LightingDesign

By reading this book, you will develop the skills to perceive a space and its contents in
light,andbeabletodevisealayoutofluminairesthatwillprovidethatlitappearance.
WrittenbyrenownedlightingexpertChristopher(Kit)Cuttle,thebook:
explainsthedifferencebetweenvisionandperception,whichisthedistinction
betweenprovidinglightingtomakethingsvisible,andprovidingittoinfluencethe
appearanceofeverythingthatisvisible;
demonstrateshowlightingpatternsgeneratedbythree-dimensionalobjects
interactingwithdirectionallightingarestronglyinfluentialuponhowthevisual
perceptionprocessenablesustorecogniseobjectattributes,suchaslightness,
colourfulness,textureandgloss;
revealshowadesignerwhounderstandstheroleoftheselightingpatternsinthe
perceptualprocessmayemploythemeithertoreveal,ortosubdue,ortoenhancethe
appearanceofselectedobjectattributesbycreatingappropriatespatialdistributionsof
light;
carefullyexplainscalculationaltechniquesandprovideseasy-to-usespreadsheets,so
thatlayoutsoflampsandluminairesarederivedthatcanbereliedupontoachieve
therequiredilluminationdistributions.
Practical lighting design involves devising three-dimensional light fields that create
luminoushierarchiesrelatedtothevisualsignificanceofeachelementwithinascene.By
providing you with everything you need to develop a design concept – from the
understandingofhowlightinginfluenceshumanperceptionsofsurroundings,throughto
engineering efficient and effective lighting solutions – Kit Cuttle instils in his readers a
new-foundconfidenceinlightingdesign.
Christopher ‘Kit’ Cuttle, MA, FCIBSE, FIESANZ, FIESNA, FSLL, is visiting lecturer in
Advanced Lighting Design at the Queensland University of Technology, Brisbane,
Australia, and is author of two books on lighting (Lighting by Design, 2nd edition,
ArchitecturalPress,2008;andLight for Art’s Sake, Butterworth Heinemann,2007).
His previous positions include Head of Graduate Education in Lighting at the Lighting

ResearchCenter,RensselaerPolytechnicInstitute,Troy,NewYork;SeniorLectureratthe
Schools of Architecture at the University of Auckland and the Victoria University of
Wellington, both in New Zealand; Section Leader in the Daylight Advisory Service,


PilkingtonGlass;andLightingDesignerwithDerekPhillipsAssociates,bothintheUK.His
recentawardsincludetheSocietyofLightandLighting’sLeonGaster2013Awardforhis
LR&T paper ‘A New Direction for General Lighting Practice’, and the Lifetime
Achievement Award presented at the 2013 Professional Lighting Design Conference in
Copenhagen.


Publisher’sNote:
Todownloadthespreadsheetsthatareusedtofacilitatethecalculationsinthisbook,goto
the e-resources link shown on the back cover of the book and click on
eResource/Downloads.


LightingDesign
Aperception-basedapproach
ChristopherCuttle


Firstpublished2015
byRoutledge
2ParkSquare,MiltonPark,Abingdon,OxonOX144RN
andbyRoutledge
711ThirdAvenue,NewYork,NY10017
RoutledgeisanimprintoftheTaylor&FrancisGroup,aninformabusiness
©2015ChristopherCuttle

