIE standard photopic observeris based on a 2-degree field of view. Also shown (c) is therelative luminous efficiency function for a 10-degree fieldof view in photopic conditions.

The unit of measurement is the candela, which isequivalent to one lumen per steradian. Luminous in-tensity is used to quantify the distribution of light froma luminaire.

Both luminous flux and luminous intensity havearea measures associated with them. The luminous fluxfalling on unit area of a surface is called the illuminance.The unit of measurement of illuminance is the lumensper square meter, or lux. The luminous intensity emittedper unit projected area in a given direction is theluminance. The unit of measurement of luminance is thecandela per square meter. The illuminance incident on asurface is the most widely used electric lighting designcriterion. The luminance of a surface is a correlate ofits brightness. Table 1 summarizes these photometricquantities and the relationship between illuminance andluminance.

Unfortunately for consistency, photometry has along history that has generated a number of differentunits of measurement for illuminance and luminance.Table 2 lists some of the alternative units, together withthe multiplying factors necessary to convert from thealternative unit to the System Internationale (SI) units oflumens per square meter for illuminance and candelasper square meter for luminance. The SI units will beused throughout this chapter.

Table 3 shows some illuminances and luminancestypical of commonly occurring situations.

2.2 Colorimetric QuantitiesThe photometric quantities described above do not takeinto account the wavelength combination, that is, thecolor of the light being measured. There are two ap-proaches to characterizing color, the color order systemand the CIE colorimetry system.

2.2.1 Color Order SystemsA color order system is a physical, three-dimensionalrepresentation of color space. It is three dimensionalbecause colors have three separate subjective attributes;hue, brightness, and strength. Hue tells us whetherthe color is primarily red or yellow or green or blue.Brightness tells us to what extent the color transmitsor reflects light. Strength tells us whether the color isstrong or weak.

There are several different color order systems usedin different parts of the world (Wyszecki and Stiles,1982). Probably the most widely used is the MunsellBook of Color available from the Munsell ColorCompany. Figure 2 shows the three-dimensional colorspace of the Munsell system. The position of any coloris identified by an alphanumeric code made up of threeterms: hue, value, and chroma (e.g., a strong red isgiven the alphanumeric 7.5R/4/12). Hue, value, andchroma are related to the three attributes of color:hue, brightness, and strength, respectively. Buildingmaterials, such as paints, plastic, and ceramics, arecommonly classified in terms of a color order system.

2.2.2 CIE Colorimetric SystemSometimes, it is necessary to quantify the color of alight or a surface before either exists. To meet this

ILLUMINATION 675

Table 1 Photometric Quantities

Quantity Definition Units

Luminous flux That quantity of radiant flux which expresses its capacity toproduce visual sensation

lumen (lm)

Luminous intensity The luminous flux emitted in a very narrow cone containingthe 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 meter−2

(lm m−2)Luminance The luminous flux emitted in a given direction divided by the

product of the projected area of the source elementperpendicular to the direction and the solid anglecontaining that direction, i.e. luminous flux/unit solidangle/unit area

candela meter−2

(cd m−2)

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

For a matte surface luminance = illuminance × reflectanceπ

Luminance factor The ratio of the luminance of a reflecting surface, viewed in agiven direction to that of a perfect white uniform diffusingsurface identically illuminated:

For a nonmatte surface for aspecific viewing directionand lighting geometry

luminance = illuminance × luminance factorπ

Table 2 Common Photometric Units of Measurement for Illuminance and Luminance and Factors Necessaryto Change Them to SI Units

Multiplying Factorto Convert

Quantity Unit Dimensions to SI Unit

Illuminance (Sl unit = lumen meter−2) lux lumen meter−2 1.00meter candle lumen meter−2 1.00phot lumen centimeter−2 10,000.00foot candle lumen foot−2 10.76

Luminance (SI unit = candela meter−2) nit candela meter−2 1.00stilb candela centimeter−2 10,000.00

— candela inch−2 1,550.00— candela foot−2 10.76

apostilba lumen meter−2 0.32blondela lumen meter−2 0.32lamberta lumen centimeter−2 3,183.00foot-lamberta lumen foot−2 3.43

aThese four items are based on an alternative definition of luminance. This definition is that if the surface can be consideredas perfectly matte, its luminance in any direction is the product of the illuminance on the surface and its reflectance. Thus,the luminance is described in lumens per unit area. This definition is deprecated in the SI system.

Table 3 Typical Illuminance and Luminance Values

Illuminanceon Horizontal Luminance

Situation Surface (lm/m2) Typical surface (cd/m2)

Clear sky in summer in northern temperate zones 150,000 Grass 2,900Overcast sky in summer in northern temperate zones 16,000 Grass 300Textile inspection 1,500 Light grey cloth 140Office work 500 White paper 120Heavy engineering 300 Steel 20Residential road lighting 10 Asphalt road surface 0.2Moonlight 0.1 Asphalt road surface 0.002


White

10

9

8

7

6

5

4

2

1

10Y10YR

5YR

10R

10P10B

5B

6 5 54 43 32 21 1

Hue scale

10RP

5R

5RP

5P5PB

10BG

10G

10GY5GY

5G

5BG

Value scale

6

Chroma scale

5Y

Black

Figure 2 Organization of Munsell color 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.

need and to provide a more accurate characterizationof color, the CIE has developed a system of colorimetryranging from the simple to the complex (CIE, 1978,1995, 2004a,b). The most fundamental characteristicof light is the spectral power distribution reaching theeye. It is this spectral power distribution that largelydetermines the color seen, although the perception ofcolor is also influenced by the surroundings (Purves andBeau Lotto, 2003). Unfortunately, the implications ofcomparisons between spectral power distributions aredifficult to comprehend. The CIE has developed twothree-dimensional color spaces, both based on mathe-matical manipulations applied to spectral distributions(Robertson, 1977; CIE, 1978). These color spaces, Laband Luv, provide a convenient means of quantifyingcolor, the Lab space used mainly for object colors andthe Luv space for self-luminous colors. If two colorshave the same coordinates in one of these color spaces,under the same observing conditions they will appearthe same. The distance between two colors in the colorspace is related to how easily they can be distinguished.

An earlier CIE color space, the 1964 Uniform ColorSpace, is used in the calculation of the CIE generalcolor-rendering index, a single number index whichis widely applied to light sources to indicate howaccurately they render colors relative to some standard(CIE, 1995). Specifically, the positions in color spaceof 8 test colors, under a reference light source andunder the light source of interest, are calculated. Theseparation between the two positions of each test colorare calculated, the separations for all the test colors aresummed and scaled to give a value of 100 when there isno separation for any of the test colors, i.e. for perfectcolor rendering.

It should be noted that the CIE general color-rendering index is a very crude metric. Different lightsources have different reference light sources and the

summation means that light sources that render the testcolors differently can have the same color-renderingindex. Much more sophisticated are the color appearancemodels now available (Hunt, 1991; CIE, 2004b), buttheir existence has had little impact on lighting prac-tice. Rather, a two-dimensional color surface is stillwidely used to characterize the color appearance of lightsources and to define the acceptable color characteristicsof light signals (CIE, 1994). This is the CIE 1931chromaticity diagram shown in Figure 3. Essentially it isa slice through the color space at a fixed luminance. Thecurved boundary of the chromaticity diagram consists ofthe colors produced by single wavelengths. The equal-energy point in the center of the diagram correspondsto a colorless surface. The further the coordinates of acolor are from the equal-energy point and the closer theyare to the boundary, the greater the strength of the color.Figure 3 also shows several areas in which a signal lightneeds to fall if it is to be perceived as the specifiedcolor. The color appearance of nominally white lightsources is conventionally described by their correlatedcolor temperature. This is the temperature of the fullradiator that is closest to the coordinates of the lightsource on the CIE 1931 chromaticity diagram (Wyszeckiand Stiles 1982). A useful summary of these colorimetrysystems is given in the tenth edition of the LightingHandbook of the Illuminating Engineering Society ofNorth America (IESNA, 2011).

2.3 Instrumentation

The instrumentation for measuring photometric andcolorimetric quantities can be divided into laboratoryand field equipment. Laboratory equipment tends to belarge and/or sophisticated and hence expensive. Fieldequipment is small and portable. The luminous fluxfrom a light source, the luminous intensity distribution

ILLUMINATION 677

0.9

0.8

0.7

0.6

0.5

y0.4

0.3

0.2

0.1

0.00.0 0.1

460470

480

490

500

510

520530

540

550

560

570

580

590600

610620

640700

Red

White

Yellow

Green

Violet

Blue

0.2 0.3 0.4x

0.5 0.6 0.7 0.8

Figure 3 CIE 1931 chromaticity diagram. The boundary curve is the spectrum locus with the wavelengths (nm) marked.The filled circle is the equal-energy point. The enclosed areas indicate the chromaticity coordinates of light signals thatwill be identified as the specified colors.

of a luminaire, and light source color properties areconventionally measured in the laboratory.

The two most widely used field instruments are theilluminance meter and the luminance meter. Illuminancemeters have three important characteristics: sensitivity,color correction, and cosine correction. Sensitivity refersto the range of illuminances covered, the range desiredbeing dependent on whether the instrument is to beused to measure daylight, interior lighting, or night-time exterior lighting. Color correction means that theilluminance meter has a spectral sensitivity matchingthe CIE standard photopic observer. Cosine correctionmeans that the illuminance meter’s response to lightstriking it from directions other than the normal followsa cosine law.

The luminance meter is designed to measure theaverage luminance over a specified area. The luminancemeter has an optical system that focuses an image on adetector. Looking through the optical system allows theoperator to identify the area being measured and usuallydisplays the luminance of the area. The important char-acteristics of a luminance meter are its spectral response,its sensitivity, and the quality of its optical system.Again, a good luminance meter has a spectral responsematching the CIE standard photopic observer. The sen-sitivity needed depends on the conditions under whichit will be used. The quality of its optical system canbe measured by its sensitivity to light from outside themeasurement area (CIE, 1987).

Procedures for using illuminance or luminancemeters in the field and for light measurements in the

laboratory are described and referenced in the guidancepublished by national bodies [IESNA, 2011; CharteredInstitution of Building Services Engineers (CIBSE),2009; Society of Light and Lighting (SLL), 2009]. Itshould be noted that virtually all commercial instrumen-tation used to measure illuminance and luminance usesthe CIE standard photopic observer as the basis of theinstrument’s spectral sensitivity, even when the instru-ment is designed to be used in mesopic and scotopicconditions.

Recently, another approach has been developed forrapidly acquiring the distribution of luminances over alarge area. This approach uses multiple images capturedby a digital camera and is called high-dynamic-range(HDR) imaging (Inanici, 2006). At the moment, HDRimaging is mainly being used for capturing luminancedistributions that are subject to large and suddenchanges, for example, sky luminances.

3 PRODUCTION OF ILLUMINATION

Illumination is produced naturally by the sun andartificially by electric light sources.

3.1 Daylight, Sunlight, and SkylightNatural light is light received on Earth from the sun,either directly or after reflection from the moon. Theprime characteristic of natural light is its variability.Natural light varies in magnitude, spectral content, anddistribution with different meteorological conditions, at


different times of day and year, and at different latitudes.Moonlight is of little interest as a source of illumination,but daylight is used, and strongly desired, for thelighting of buildings. Daylight can be divided into twocomponents, sunlight and skylight. Sunlight is lightreceived at Earth’s surface, directly from the sun. Sun-light produces strong, sharp-edged shadows. Skylight islight from the sun received at Earth’s surface after scat-tering in the atmosphere. Skylight produces only weak,diffuse shadows. The balance between sunlight andskylight is determined by the nature of the atmosphereand the distance that the light passes through it. Thegreater is the amount of water vapor and the longer thedistance, the higher is the proportion of skylight.

The illuminances on Earth’s surface produced bydaylight can cover a large range, from 150,000 lx ona sunny summer day to 1000 lx on a heavily overcastday in winter. Several models exist for predicting thedaylight incident on a plane at different locations fordifferent atmospheric conditions (Robbins, 1986). Thesemodels can be used to predict the contribution of day-light to the lighting of interiors. Alternatively, there arenow available sets of measured illuminance or irradiancedata for many different sites around the world. Thesemake it possible to do climate-based modeling of day-light availability in interiors and the impact of daylighton the energy use of buildings (Mardaljevic et al., 2009).

The spectral composition of daylight also varies withthe nature of the atmosphere and the path length throughit. The correlated color temperature of daylight can varyfrom 4000 K for an overcast day to 40,000 K for aclear blue sky. For calculating the appearance of objectsunder natural light, the CIE recommends the use of oneof three different spectral distributions corresponding tocorrelated color temperatures of 5503, 6504, and 7504 K(Wyszecki and Stiles, 1982).

3.2 Electric Light SourcesThe lighting industry makes several thousand differ-ent types of electric lamps. Those used for providingillumination can be divided into three classes: incan-descent, discharge, and solid state. Incandescent lampsproduce light by heating a filament. Discharge lampsproduce light by an electric discharge in a gas. Solid-state lamps produce light by the passage of an electriccurrent through a semiconductor. Incandescent lampscan operate directly from mains electricity. Dischargelamps all require control gear between the lamp and theelectricity supply, because different electrical conditionsare required to initiate the discharge and to sustain it.Solid-state lamps require devices, called drivers, to limitthe current through the semiconductor.

Electric light sources can be characterized on severaldifferent dimensions:

• Luminous Efficacy . The ratio of luminous fluxproduced to power supplied (lumens per watt). Ifthe lamp needs control gear, the watts suppliedshould include the power demand of the controlgear.

• Correlated Color Temperature (CCT). A measureof the color appearance of the light produced,measured in degrees Kelvin (see Section 2.2.2).

• CIE General Color-Rendering Index (CRI). Ameasure of the ability to render colors accurately(see Section 2.2.2).

• Lamp Life. The number of burning hours untileither lamp failure or a stated percentage reduc-tion in light output occurs. Lamp life can varywidely with switching cycle.

• Run-Up Time. The time from switch-on to fulllight output.

• Restrike Time. The time delay between the lampbeing switched off before it will reignite.

Table 4 summarizes these characteristics for twoincandescent lamp types, seven discharge lamp types, andone solid-state type that are widely used for illuminationand gives the most common applications for each lamptype. The values in Table 4 should be treated as indicativeonly. Details about the characteristics of any specificlamp should always be obtained from the manufacturer.

3.3 Control of Light Distribution

Being able to produce light is only part of what is nec-essary to produce illumination. The other part is tocontrol the distribution of light from the light source.For daylight, this is done by means of window shape,placement, and glass transmittance (Robbins, 1986). Forelectric light sources, it is done by placing the lightsource in a luminaire. The luminaire provides electricaland mechanical support for the light source and controlsthe light distribution. The light distribution is controlledby using reflection, refraction, or diffusion, individuallyor in combination (Simons and Bean, 2000). One factorin the choice of which method of light control toadopt in a luminaire is the balance desired betweenthe reduction in the luminance of the light sourceand the precision required in light distribution. Highlyspecular reflectors can provide precise control of lightdistribution but do little to reduce source luminance.Conversely, diffusers make precise control of lightdistribution impossible but do reduce the luminanceof the luminaire. Refractors are an intermediate case.The light distribution provided by a specific luminaireis quantified by the luminous intensity distribution.All reputable luminaire manufacturers provide luminousintensity distributions for their luminaires.

3.4 Control of Light Output

The control of daylight admitted through a window isachieved by mechanical structures, such as light shelves,or by adjustable blinds (Littlefair, 1990). Whenever thesun or a very bright sky is likely to be directly visiblethrough a widow, some form of blind will be required.Blinds can take various forms, horizontal, Venetian,vertical, and roller being the most common. Blinds canalso be manually operated or motorized, either undermanual control or under photocell control. Probablythe most important feature to consider when selectinga blind is the extent to which it preserves a view ofthe outside. Roller blinds that can be drawn down to aposition where the sun and/or sky is hidden but the lowerpart of the widow is still open are an attractive option.

ILLUMINATION 679

Table 4 Properties of Some Electric Light Sources Widely Used for Illumination

Luminous Lamp Run-up RestrikeEfficacy Life Time Time

Source (lm/W) CCT (K) CRI (h) (min) (min) Application

IncandescentTungsten 8–14 2500–2700 100 1000 Instant Instant ResidentialTungsten–halogen 15–25 2700–3200 100 1500–5000 Instant Instant Residential,

retail, display

DischargeFluorescent 20–96 2700–17,000 50–98 8000–19,000 0.5 Instant CommercialCompact fluorescent 20–70 2700–6500 80–90 5000–15,000 0.5–1.5 Instant Commercial,

retail,residential

Mercury vapor 33–57 3200–3900 40–50 8000–10,000 4 3–10 Older industrialand road

Metal halide 60–98 3000–6000 60–93 2000–10,000 1–8 5–20 Industrial,commercial,retail and road

Low-pressure sodium 70–180 N/A N/A 15,000–20,000 10–20 1 RoadHigh-pressure sodium 53–142 1900–2150 19–65 10,000–20,000 3–7 0–1 Industrial, roadInduction 47–80 2550–4000 80 60,000 Instant Instant Road

Solid stateWhite light-emittingdiode (LED) usingphosphor

21–33 3000–4000 70–83 40,000 Instant Instant Residential,retail

Roller blinds made of a mesh material can preservea view through the whole window while reducing theluminance of the view out. Such blinds are an attractiveoption where the problem is an overbright sky but willbe of limited value when a direct view of the sun is theproblem. The same applies to low-transmission glass.

For electric light sources, control of light output isprovided by switching or dimming systems. Switchingsystems can vary from the conventional manual switchto sophisticated daylight control systems that dim lampsnear windows when there is sufficient daylight. Timeswitches are used to switch off all or parts of a lightinginstallation at the end of the working day. Occupancysensors are used to switch off lighting when there isnobody in the space. Such switching systems can reduceelectricity waste, but they will be irritating if they switchlighting off when it is required and they may shortenlamp life if switching occurs frequently. The factors tobe considered when selecting a switching system arewhether to rely on a manual or an automatic systemand, if it is automatic, how to match the switching tothe activities in the space. If your interest is primarilyin reducing electricity consumption, a good principle isto use automatic switch-off and manual switch-on. Thisprinciple uses human inertia for the benefit of reducingenergy consumption.

As for dimming systems, these all reduce light out-put and energy consumption, but a different system is re-quired for each lamp type. The factors to consider whenevaluating a dimming system are the range over whichdimming can be achieved without flicker or the lamp

extinguishing, the extent to which the color propertiesof the lamp change as the light output is reduced, andany effect dimming has on lamp life and energy con-sumption. There are large individual differences inpreferred illuminances so whether or not giving peopleindividual control of dimming will reduce energyconsumption depends on the maximum illuminanceprovided (Boyce et al., 2006b).

4 FUNCTIONAL CHARACTERISTICSOF HUMAN VISUAL SYSTEM

4.1 Visual System Structure

Illumination is important to humans because it alters thestimuli to the visual system and the operating state ofthe visual system itself. Therefore, an understanding ofthe capabilities of the visual system and how they varywith illumination is important to an understanding of theeffects of illumination. The visual system is composedof the eye and brain working together. Light entering theeye is brought to focus on the retina by the combinedoptical power of the air/cornea surface and the lens ofthe eye. The retina is really an extension of the brain,consisting of two different types of photoreceptors andnumerous nerve interconnections. At the photoreceptors,the incident photons of light are absorbed and convertedto electrical signals. The nerve interconnections takethese signals and carry out some basic image processing.The processed image is transmitted up the optic nerve ofeach eye to the optic chiasma, where nerve fibers fromthe two eyes are combined and transmitted to the left


Near vision

Distant vision

Lens rounded

CorneaRetina Fovea

Sclera

Optic nerve

Blindspot

Ciliarymuscle

Iris contracted

Pupil

Iris openedLens flattened

Retina

Opticchiasma

Visual areaof cortex

Optic NerveLateralgeniculate body

Figure 4 Section through the eye adjusted for near and distant vision and a schematic diagram of the binocular nervepathways of the visual system.

and right parts of the visual cortex. It is in the visualcortex that the signals from the eye are interpreted interms of past experience (Figure 4).