TherightofChristopherCuttletobeidentifiedasauthorofthisworkhasbeenassertedbyhiminaccordancewith
sections77and78oftheCopyright,DesignsandPatentsAct1988.
Allrightsreserved.Nopartofthisbookmaybereprintedorreproducedorutilisedinanyformorbyanyelectronic,
mechanical,orothermeans,nowknownorhereafterinvented,includingphotocopyingandrecording,orinany
informationstorageorretrievalsystem,withoutpermissioninwritingfromthepublishers.
Trademarknotice:Productorcorporatenamesmaybetrademarksorregisteredtrademarks,andareusedonlyfor
identificationandexplanationwithoutintenttoinfringe.
BritishLibraryCataloguinginPublicationData
AcataloguerecordforthisbookisavailablefromtheBritishLibrary
LibraryofCongressCataloginginPublicationData
Cuttle,Christopher.
Lightingdesign:aperception-basedapproach/ChristopherCuttle.
pagescm
Includesbibliographicalreferencesandindex.
ISBN978-0-415-73196-6(hardback:alk.paper)—ISBN978-0-415-73197-3
(pbk.:alk.paper)—ISBN978-1-315-75688-2(ebook)1.Lighting,
Architecturalanddecorative—Design.2.Visualperception.I.Title.
NK2115.5.L5C882015
747’.92—dc23
2014009980
ISBN:978-0-415-73196-6(hbk)
ISBN:978-0-415-73197-3(pbk)
ISBN:978-1-315-75688-2(ebk)
TypesetinBembo
bySaxonGraphicsLtd,Derby


Contents

Listoffigures

Listoftables
Acknowledgements
Introduction
1Theroleofvisualperception
2Ambientillumination
3Illuminationhierarchies
4Spectralilluminationdistributions
5Spatialilluminationdistributions
6Deliveringthelumens
7Designingforperception-basedlightingconcepts
Appendix
Index


Figures

1.1TheCheckerShadowIllusion.SquaresAandBareidentical
1.2AwhitesheethasbeendrawnovertheCheckerShadowIllusion,withcut-outsfor
squaresAandB,andnowtheyappeartobeidentical
1.3Previouslythecylindricalobjectappearedtobeuniformlygreen
1.4Theobjectattributesofthisbuildingareclearlyrecognisable(ChartresCathedral,
France)
1.5ChartresCathedral,Francebutavastlydifferentappearance
2.1Tostartthethoughtexperiment,imaginearoomforwhichthesumofceiling,walls,
andfloorareais100m2
2.2Totheroomisaddedaluminaire
2.3Allroomsurfacesaregivenaneutralgreyfinishsothatρrs=0.5
2.4Roomsurfacereflectanceisincreasedsothatρrs=0.8
2.5Roomsurfacereflectanceisreducedtozero,soρrs=0
2.6Thefinalstageofthethoughtexperiment

2.7ReflectanceplottedagainstMunsellValue
2.8Usinganinternallyblackenedtubemountedontoalightmetertoobtaina
measurementofsurfacereflectance
2.9Thevalueofthereflectance/absorptanceratioisproportionaltomeanroomsurface
exitance,MRSE
3.1Demonstrationset-upforgainingassessmentsofnoticeable,distinct,strongand
emphaticilluminationdifferences
3.2Flowchartforachievingmeanroomsurfaceexitance,MRSE,andtask/ambient
illumination,TAIR,designvalues
4.1RelativesensitivityfunctionsforV(λ),andthethreeconetypes;long-,medium-and
short-wavelength;L(λ),M(λ)andS(λ)
4.2TheVB3(λ)spectralsensitivityofbrightnessfunctionfordaytimelightlevels


4.3TheV(λ)andV′(λ)relativeluminousefficiencyfunctionsrelatetophotopicand
scotopicadaptationrespectively
4.4Rea’sproposedVC(λ)functionfortherelativecircadianresponse
4.5Theblack-bodylocus(solidline)plottedontheCIE1931(x,y)chromaticitychart
4.6ThereciprocalmegaKelvinscale(MK−1)comparedwiththeKelvin(K)scale
4.7Contoursofperceivedleveloftint
4.8Kruithof’schartrelatingcorrelatedcolourtemperature(TC)andilluminance(E)to
colourappearance
4.9OutputfromCIE133W.execomputerprogramtocalculateCRIs,foraWarmWhite
halophosphatefluorescentlamp
4.10Colour-mismatchvectordataforahalophosphateCoolWhitecolour33fluorescent
lamp
4.11GamutareasforsomefamiliarlightsourcesplottedontheCIE1976UCS(uniform
chromaticityscale)diagram
4.12TheGretagMacbethColorCheckercolourrenditionchartbeingexaminedunder
daylight