Many of the capabilities of the visual system canbe understood from the organization of the retina. Thetwo types of visual photoreceptors, called rods andcones from their anatomical appearance, have differentwavelength sensitivities and different absolute sen-sitivities to light and are distributed differently acrossthe retina.

Rods are the more sensitive of the two and effec-tively provide a night retina. Cones are less sensitive tolight and operate during daytime. In fact, there are threetypes of cones, each with a different spectral sensitivity.These cones are commonly called long-, middle-, andshort-wavelength cones, from their regions of maximumspectral sensitivity. These three cone types combinetogether to give the perception of color. Figure 5 showsthe distribution of rods and cones across the retina.Cones are concentrated in a small central area of the

retina called the fovea that lies where the visual axisof the eye meets the retina, although there are conesdistributed evenly across the rest of the retina. Rodsare absent from the fovea, reaching their maximumconcentration about 20◦ from the fovea. This variationin concentration of rods and cones with deviation fromthe fovea is amplified by the number of photoreceptorsconnected to each optic nerve fiber. In the fovea, theratio of photoreceptors to optic nerve fibers is close to1 but increases rapidly as the deviation from the foveaincreases. The net effect of this structure is to providedifferent functions for the fovea and the periphery.The fovea is the part of the retina which providesfine discrimination of detail. The rest of the retina isprimarily devoted to detecting changes in the visualenvironment that require the attention of the fovea.

4.2 Wavelength Sensitivity

The rod and cone photoreceptors have different absolutespectral sensitivities (Figure 6). The spectral response of

ILLUMINATION 681

200

150

50

0100 80 60

Nasal retina Fovea

ConesRods

Angular eccentricity (deg)

Temporal retina40 20 20 40 60 800

100

Den

sity

(th

ousa

nds

mm

−2)

Figure 5 Density of rod and cone photoreceptors acrossthe retina on a horizontal meridian. (After Osterburg, G.,Acta Ophthalmologica, Vol. 13, Suppl. 6, 1935.)

the cones lies between 380 and 780 nm with the peaksensitivity occurring at 555 nm. The spectral responseof the rods lies between 380 and 780 nm with the peakat 507 nm. The peak sensitivity of the rods is muchgreater than that of the cones. These spectral sensitivitiesform the basis of the CIE standard observers and hencethe photometric quantities discussed in Section 2.1. Byadjusting the spectral emission of a light source to liewithin the most sensitive part of the spectral response ofthe visual system, lamp manufacturers are able to varythe number of lumens emitted for each watt of powerapplied.

4.3 Adaptation

The visual system can operate over a range of about12 log units of luminance, from a luminance of 10−6 to106 cd/m2, from starlight to bright sunlight. But it cannotcover this range simultaneously. At any instant in time,the visual system can cover a range of 2 or 3 log unitsof luminance. Luminances above this limited range areseen as glaringly bright, those below as undifferentiatedblack. The capabilities of the visual system dependon where in the complete range of luminances it isadapted. Three different functional ranges of luminanceare conventionally identified: the photopic, mesopic,and scotopic. Table 5 summarizes the visual systemcapabilities in each of these functional ranges.

2

400

Rods

Peripheral conesFoveal cones

500

Wavelength (nm)

600 700

1

0

Log

rela

tive

spec

tral

sen

sitiv

ity

−1

−2

−3

−4

−5

Figure 6 Log relative spectral sensitivity of rod and conephotoreceptors plotted against wavelength. (After Wald,G., Science, Vol. 101, p. 653, 1945.)

The visual system continuously adjusts its state ofadaptation through three mechanisms: neural, mechan-ical, and photochemical. These three mechanisms dif-fer in their speed and range of adjustment. The neuralmechanism, which is based in the retina, operates in mil-liseconds and covers a range of two to three log unitsin luminance. The mechanical mechanism involves theexpansion and contraction of the iris. The consequentchanges in pupil size take about a second but coverless than one log unit in luminance. The photochem-ical mechanism covers the whole range of luminancebut is slow, the changes taking minutes. The exact timewill depend on the starting and finishing luminances. Ifboth starting and finishing luminances for the adaptationare greater than 3 cd/m2, only cones are involved. Asthe time constant for cones is of the order of 2–3 min,adaptation takes only a few minutes. When the startingluminance is in the operating range of the cones andthe finishing luminance is within the operating range ofthe rods, a two-stage adaptation process occurs involv-ing both cones and rods. As rods have a time constantaround 7–8 min, the adaptation time is much longer.Complete adaptation from a high photopic luminance todarkness can take up to an hour.

Table 5 Functional Ranges of Visual System Capabilities

Operating Luminance Active Peak WavelengthState Range (cd/m2) Photoreceptors Sensitivity Characteristics

Photopic > 3 Cones 555 nm Good color vision Good resolutionScotopic < 0.001 Rods 507 nm No color vision Poor resolution Fovea ‘‘blind’’Mesopic > 0.001 and <3 Cones and rods 555 nm in fovea,

between 555 nm and507 nm elsewhere

Reduced color vision, reduced resolutionrelative to photopic


Interior lighting is almost always sufficient for thevisual system to be operating in the photopic region.Exterior lighting on roads and in urban areas is usuallysufficient to keep the visual system operating in thelow-photopic or high-mesopic regions. It is in very ruralareas, at sea, or underground, where there is neitherexterior lighting nor moonlight, that the visual systemreaches the scotopic state. The speed of adaptationis important where a large and sudden change in theluminance occurs. Examples of situations where thishappens are the entrance to road tunnels during daytime(Bourdy et al., 1987) and the onset of emergencylighting during a power failure (Boyce, 1985). Theseproblems are overcome either by installing a gradualreduction in luminance, which allows more time foradaptation to occur, or by setting a minimum luminancewithin the neural adaptation range.

4.4 Color VisionWhen photopically adapted, the visual system candiscriminate many thousands of colors. This ability todiscriminate colors reduces as the adaptation luminancedecreases through the mesopic region and vanishesin the scotopic vision. This is because color vision ismediated by the cone photoreceptors. Different lightsources have different spectral emissions and hencerender colors differently. To ensure good color dis-crimination, it is necessary to use a light source thathas a high CIE general color-rendering index and thatproduces sufficient light to ensure the visual system isoperating in the photopic state.

4.5 Receptive Field Size and EccentricityThe retina is organized in such a way that increasingnumbers of photoreceptors are connected to each opticnerve fiber as the deviation from the fovea increases.

This feature of the visual system is important whendetection of a stimulus is necessary and it can occuranywhere in the visual field. The visual system willnormally operate by first detecting the stimulus off-axis, that is, in the peripheral visual field, and thenturning the eye so that the stimulus is brought onto thefovea for detailed examination. In order to identify astimulus off-axis, the stimulus should be clearly differentfrom its background, in luminance or color, and shouldchange in space or time, that is, it should either moveor flicker. A flickering light is commonly used to drawdrivers’ attention to important signs placed beside orabove the road.

4.6 Meaningful Stimulus ParametersAny stimulus to the visual system can be describedby five parameters: its visual size, luminance contrast,chromatic contrast, retinal image quality, and retinal illu-mination. These parameters are important in determiningthe extent to which the visual system can detect andidentify the stimulus.

4.6.1 Visual SizeThe visual size of a stimulus describes how big thestimulus is. The larger a stimulus is, the easier it is todetect.

There are several different ways to express the size ofa stimulus presented to the visual system, but all of themare angular measures. The visual size of a stimulus fordetection is best given by the solid angle the stimulussubtends at the eye. The solid angle is given by thequotient of the areal extent of the object and the squareof the distance from which it is viewed. The larger thesolid angle is, the easier the stimulus is to detect.

The visual size for resolution is usually given as theangle the critical dimension of the stimulus subtendsat the eye. What the critical dimension is depends onthe stimulus. For two points, the critical dimension isthe distance between the two points. For two lines, itis the separation between the two lines. For a Landoltring, it is the gap width. The larger is the visual size ofdetail in a stimulus, the easier it is to resolve the detail.

For complex stimuli, the measure used to expresstheir dimensions is the spatial frequency distribution.Spatial frequency is the reciprocal of the angular sub-tense of a critical detail, in cycles per degree. Com-plex stimuli have many spatial frequencies and hencea spatial frequency distribution. The match between thespatial frequency distribution of the stimulus and thecontrast sensitivity function of the visual system (seeSection 5.2) determines if the stimulus will be seen andwhat detail will be resolved.

Lighting can change the visual size of three-dimensional stimuli by casting shadows that extend ordiminish the apparent visual size of the stimulus.

4.6.2 Luminance ContrastThe luminance contrast of a stimulus quantifies its lumi-nance relative to its background. The higher the lumi-nance contrast is, the easier it is to detect the stimulus.There are two different forms of luminance contrast.For stimuli that are seen against a uniform background,luminance contrast is defined as

C = |Lt − Lb |/Lbwhere

C = luminance contrastLb = Luminance of the backgroundLt = Luminance of the detail

This formula gives luminance contrasts that rangefrom 0 to 1 for stimuli that have details darker than thebackground and from 0 to infinity for stimuli that havedetails brighter than the background. It is widely usedfor the former, for example, printed text.

For stimuli which have a periodic pattern, forexample, a grating, the luminance contrast or modulationis given by

C = (Lmax − Lmin)/(Lmax + Lmin)where

C = Luminance contrastLmax = maximum luminanceLmin = minimum luminance

This formula gives luminance contrast that rangesfrom 0 to 1.

ILLUMINATION 683

Lighting can change the luminance contrast of a stim-ulus by producing disability glare in the eye or veilingreflections from the stimulus or by changing the incidentspectral radiation when colored stimuli are involved.

4.6.3 Chromatic Contrast

Luminance contrast uses the total amount of lightemitted from a stimulus and ignores the wavelengths ofthe emitted light. It is the wavelengths emitted from thestimulus that largely determine its color. It is possibleto have a stimulus with zero luminance contrast that canstill be detected because it differs from its backgroundin color, that is, it has chromatic contrast. There isno widely accepted measure of chromatic contrast,although various suggestions have been made (Tansleyand Boynton, 1978). Fortunately, chromatic contrastonly becomes important for detection when luminancecontrast has reached a low level, typically less than 0.2(O’Donell and Colombo, 2008).

Lighting can alter chromatic contrast by using lightsources with different spectral emission characteristics.

4.6.4 Retinal Image Quality

As with all image-processing systems, the visualsystem works best when it is presented with a clear,sharp image. The sharpness of the stimulus can bequantified by the spatial frequency distribution of thestimulus; a sharp image will have high spatial frequencycomponents present, a blurred image will not.

The sharpness of the retinal image is determinedby the stimulus itself, the extent to which mediumthrough which it is transmitted scatters light, and theability of the visual system to focus the image on theretina. Lighting can do little to alter any of these factors,although it has been shown that light sources that arerich in the short wavelengths produce smaller pupilsizes and these tend to improve visual acuity slightly(Berman et al., 2006). The suggested explanation is thatthe smaller pupil sizes produce greater depth of field andhence better retinal image quality (Berman et al., 1993).

4.6.5 Retinal Illumination

The retinal illumination determines the state of adapta-tion of the visual system and therefore alters its capa-bilities. The retinal illumination is determined by theluminance in the visual field modified by the pupil size.Retinal illumination is measured in trolands, a quan-tity formed from the product of the luminance of thevisual field and the pupil size (Wyszecki and Stiles,1982). Illuminances and surface reflectances determinethe luminances of the visual field. Luminances and lightspectrum determine pupil size.

5 EFFECTS ON THRESHOLD VISUALPERFORMANCE

Qualitatively, threshold visual performance is the per-formance of a visual task close to the limits of whatis possible. Quantitatively, it is the performance of atask at a level such that it can be correctly carried out

on 50% of the occasions it is undertaken. Thresholdvisual performance is affected by many different vari-ables. For example, visual acuity is affected by the formof the target used, the luminance contrast of the target,the duration for which it is presented, where in the visualfield it appears, and the luminance of the surround rel-ative to the luminance of the immediate background. Inthis discussion of threshold visual performance, atten-tion will be limited to the effects of variables that arecontrolled by the lighting system, that is, the adaptationluminance and the spectral content of the light. Informa-tion on the influence of other variables can be obtainedfrom Boff and Lincoln (1988). In the data presented itwill be assumed that the observer is fully adapted to theprevailing luminance, the image of the target is on thefovea, the target is presented for an unlimited time, andthat observer is correctly refracted.

5.1 Visual Acuity

Visual acuity is the limit in the ability to resolve detail.Visual acuity has been frequently measured usinggratings or Landolt C’s. Visual acuity can be quantifiedas the angle subtended at the eye by the size of detailthat can be correctly detected on 50% of the occasionsit is presented.

No matter what target is used visual acuity improves,that is, the size of detail that can be resolved decreasesas adaptation luminance increases. Figure 7 shows thatas adaptation luminance increases from scotopic to pho-topic conditions, visual acuity increases, asymptoticallyapproaching a maximum at high luminances. Table 3gives some luminances typically found in interior andexterior lighting installations. Given a value for theadaptation luminance, Figure 7 can be used to deter-mine if detail of a given size can be resolved or not.A useful rule of thumb is that the detail needs to befour times bigger than the visual acuity limit if it is tobe resolved sufficiently quickly to avoid affecting visualperformance (Bailey et al., 1993) .

As for light spectrum, for a lamp producing whitelight, then, at the same luminance, a spectrum thatproduces a smaller pupil size will enhance visual acuityslightly (Berman et al., 2006).

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Figure 7 Effect of adaptation luminance on gap size ofLandolt C target which can just be resolved. (After Shlaer,S., Journal of General Physiology, Vol. 21, p. 165, 1937.)


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Figure 8 Effect of adaptation luminance on contrast sensitivity function. Contrast sensitivity plotted against spatialfrequency (cycles/degree) for different adaptation luminances. (After Savoy, R. L., and MaCann, J. J., Journal of the OpticalSociety of America, Vol. 65, p. 343, 1975.)

5.2 Contrast Sensitivity Function

Contrast sensitivity is the reciprocal of the luminancecontrast that can be detected on 50% of the occasionsit is presented. Contrast sensitivity is usually measuredusing a sinusoidal grating target. The contrast sensitivityfunction is contrast sensitivity plotted against the spatialfrequency of the sinusoidal target.

Figure 8 shows the effect of adaptation luminance onthe contrast sensitivity function. It shows that as theadaptation luminance increases from scotopic to pho-topic conditions, the contrast sensitivity increases forall spatial frequencies; the spatial frequency at whichthe peak contrast sensitivity occurs increases and thehighest spatial frequency that can be detected also in-creases. Figure 8 can be used to determine if a giventarget will be visible by breaking the target into its spa-tial frequency components and determining if any of thecomponents are within the limit set by the contrast sen-sitivity function (Sekular and Blake, 1994). The targetwill only be visible if at least one of its componentsfalls within this limit, although it should be noted thatthe appearance of the target will be different dependingon which component or components are visible. As arule of thumb, for a target to be easily seen, it is neces-sary for the luminance contrast to be at least twice thecontrast threshold.

As for the light spectrum, the results of Berman et al.(2006) imply that the contrast sensitivity function issomewhat influenced by different white-light spectra.

5.3 Temporal Sensitivity Function

The temporal sensitivity function shows percentagemodulation amplitude plotted against the frequency ofthe modulation. Figure 9 shows the effect of adaptationluminance on the temporal sensitivity function. It shows

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Figure 9 Effect of adaptation luminance on temporalsensitivity function. Percentage modulation amplitudeplotted against frequency (Hz) for different levels of retinalillumination. The retinal illuminations are: filled square =0.06 trolands; open square = 0.65 trolands; open, invertedtriangle = 7.1 trolands; open, upright triangle = 77trolands; open circle = 850 trolands; filled circle = 9300trolands. (After Kelly, D. H., Journal of the Optical Societyof America, Vol. 51, p. 422, 1961.)

that as the adaptation luminance increases from mesopicto photopic conditions, the temporal sensitivity increasesfor all frequencies; the frequency at which the peaktemporal sensitivity occurs increases, and the highestfrequency that can be detected also increases. Figure 9can be used to determine if a given temporal variationwill be visible by breaking the waveform representing

ILLUMINATION 685

the light fluctuation into its frequency components anddetermining if any of the components are within the limitset by the temporal sensitivity function. The fluctuationwill only be visible if at least one of its frequencycomponents falls within this limit.

Temporal fluctuation in luminous flux (i.e., flicker) isundesirable in lighting installations. To eliminate flicker,it is necessary to increase the frequency and/or decreasethe modulation sufficiently to take their combinationoutside the limits set by the temporal sensitivityfunction. In practice, this is easily done. Incandescentlamps have sufficient thermal inertia to ensure that, eventhough the frequency of the fluctuation is only twice thesupply frequency (120 Hz for a 60-Hz electrical supply),the modulation is small so there is little chance of seeingflicker from such a lamp. Discharge and solid-statelamps do not have thermal inertia so their modulationcan be high where there is no phosphor used to modifythe spectrum of the light emitted. Where a phosphor

is used, the persistence of the phosphor will tend toreduce the modulation. To ensure that discharge andsolid-state lamps do not produce visible flicker, it is bestto use a control gear that operates at a high frequencyor, alternatively, in the case of solid-state lamps, zerofrequency, that is, from a direct-current supply.

5.4 Color Discrimination

The ability to discriminate between two colors of thesame luminance depends on the difference in spectralpower distribution of the light received at the eye.Figure 10 shows the MacAdam ellipses, the areaaround a number of chromaticities, each magnified 10times, within which no discrimination of color can bemade, even under side-by-side comparison conditions(Wyszecki and Stiles, 1982).

The effect of illuminance on the ability to discrim-inate between colors is limited in the photopic region,

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Figure 10 MacAdam ellipses plotted on the CIE 1931 chromaticity diagram. The boundary of each ellipse represents 10times the standard deviation of color matches made for the indicated chromaticity. (After MacAdam, D. L., Journal of theOptical Society of America, Vol. 32, p. 247, 1942.)


an illuminance of 300 lx being sufficient for good colorjudgment work (Cornu and Harlay, 1969). As the visualsystem enters the mesopic region, the ability to dis-criminate colors deteriorates and ultimately fails as thescotopic region is reached.

The effect of the light spectrum is much moreimportant. The position of a color on the CIE 1931chromaticity diagram is determined by the spectrum ofthe light and, if it is reflected from or transmitted througha surface, the spectral reflectance or transmittance ofthat surface. Therefore, by changing the light spectrumemitted by the lamp, it is possible to make colors easilydiscriminable or difficult to discriminate. The carefulchoice of light source is important wherever good colordiscrimination is important.

5.5 InteractionsThe fact that there are many other variables besidesadaptation luminance and light spectrum that influ-ence threshold visual performance has been men-tioned earlier. It is now necessary to introduce anothercomplication, namely, interaction between the vari-ous components of visual system performance. As anexample, consider the effect of luminance contrast onvisual acuity. Visual acuity is conventionally measuredusing targets with a high luminance contrast. However,as the luminance contrast of the target is decreased,visual acuity also worsens. Similarly, the temporalsensitivity function as presented applies to a uniformluminance field. If the field has a pattern and hence adistribution of spatial frequencies, the temporal sensi-tivity function may be changed (Koenderink and VanDoorn, 1979).

Put crudely, what this means is that as visualperformance gets closer to threshold, almost everythingabout the stimulus presented to the visual systembecomes important. Further details on some of the inter-actions that occur are given in Boff and Lincoln (1988)

5.6 Approaches to Improving ThresholdVisual PerformanceWorking close to threshold is not easy. In fact, it canbe argued that the main function of anyone designingillumination is to provide conditions that avoid the needto use the visual system close to threshold. However, ifthis is required, then the following steps can be takento improve threshold visual performance. Not all of thefollowing steps will be possible in every situation andnot all are appropriate for every problem. The discussionabove should indicate which approach is likely to bemost effective.

Changing the Task:• Increase the size of the detail in the task.• Increase the luminance contrast of the detail in

the task.• Present the task so that it can be looked at

directly, that is, with the fovea.• Change the color of the target to make it more

conspicuous• Reduce the velocity of the task.• Present the task for a longer time.

Changing the Environment:• Increase the adaptation luminance.• Select a lamp with better color properties.• Design the lighting so that it is free from disabil-

ity glare and veiling reflections (see Section 7).

6 EFFECTS ON SUPRATHRESHOLD VISUALPERFORMANCE

Suprathreshold visual performance is the performanceof tasks that are easily visible because the stimulithey present to the visual system are well above thoseassociated with threshold conditions. This raises thequestion as to why lighting conditions make a differenceto task performance once what has to be seen is clearlyvisible. The answer is that although the stimuli areclearly visible, lighting influences the speed with whichthe visual information extracted from the stimuli can beprocessed. The aspect of lighting which determines thiseffect is the retinal illumination. The retinal illuminationis determined by the luminance of the visual field thatis viewed and hence by the illuminance on the surfaceswhich form that field.

6.1 Relative Visual Performance Modelfor On-Axis DetectionThe relative visual performance (RVP) model of visualperformance is an empirical model of the reaction timefor the detection of different visual stimuli seen on thefovea for a range of adaptation luminances, luminancecontrasts, and visual sizes (Rea and Ouellette, 1988,1991). Figure 11 shows the form of the RVP modelfor four different visual size tasks, each surface beingfor a range of contrasts and retinal illuminances. Theoverall shape of the relative visual performance surfacehas been described as a plateau and an escarpment(Boyce and Rea, 1987). In essence what it showsis that the visual system is capable of a high levelof visual performance over a wide range of visualsizes, luminance contrasts, and retinal illuminations(the plateau), but at some point either visual size orluminance contrast or retinal illumination will becomeinsufficient and visual performance will rapidly collapse(the escarpment) toward threshold. The existence of aplateau of visual performance, or rather a near plateaubecause there is really a slight improvement in visualperformance across the plateau, implies that for a widerange of visual conditions visual performance changesvery little with changes in the lighting conditions.

The RVP model of suprathreshold visual perfor-mance provides a quantitative means of predicting theeffects of changing either task size or contrast or theadaptation luminance on visual performance. It has beendeveloped using rigorous methodology and has beenvalidated against independently collected data (Eklundet al., 2001; Boyce, 2003). However, it is important tonote that it should only be applied to a limited rangeof tasks. Specifically, it is most appropriate for taskswhere task performance is dominated by the visual com-ponent (see Section 6.3), which do not require the useof off-axis vision to any extent; present stimuli to thevisual system that can be completely characterized by

ILLUMINATION 687

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Figure 11 Relative visual performance surfaces plotted against retinal illumination, in trolands, and luminance contrastfor four stimuli subtending four different solid angles, measured in microsteradians. (After Rea, M. S., and Ouellette, M. J.,Lighting Research and Technology, Vol. 23, p. 135, 1991.)

visual size, luminance contrast, and background lumi-nance; and have values for these variables that fallwithin the ranges used to develop the model. Where thetask involves chromatic contrast as well as luminancecontrast, the RVP model is likely to be misleading andthe light spectrum used for illumination will be impor-tant. Where the task is achromatic, the light spectrumis not likely to be important for suprathreshold visualperformance unless performance is limited by the sizeof detail that needs to be seen (Boyce et al., 2003).

6.2 Visual Search

One class of tasks for which the RVP model is notapplicable is comprised of those in which the objectto be detected can appear anywhere in the visual field.These tasks involve visual search. Visual search istypically undertaken through a series of eye fixations,the fixation pattern being guided either by expectationsabout where the object to be seen is most likely to appearor by what part of the visual scene is most important.Typically, the object to be detected is first detectedoff-axis and then confirmed or resolved by an on-axis

fixation. The speed with which a visual search task iscompleted depends on the visibility of the object to befound, the presence of other objects in the search area,and the extent to which the object to be found is differentfrom the other objects. The simplest visual search task isone in which the object to be found appears somewherein an otherwise empty field, for example, paint defectson a car body. The most difficult visual search task isone where the object to be found is situated in a clutteredfield and the clutter is very similar to the object to befound, for example, searching for a face in a crowd.

The lighting conditions necessary to achieve fastvisual search are similar to those used to improve fovealthreshold visual performance. By improving fovealthreshold visual performance, the peripheral thresholdvisual performance is also improved so the object to befound is made more visible. The lighting required forfast visual search will have to be matched to the physicalcharacteristics of the object to be found. For example, ifthe object is two dimensional and of matte reflectancelocated on a matte background, increasing the adaptationluminance is about the only option. However, if theobject is three dimensional and has a specular reflectance


component, then light distribution can be used toincrease the apparent size by casting shadows and theluminance contrast of the object by producing highlightson or around the object—changes which will be muchmore effective than simply increasing the adaptationluminance. Likewise, if the object is distinguished fromits background primarily by color, the light spectrumused is an important consideration. It is this needto match the lighting conditions to the nature of theobjects to be found which makes the design of lightinginstallations for visual inspection tasks so difficult anddiverse (IESNA, 2011; Boyce, 2003).

The extent to which a lighting installation is effectivein revealing an object can be estimated from the object’svisibility lobe (Inditsky et al., 1982). The visibility lobeis the distribution of the probability of detecting theobject within one eye fixation pause. This probabilityis a maximum when the object is viewed on-axis anddecreases with increasing deviation from the fovea.The probability distribution is assumed to be radiallysymmetrical about the visual axis, resulting in circlesaround the fixation point, each circle having a givenprobability of detection within one fixation pause. Forobjects which appear on a uniform field, the visibilitylobe is based on the detection of the object. For objects

which appear among other similar objects, the visibilitylobe is based on the discriminability of the object fromthe others surrounding it. Visual search will be fastestfor objects which have the largest visibility lobe.

6.3 Visual Performance, Task Performance,and Productivity

Figure 12 shows the relationships between the stimulito the visual system and their impact on visual perfor-mance, task performance, and productivity. The stimulito the visual system, including the retinal illumination,determine the operation of the visual system and hencethe level of visual performance achieved. This visualperformance then contributes to task performance. Itis important to point out that visual performance andtask performance are not necessarily the same. Taskperformance is the performance of the complete task.Visual performance is the performance of the visualcomponent of the task. Task performance is what isneeded in order to measure productivity and to establishcost–benefit ratios. Visual performance is the only thingthat changing the lighting conditions can affect directly.

Most apparently visual tasks have three components:visual, cognitive, and motor. The visual componentrefers to the process of extracting information relevant

Visualstimulus

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Figure 12 Schematic of relationships between stimuli to the visual system and their impact on visual performance,task performance, and productivity. The arrows indicate the direction of the effects. The dotted arrow between visualperformance and visual size indicates that, if visual performance is poor, a common response is to move closer to thestimulus to increase its visual size.

ILLUMINATION 689

to the performance of the task using the sense ofsight. The cognitive component is the process by whichsensory stimuli are interpreted and the appropriate actiondetermined. The motor component is the process bywhich the stimuli are manipulated to extract informationand/or the actions decided upon are carried out.Every task is unique in its balance between visual,cognitive, and motor components and hence in the effectlighting conditions have on task performance. It is thisuniqueness which makes it impossible to generalizefrom the effect of lighting on the performance of onetask to the effect of lighting on the performance ofanother. The RVP model for on-axis tasks and the visualsearch models discussed above can be used to quantifythe effects of lighting conditions on visual performance,but there is no general model to translate those resultsto task performance.

6.4 Approaches to Improving SuprathresholdVisual Performance

The main purpose of lighting installations is to ensurethat people can perform the work they need to doquickly, easily, comfortably, and safely. To achieve thisdesirable aim, it is necessary to provide lighting whichensures people are working on the plateau of visual per-formance and not on the escarpment. The RVP model ofvisual performance provides a simple means of checkingwhether lighting is adequate for the visual performanceof many on-axis tasks. The visibility lobe provides anapproach to quantifying the effect of lighting conditionson visual search tasks. Alternatively, most countrieshave well-established recommendations for the illumi-nances to be provided for working interiors (IESNA,2011; CIBSE, 2009). Most of these recommendationseasily exceed what would be deduced as necessaryfrom a consideration of visual performance alone.

Although the discussion above has focused onlighting conditions, it is important to recognize thatimproving suprathreshold visual performance can beachieved by changing the characteristics of the task aswell as the lighting. The following list is divided intotwo parts: task changes and lighting changes. Not allof the following suggestions will be possible in everysituation and not all are appropriate for every problem.

Changing the Task:

• Increase the size of the detail in the task.• Increase the luminance contrast of the detail in

the task.• For off-axis tasks in a cluttered field, make the

object to be detected clearly differ from the sur-rounding objects on as many different dimen-sions as possible, for example, size, contrast,color, and shape.

• Ensure the object presents a clear, sharp imageon the retina.

Changing the Environment:• Increase the adaptation luminance.• Where the task involves color, select a lamp with

better color properties.

• Design the lighting so that it is free from disabil-ity glare and veiling reflections (see Section 7).

• Design the lighting to increase the apparent sizeor luminance contrast of the object.

7 EFFECTS ON COMFORT

Lighting installations are rarely designed for visualperformance alone. Visual comfort is almost always aconsideration. The aspects of lighting which cause visualdiscomfort include those relevant to visual performanceand extend beyond them. This is because the factorsrelevant to visual performance are generally restrictedto the task and its immediate area, whereas the factorsaffecting visual discomfort can occur anywhere withinthe lit space.

7.1 Symptoms and Causes of VisualDiscomfortVisual discomfort can give rise to an extensive listof symptoms. Among the more common are red,sore, itchy, and watering eyes; headaches and migraineattacks; and aches and pains associated with poorposture. Visual discomfort is not the only possiblesource of these symptoms. All can have other causes.It is this vagueness which makes it essential to considerthe nature of the visual environment before ascribingany of these symptoms to the lighting conditions.

Features of the visual environment that can causevisual discomfort are as follows:

Visual Task Difficulty . The visual system is de-signed to extract information from the visualenvironment. Any visual task that is close tothreshold contains information that is difficult toextract. The usual reaction to a high level ofvisual difficulty is to bring the task closer toincrease its visual size. As the task is broughtcloser, the accommodation mechanism of the eyehas to adjust to keep the retinal image sharp. Thisadjustment can lead to muscle fatigue and hencesymptoms of visual discomfort.

Under- and Overstimulation. The visual system isdesigned to extract information from the visualenvironment. Discomfort occurs either whenthere is no information to be extracted or whenthere is an excessive amount of repetitive infor-mation. Examples of no information occur whendriving in fog or in a “whiteout” snowstorm.In both cases, the visual system is searchingfor information which is hidden but which mayappear suddenly and require a rapid response. Thestress experienced while driving in these condi-tions is a common experience. As for overstimu-lation, the important point is not the total amountof visual information, but rather the presence oflarge areas of the same spatial frequency. Wilkins(1993) has associated the presence of large areasof specific spatial frequencies in printed text withthe occurrence of headaches, migraines, and read-ing difficulties.


Distraction . The visual system is designed to extractinformation from the visual environment. To dothis, it has a large peripheral field which detectsthe presence of objects which are then examinedusing the small, high-resolution fovea. For thissystem to work, objects in the peripheral field thatare bright, moving, or flickering have to be easilydetected. If, upon examination, these bright, mov-ing, or flickering objects prove to be of little inter-est, they become sources of distraction becausetheir attention-gathering power is not diminishedafter one examination. Ignoring objects that auto-matically attract attention is stressful and can leadto symptoms of visual discomfort.

Perceptual Confusion . The visual system is designedto extract information from the visual environ-ment. The visual environment consists of a pat-tern of luminances developed from the differencesin reflectance of the surfaces in the field of viewand the distribution of illuminance on those sur-faces. Perceptual confusion occurs when there isa pattern of luminances present which is solelyrelated to the illuminance distribution and con-flicts with the pattern of luminance associatedwith the reflectances of the surfaces.

7.2 Lighting Conditions That Can CauseDiscomfortThere are many different aspects of lighting that cancause discomfort. Insufficient light for the performanceof a task has been discussed earlier and will not bediscussed again. Rather, attention will be devoted toflicker, glare, shadows, and veiling reflections. It shouldbe noted that whether or not these aspects of lightingcause discomfort will depend on the context. All can beused to positive effect in some contexts.

Flicker A lighting installation that produces visibleflicker will be almost universally disliked unless it isbeing used for entertainment. Large individual differ-ences in the sensitivity to flicker imply that a clear safetymargin is necessary. This can be achieved by high-frequency operation and/or the mixing of light fromlamps powered from different phases of the electricitysupply. The same approaches, which will result in achanged frequency and/or a reduced percentage modula-tion, can be used to diminish any stroboscopic illusions.The use of high-frequency control gear has been asso-ciated with a reduction in the prevalence of headachesunder fluorescent lighting (Wilkins et al. 1989).

Glare Glare occurs in two ways. First, it is possible tohave too much light. Too much light produces a simplephotophobic response in which the observer screws uphis eyes, blinks, or looks away. Too much light israre indoors but is common in full sunlight. Second,glare occurs when the range of luminances in a visualenvironment is too large. Glare of this sort can have twoeffects: a reduction in threshold visual performance anda feeling of discomfort. Glare which reduces thresholdvisual performance is called disability glare. It is due to

light scattered in the eye reducing the luminance contrastof the retinal image on the fovea. The magnitude ofdisability glare can be estimated by calculating theequivalent veiling luminance (IESNA, 2011).

The effect of disability glare on the luminancecontrast of the object being looked at can be determinedby adding the equivalent veiling luminance to allelements in the formulas for luminance contrast (seeSection 4.6.2). Disability glare is rare in interior lightingbut is common on roads at night from oncomingheadlights and during the day from the sun. Usuallydisability glare also causes discomfort, but it is possibleto have disability glare without discomfort when theglare source is large in area. This can be seen by lookingat a picture hung on a wall adjacent to a window. Thepicture will usually be much easier to see when the eyeis shielded from the window.

As for discomfort glare, this, by definition, doesnot cause any shift in threshold visual performance butdoes cause discomfort. There are many different nationalsystems for predicting the magnitude of discomfortglare produced by interior lighting installations (IESNA,2011; CIBSE, 2009; CIE, 2002). All these systemsare based on formulas that imply that discomfort glareincreases as the luminance and solid angle of the glaresource increase and decreases as the luminance of thebackground and the deviation from the glare sourceincrease. Lighting equipment manufacturers use theseformulas to produce tabular estimates of the level ofdiscomfort glare produced by a regular array of theirluminaires for a range of standard interiors. These tablesprovide all the precision necessary for estimating theaverage level of discomfort glare likely to occur in aninterior, although the precision with which they predictan individual’s sense of discomfort is low (Stone andHarker, 1973).

Shadows Shadows are cast when light coming froma particular direction is intercepted by an opaque object.If the object is big enough, the effect is to reducethe illuminance over a large area. This is typically theproblem in industrial lighting where large pieces ofmachinery cast shadows in adjacent areas. The effectof these shadows can be overcome either by increasingthe proportion of interreflected light by using highreflectance surfaces or by providing local lighting in theshadowed area. If the object is smaller, the shadow canbe cast over a meaningful area, which in turn can causeperceptual confusion, particularly if the shadow moves.An example of this is the shadow of a hand cast ona blueprint. This problem can be reduced by increasingthe interreflected light in the space or by providing locallighting which can be adjusted in position.

Although shadows can cause visual discomfort, itshould be noted that they are also an essential elementin revealing the form of three-dimensional objects.Techniques of display lighting are based around theidea of creating highlights and shadows to change theperceived form of the object being displayed.

The number and nature of shadows produced by alighting installation depends on the size and numberof light sources and the extent to which light is

ILLUMINATION 691

interreflected around the space. The strongest shadowis produced from a single point source in a black room.Weak shadows are produced when the light sources arelarge in area and the degree of interreflection is high.

Veiling Reflections Veiling reflections occur whena source of high luminance, usually a luminaire or awindow, is reflected from a specularly reflecting surface,such as a glossy printed page or a display screen. Theluminance of the reflected image changes the luminancecontrast of the printed text or the display. The extent towhich this changes visual performance can be estimatedusing the RVP model, but the extent to which it causesdiscomfort is different. Bjorset and Fredericksen (1979)have shown that a 20% reduction in luminance contrastis the limit of what is acceptable, regardless of the lumi-nance contrast without veiling reflections (Figure 13).

The two factors that determine the magnitude of veil-ing reflections are the specularity of the material beingviewed and the geometry between the observer, theobject, and any sources of high luminance. If the objectis completely diffusely reflecting, no veiling reflectionsoccur, but if it has a specular reflection component,veiling reflections can occur. The positions where theyoccur are those where the incident ray corresponding tothe reflected ray which reaches the observer’s eye fromthe object comes from a source of high luminance.This means that the strength and magnitude of veilingreflections can vary dramatically within a singlelighting installation (Boyce and Slater, 1981).

Like shadows, veiling reflections can also be usedpositively, but when they are, they are conventionallycalled highlights. Display lighting of specularly reflect-ing objects is all about producing highlights to revealthe specular nature of the surface.

7.3 Comfort, Performance, and ExpectationsWhile lighting conditions that make it difficult toachieve good visual performance will almost always

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be considered uncomfortable, lighting conditions thatallow a high level of visual performance may also beconsidered uncomfortable. Figure 14 shows the meandetection speed for finding a number from many laidout at random on a table, and the percentage of peopleconsidering the lighting good. As might be expected,increasing the illuminance on the table increases meandetection speed and the percentage considering thelighting good. However, as the illuminance exceeds2000 lx, the percentage considering the lighting gooddeclines even though the mean detection speed continuesto increase. This result indicates that if you wishto achieve a satisfactory lighting installation it isnecessary to provide lighting which allows easy visualperformance and avoids discomfort and that visualdiscomfort is more sensitive to lighting conditions thanvisual performance.

There is another aspect of visual comfort which distin-guishes it from visual performance. Visual performanceis determined solely by the capabilities of the visual sys-tem. Visual comfort is linked to peoples’ expectations.Any lighting installation which does not meet expec-tations may be considered uncomfortable even thoughvisual performance is adequate; and expectations canchange over time. Figure 12 also demonstrates anotherpotential impact of visual comfort. Lighting conditionswhich are considered uncomfortable may influence taskperformance by changing motivation even when theyhave no effect on the stimuli presented to the visualsystem and hence on visual performance.

7.4 Approaches to Improving Visual ComfortIn order to ensure visual comfort it is necessary toensure that the lighting allows a good level of visualperformance, does not cause distraction, and allowssufficient stimulation without perceptual confusion. Thiscan be done by

• Identifying the visual tasks to be performedand then determining the characteristics of thelighting needed to allow a high level of visualperformance of the tasks (see Sections 5 and 6)

• Eliminating flicker from the lighting by usingappropriate control gear for discharge and solidstate lamps. If this is not possible, reduce themodulation of the flicker by mixing light fromsources operating on different phases of theelectricity supply

• Reducing disability glare by careful selection,placing, and aiming of luminaires so as to reducethe luminous intensity of the luminaires close tothe common lines of sight

• Reducing discomfort glare by careful selectionand layout of luminaires. Use the appropriatenational discomfort glare system to estimatethe magnitude of discomfort glare. Using highreflectance surfaces in the space will help reducediscomfort glare by increasing the backgroundluminance against which the luminaires are seen

• Considering the density and areal extent of anyshadows which are likely to occur. If shadows


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Figure 14 Mean detection speed for locating a specified number from amongst others at different illuminances, andthe percentage of observers who consider the lighting good at each illuminance. (after Muck, E., and Bodmann, H. W.,Lichttechnik, Vol. 13, p. 502, 1961.)

are undesirable and large area shadows are likelyto occur, use high reflectance surfaces in thespace to increase the amount of interreflectedlight and use more lower-wattage lamps tosupply the desired illuminance. If shadowscannot be avoided because of the extent ofobstruction in the space, be prepared to providesupplementary task lighting in the shadowedareas. If dense, small area shadows occur in theimmediate work area, use adjustable task lightingto moderate their impact

• Considering the extent to which veiling reflec-tions (or highlights) are desirable. If they areundesirable, veiling reflections can be reduced by• Reducing the specular reflectance of the

surface being viewed• Changing the geometry between the viewer,

the surface being viewed, and the offendingzone

• Reducing the luminance of the offendingzone

• Increasing the amount of inter-reflected lightin the space

8 INDIVIDUAL DIFFERENCES

Differences between individuals in visual capabilitiesare common and are usually dealt with by providinglighting which is more than adequate for visual per-formance and visual comfort. However, there are three

sources of individual differences which are both com-mon and consistent enough in direction to deserve spe-cial consideration. They are the effects of age, partialsight, and defective color vision.

8.1 Changes with AgeAs the visual system ages, a number of changes in itsstructure and capabilities occur. Usually, the first tooccur is an increase in the near point, i.e., the shortestdistance at which a clear, sharp retinal image can beachieved. This increase occurs due to an increase inthe rigidity of the lens with age. This change, calledpresbyopia, is why the majority of people over fiftyhave to wear glasses or contact lenses to read.

While the increasing rigidity of the lens, and otherforms of focusing difficulty, can be compensated byadjusting the optical power of the eye’s optical systemwith lenses, the other changes that occur in the eye can-not. As the visual system ages, the amount of light reach-ing the retina is reduced, more of the light entering theeye is scattered, and the color of the light is altered bypreferential absorption of the short, visible wavelengths.The rate at which these changes occur accelerates afterabout sixty. The consequences of these changes withage are reduced visual acuity, reduced contrast sensitiv-ity, reduced color discrimination, increased time takento adapt to large and sudden changes in adaptation lumi-nance, and increased sensitivity to glare (Boyce, 2003).

Lighting can be used to compensate for thesechanges, to some extent. Older people benefit fromhigher illuminances than are needed by young people

ILLUMINATION 693

(Smith and Rea, 1979), but simply providing more lightmay not be enough. The light has to be provided insuch a way that both disability and discomfort glare arecarefully controlled and veiling reflections are avoided.Where elderly people are likely to be moving from awell lit area to a dark area, such as from a supermarketto a parking lot, a transition zone with a graduallyreducing illuminance is desirable. Such a transitionzone allows their visual system more time to make thenecessary changes in adaptation.

8.2 Helping People with Low Vision

Low vision is a state of vision that falls between normalvision and total blindness. The World Health Organi-zation has a system for classifying vision from normalto total blindness. Low vision occurs when the visualacuity in the better eye is less than 6/18 or the visualfield is less than 10 degrees. A visual acuity of 6/18means that the individual can just resolve details at 6 mwhich people with normal vision can resolve at 18 m.

While some people are born with low vision, themajority of people with low vision are elderly. Kahnand Moorhead (1973) found that among those withlow vision, 20 percent reached this state between birthand 40 years, 21 percent between 41 and 60 yearsand 59 percent after 60 years of age. Surveys in theUnited States and the United Kingdom suggest that thepercentages of the total population who are classifiedas having low vision are in the range 0.5 to 1 percent.This percentage increases markedly in less developedcountries (Tielsch, 2000).

The three most common causes of low vision arecataract, macular degeneration, and glaucoma. Thesecauses involve different parts of the eye and havedifferent implications for how lighting might be usedto help.

Cataract is an opacity developing in the lens. Theeffect of cataract is to absorb and scatter more light asthe light passes through the lens. This change results inreduced visual acuity and reduced contrast sensitivityover the entire visual field and greater sensitivity toglare. The extent to which more light can help aperson with cataract depends on the balance betweenabsorption and scattering. More light will help overcomethe increased absorption but if scattering is high, theconsequent deterioration in the luminance contrast ofthe retinal image will reduce visual capabilities. Thereis really little alternative to testing the effectiveness ofadditional light on an individual basis. What is true foreveryone with cataract is that they will be very sensitiveto glare from luminaires and windows. Careful selectionof luminaires and window treatments to limit glare isdesirable. The use of dark backgrounds against whichobjects are to be seen will also help.

Macular degeneration occurs when the macular of theretina, which is just above the fovea, becomes opaquedue to bleeding or atrophy. An opacity immediatelyin front of the fovea implies a serious reductionin visual acuity and in contrast sensitivity at highspatial frequencies. It also implies that the ability todiscriminate colors will be reduced. Typically, thesechanges make reading difficult if not impossible.

However, peripheral vision is unaffected so the abilityto find ones way around is unchanged. Providing morelight, usually by way of a task light, will help people inthe early stage of macular deregulation to read, althoughas the deterioration progresses additional light will beless effective. Increasing the size of the retinal image bymagnification or by getting closer is helpful at all stages.

Glaucoma is shown by a progressive narrowing ofthe visual field. Glaucoma is due to an increase inintraocular pressure which damages the retina and theanterior optic nerve. Glaucoma will continue until com-plete blindness occurs unless the intraocular pressure isreduced. As glaucoma develops it leads to a reduction invisual field size, reduced contrast sensitivity, poor nightvision, and slowed transient adaptation but the resolu-tion of detail seen on-axis is unaffected until the finalstage. Lighting has limited value in helping people inthe early stages of glaucoma, because where damage hasoccurred the retina has been destroyed. However, con-sideration should be given to providing enough light forexterior lighting at night to enable the fovea to operate.

While the extent to which providing more light ishelpful will depend on the specific cause of low vision,there is one approach that is generally useful. Thisapproach is to simplify the visual environment and tomake its salient details more visible. As an example,consider the problem of how to set a table so that aperson with low vision can eat with confidence. Theplate containing the food and the associated cutlery canbe made more visible by using a contrasting tablecloth,e.g., a dark tablecloth with a white plate and cutlery.The food on the plate can be made easier to identifyby using an overlarge plate so that individual fooditems can be separated from each other. The wholescene can be simplified can be using solid colors ratherthan patterns. This same approach of simplification andenhanced visibility can be applied to whole rooms, forexample, by painting a door frame in a contrasting colorto the door so that the door is easily identified.

8.3 Consequences of Defective Color Vision

About 8 percent of males and 0.5 percent of femaleshave some form of defective color vision (McIntyre,2002). For most activities this causes few problems,either because the exact identification of color isunnecessary or because there are other cues by which thenecessary information can be obtained. Where defectivecolor vision does become a problem is where color is thesole means used to identify significant information as,for example, in some forms of electrical wiring. Peoplewith defective color vision will have difficulty with suchactivities (Steward and Cole, 1989).

Where self-luminous colors are used as signals, careshould be taken to restrict the range of colored lightsused to those which can be distinguished by people withthe most common forms of color defect. For example,the CIE has recommended areas on the CIE 1931chromaticity diagram within which red, green, yellow,blue, and white signal lights should lie. These areas aredesigned so that the red signal will be named as red andthe green as green by people with the most commonforms of defective color vision (CIE, 1994).


9 OTHER EFFECTS OF LIGHT ENTERINGTHE EYE

Although making things visible is the most obviouseffect of light entering the eye, there are two otherways in which light can affect us. The first is throughthe circadian system. The second is through thepsychological impact of what is visible (CIE, 2009).

9.1 Human Circadian System

Light entering the eye does more than stimulate thevisual system. It also influences the human circadiansystem and hence our biological rhythms. The humancircadian system has three components: an internaloscillator, a number of external oscillators that entrainthe internal oscillator, and a messenger hormone, mela-tonin, that carries the internal “time” information to allparts of the body through the bloodstream (Djik et al.,1995). The light – dark cycle is one of the most potentof the external stimuli for entrainment. By varying theamount of light exposure and when it is presented, it ispossible to shift the phase of the circadian system clock,either forward or backward, as required. In addition, itis possible to have an immediate alerting effect by expo-sure to light during circadian night (Boyce, 2003). Theamount of light required to produce phase shifts or animmediate alerting effect is within the range of currentlighting practice. However, the spectral sensitivity of thecircadian system is not the same as the visual system,the peak sensitivity being about 480 nm (Brainard et al.,2001; Thapan et al., 2001). This is because a differentphotoreceptor is used by the circadian system (Bersonet al., 2002), although there is evidence that there issome interaction between the circadian photoreceptorand the visual photoreceptors (Bullough et al., 2008).This means that the effectiveness of light sources forstimulating the circadian system cannot be evaluatedusing the CIE photometry system (see Section 2).

The growing understanding of the importance of thelight dark cycle for circadian rhythms has significancefor human health and well-being. One application whereexposure to light has been of interest is for shift-work. The short term problems of shift work arefatigue, produced by poor quality sleep, and maintainingalertness during work. Long term, there is evidencethat shift workers have a higher risk of cardiovasculardisease, gastrointestinal ailments, and emotional andsocial problems. The short term problems are believedto occur because of a mismatch between the demandsof the work and the state of the worker’s circadianrhythm. Put plainly, the workers are expected to workwhen their physiology is telling them to sleep and sleepwhen their physiology is telling them to be awake.Light is useful in alleviating this problem because it canmore rapidly shift the human circadian rhythm so thatit better matches the functional requirements, but to dothis requires control of light exposure for the completetwenty-four hours (Eastman et al. 1994). As for theimmediate alerting affect, improvements in alertnessand cognitive performance have been found followingexposure to high light levels during night shift work,together with physiological changes indicative of the

state of the circadian rhythm (French et al., 1990; Boyceet al., 1997).

Another human health problem that has been shownto be sensitive to light exposure is seasonal affectivedisorder (SAD). People with this condition typicallyexperience decreased energy and stamina, depression,feelings of despair and a greater need for sleep duringthe winter months. Light therapy, in which the patient isexposed to a high illuminance for a set period each day,has been shown to alleviate these symptoms in manypatients (Lam and Levitt, 1999).

The uses of light to alleviate the problems of shiftwork and to treat seasonal depression are just themost advanced examples of the influence of lighton human well-being. Other applications of lighttherapy include the treatment of sleep disorders, moregeneral, non-seasonal depression and jet lag as wellas the alleviation of the fractured sleep/wake cycles ofAlzheimer’s patients. Also of interest is what damagingside-effects exposure to light during circadian nightmight have (Brainard et al., 1999; Figueiro et al.,2006). Until a clearer understanding of the positive andnegative impacts of exposure to light during circadiannight is achieved, it would be wise to treat attemptsto use light exposure to manipulate such a fundamentalpart of our physiology as the circadian system withcaution (Boyce, 2006).

9.2 Positive and Negative Affect

Psychology is a vast field and the psychology of lightingis only a small part of it. The area relevant to lightingpractice that has been most consistently studied is thatof perception. Studies have been undertaken in abstractsituations and have lead to quantitative relations beingproposed between simple sensations such as brightnessand photometric measurements such as luminance(Boyce, 2003). Other studies have been undertaken inrooms with complete lighting installations and have leadto an understanding of the link between the perception ofgloom and such photometric characteristics of the roomas reflectance and illuminance distributions (Shepherdet al., 1989). Yet others have tried to establish if lightinggenerates cues by which people interpret a room, forexample, does lighting the walls enhance the perceptionof spaciousness (Flynn et al., 1973).

While such studies have certainly influenced light-ing design they cannot be said to constitute a coherentbody of knowledge. Further, they cannot form a basis forlighting practice until the impacts of specific perceptionsare understood. To understand the consequences of per-ception of lighting it is necessary to take a broader view.This view centers around positive affect. Positive affect,defined as pleasant feelings induced by commonplaceevents or circumstances, has been found to influencecognition and social behavior (Isen and Baron, 1991).Specifically, positive affect has been shown to increaseefficiency in making some type of decisions, and to pro-mote innovation and creative problem solving. It alsochanges the choices people make and the judgments theydeliver. For example, it has been shown to alter peoples’preference for resolving conflict by collaboration rather

ILLUMINATION 695

than avoidance and also to change their opinions of thetasks they perform.

Given these usually desirable outcomes of positiveaffect, it is necessary to ask what can generate positiveaffect. The answer is both small and wide. Small,because the stimuli which have been shown to generatepositive affect are low-level stimuli, ranging fromreceiving a small but unexpected gift from a manufac-turer’s representative to being given positive feedbackabout task performance. Wide, because positive affectcan be influenced by the physical environment, theorganizational structure, and the organizational culture.Lighting is clearly a part of the physical environmentand has been shown to influence positive affect (Baronet al., 1992; McCloughan et al., 1999) but it is onlyone of many factors that can do that.

As would be expected, it is also possible to generatenegative affect. There is considerable information on theinfluence of frustration or anger on aggression and on therelationship between anxiety and performance (Baron,1977). It seems reasonable to propose that lightingconditions that cause visual discomfort could generatenegative affect.

Positive and negative affect provide plausible routeswhereby the perception of the visual environmentmight influence the efficiency and effectiveness oforganizations. As such, they represent a very differentapproach to identifying what is the most appropriateform of lighting for organizations to the visibility basedrecommendations used in lighting practice today. Thepossibility that improving the quality of lighting beyondthat required for good visibility without discomfortwould lead to enhanced organizational performance isa topic of current interest (CIE, 1998a; Boyce et al.,2006a). Somewhat encouraging for this belief is thefinding that lighting perceived to be of better qualityhas been shown to be reliably linked to more positivefeelings of health and well-being (Veitch et al., 2008).

10 TISSUE DAMAGE

The part of the electromagnetic spectrum from 100 nmto 1 mm is called optical radiation. This part of theelectromagnetic spectrum covers ultraviolet (100–400nm), visible (400-760 nm) and infrared radiation (760nm–1 mm). Sunlight and electric light sources allemit optical radiation. In sufficient quantities, opticalradiation can cause damage to the eye and the skin.Details are given in CIE (2006).

10.1 Mechanisms for Damage to Eyeand Skin

There are two mechanisms for tissue damage to occur;photochemical and thermal. They are not mutuallyexclusive, both can occur for the same incident opticalradiation but one will have a lower damage thresholdthan the other. Photochemical damage is related tothe energy absorbed by the tissue within the repair orreplacement time of the cells of the tissue. Thermaldamage is determined by the magnitude and durationof the temperature rise.

The factors that determine the likelihood of tissuedamage are the spectral irradiance incident on thetissue, the spectral sensitivity of the tissue, the time forwhich the radiation is incident and, for thermal damage,the area over which the irradiance occurs. Spectralirradiance will be determined by the spectral radiantintensity of the source of optical radiation; the spectralreflectance and/or the spectral transmittance of materialsfrom which the optical radiation is reflected or throughwhich it is transmitted; and the distance from the sourceof optical radiation. Area is important for thermal tissuedamage because the potential for dissipating heat gainis greater for a small area than a large area.

The visual system provides an automatic protectionfrom tissue damage in the eye, for all but the highestlevels of visible radiation. This is the involuntaryaversion response produced when viewing bright light.The response is to blink and look away, thereby reducingthe duration of exposure. Of course, this involuntaryresponse only works for sources that have a highvisible radiation component, such as the sun. Sourcesthat produce large amounts of ultraviolet and infraredradiation with little visible radiation are particularlydangerous because they do not trigger the aversionresponse.

10.2 Acute and Chronic Damage to Eyeand Skin

Tissue damage can be classified according to theduration of exposure it takes to produce the damage.Acute forms of damage are detectable immediately orat least within a few hours of exposure. Chronic formsof damage only become apparent after many years.

Ultraviolet radiation incident on the skin produces animmediate pigment darkening, followed a few hours lat-ter by erythema (reddening of the skin) and, ultimately,by a tan, produced by an increase in the number, sizeand pigmentation of melanin granules. Excessive ultra-violet radiation incident on the eye can produce, a fewhours later, an inflammation of the cornea called pho-tokeratitis. This typically lasts a few days followed byrecovery. As for chronic damage, prolonged exposureto ultraviolet radiation has been shown to be associatedwith various forms of skin cancer and cataract.

Visible radiation incident on the skin will produceerythema but not tanning, and, in sufficient quantity,skin burns. Visible radiation incident on the eye reachesthe retina. This irradiance represents both an acutephotochemical and an acute thermal hazard to the eye.Photochemical damage to the retina is associated withshort wavelength light (blue light). The thermal damagecovers retinal burns. As for chronic damage, it may bethat prolonged and repeated exposure to light is involvedin the retinal aging process.

Infrared radiation incident on the skin again initiallyproduces skin reddening and, at a high enough irradi-ance, burns. Infrared radiation incident on the eye willcause heating of various elements of the eye, dependingon the spectral content of the irradiance and the spec-tral transmittance of the various components of the eye.Infrared radiation from 760 nm to 1400 nm will reach


the retina and can cause retinal burns. Longer wave-lengths will be absorbed by other components of the eye.Prolonged heating of the lens is believed to be involvedin the incidence of cataract.

10.3 Damage Potential of DifferentLight Sources

The light source with the greatest potential for tissuedamage is the sun. The sun produces copious amountsof ultraviolet, visible, and infrared radiation. Voluntarystaring at the sun is a common cause of retinal burns.Voluntary exposure of the skin to the sun commonlyproduces sunburn. However, there exist some electriclight sources which can be hazardous, some beingintended for lighting and others being used as a sourceof optical radiation for industrial processes.

The extent to which a light source represents a haz-ard can be evaluated by applying the recommendationsof the American Conference of Governmental IndustrialHygienists (ACGIH, 2009), using recommended proce-dures (IESNA, 2000, 2005, 2007; CIE, 1998b, 2006).These recommendations take several different forms,ranging from maximum permissible exposure times toirradiance limits. Application of these standards to var-ious electric light sources indicates that such sources,as conventionally used for interior lighting, rarely rep-resent a hazard (McKinlay et al., 1988; Bergman et al.,1995; Kohmoto, 1999).

10.4 Approaches to Limiting Damage

The approach to minimizing the damage caused byoptical radiation is to limit the irradiance and/orthe time of exposure. Whether any such action isnecessary can be determined by applying the ACGIHrecommendations to the situation.

For sources of optical radiation used for lighting,if the threshold limiting values are exceeded, it willoften be possible to use a different light source thatis less hazardous. If this is not possible then it isnecessary to filter the source to eliminate some of thehazardous wavelengths or to use some form of eye orskin protection to attenuate the optical radiation or tolimit the exposure time.

For sources of optical radiation used in industrial pro-cesses, the source should be installed in an enclosure,with an interlock so that opening the enclosure extin-guishes the source. If this is not possible, then appropri-ate forms of eye and skin protection are required.

11 EPILOGUE

Illumination has been a subject of study for more thanninety years. The result has been a growing understand-ing of how lighting conditions and the visual systeminteract to facilitate visual performance and dimin-ish visual discomfort. This knowledge has formed theframework around which many national illuminatingengineering organizations have built recommendationsfor lighting practice (IESNA 2011; CIBSE, 2009). Theserecommendations provide a firm basis for designingeveryday lighting installations, provided the recommen-dations are applied with thought and not by rote.

There are three current areas of study with consider-able potential to change lighting practice:

• The value of better lighting quality for theefficiency of organizations.If it can be shown that better quality lighting has

a reliable economic impact on organizationalefficiency, the economics of lighting will bedramatically changed.

• The effect of light spectrum in mesopic condi-tions.In mesopic conditions light sources which stim-

ulate the rod photoreceptors more producea perception of greater brightness and allowsuperior off-axis performance (Rea et al,2009; Fotios and Cheal, 2009). Such findingsare likely to change both the light sourcesand the design recommendations for exteriorlighting

• The non-visual effects of light.If lighting can be shown to influence the

health and capabilities of people in everydaysituations above and beyond allowing themto see, then the whole basis of lightingrecommendations may need to be changed.

REFERENCES

American Conference of Governmental Industrial Hygienists(ACGIH) (2009), “TLVs and BEIs Threshold LimitValues for Chemical Substances and Physical Agents,Biological Exposure Indices,” ACGIH, Cincinnati, OH.

Bailey, I., Clear, R., and Berman, S. (1993), “Size as a Deter-minant of Reading Speed,” Journal of the IlluminatingEngineering Society , Vol. 22, pp. 102–117.

Baron, R. A., (1977), Human Aggression , Plenum, New York.Baron, R. A., Rea, M. S., and Daniels S. G. (1992), “Effects of

Indoor Lighting (Illuminance and Spectral Distribution)on the Performance of Cognitive Tasks and InterpersonalBehaviors: The Potential Mediating Role of PositiveAffect,” Motivation and Emotion , Vol. 16, pp. 1–33.

Bergman, R. S., Parham, T. G., and McGowan, T. K. (1995),“UV Emission from General Lighting Lamps,” Journal ofthe Illuminating Engineering Society , Vol. 24, pp. 13–24.

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