5.1Thetripleobjectlightingpatternsdevice
5.2Forthethreelightingconditionsdescribedinthetext
5.3ThestrikingfirstviewoftheinterioroftheQELAboutique,Doha
5.4QELA–Thedisplaylightinginthecentralareahasstrongdownward‘flow’,with
‘sharpness’creatingcrispshadowandhighlightpatterns
5.5QELA–Inthisdisplayarea,whichisadjacenttothecentralarea,thelowermeanroom
surfaceexitance(MRSE)levelhastheeffectofstrengtheningtheshadingpatterns
5.6QELA–Inthisdisplayarea,themannequinappearsisolatedbythestrongshading
patterngeneratedbytheselectivelighting
5.7QELA–Ontheupperfloor,the‘fire’ontherightmatchesthewarmwhiteillumination
usedthroughouttheboutique
5.8ThepointPislocatedattheintersectionofthex,yandzorthogonalaxes
5.9Thethree-dimensionalilluminationdistributionaboutpointP
5.10TheilluminationsolidisnowthesumofcomponentsolidsduetosourcesS1andS2
5.11Theilluminationsolidatapointinaspacewherelightarrivesfromeverydirection
5.12Themagnitudeanddirectionof(EA–EB)maxdefinestheilluminationvector,whichis
depictedasanarrowactingtowardsthepoint


5.13Thisisthesymmetricsolid
5.14In(a),asmallsourceSprojectsluminousfluxofFlmontoadiscofradiusr,producing
asurfaceilluminanceE=F/(π.r2).In(b),thediscisreplacedbyasphereofradiusr,
givingasurfaceilluminanceE=F/(4π.r2)
5.15(a):VerticalsectionthroughPshowingilluminationvectoraltitudeangleα,and(b):
HorizontalsectionthroughPshowingazimuthangleφofthehorizontalvector
component
5.16ThepointPisonasurface,andisilluminatedbyadisc-shapedsourcethatisnormal
tothesurfaceandofangularsubtenceα
5.17Thiscomparisonsurfacehastwomountedsamplesthatresponddifferentlytothedisc
source

5.18Asthesubtenceofalargediscsourceisreduced,thesourceluminancerequiredto
maintainanilluminancevalueof100luxincreasesrapidlyassubtencefallsbelow30
degrees
5.19Forsmallsources,theincreaseinluminancerequiredtomaintain100luxincreases
dramaticallyforsubtenceangleslessthan3degrees
5.20HighlightcontrastpotentialHLCforthreevaluesoftargetreflectance
5.21Lightsourcesofsmallersubtenceangleproducelesspenumbra,increasingthe
’sharpness’ofthelighting
6.1Measuringsurfacereflectance,usinganinternallyblackenedcardboardtubefittedover
anilluminancemeter
6.2Applicationofthepoint-to-pointformula
6.3DeterminingtheilluminanceatpointPonaverticalplane
6.4ThepointPisilluminatedbytwoalternativesources
6.5ThecorrectionfactorC(D/r)tobeappliedtopointsourceilluminationformulae
6.6TheCubicIlluminationconcept
6.7ThelocationofsourceSrelativetoathree-dimensionalobjectisdefinedintermsofX,
Y,andZdimensions
6.8Assessmentoflikelyprospectsforvariousrolesforfenestrationinbuildings
6.9AsimplewayofmakinganapproximatemeasurementofMRSEusingaconventional
lightmeter
6.10Asix-photocellcubicilluminationmeter
6.11Themeasurementcubeistiltedsothatalongaxisiscoincidentwiththezaxis,and


threefacetsfaceupwardsandthreedownwards
6.12Averticalsectionthroughthetiltedcubeontheuaxis,whichliesinthesamevertical
planeastheyaxis,againstwhichitistiltedthroughtheanglea
6.13Aphotocellheadmountedonaright-anglebracket,ontoaphotographictripod
6.14Thephotocelltiltedto+35degreesrelativetothehorizontalplane
7.1Alightingdesignflowchart

7.2TAIRvaluesforthehorizontalworkingplane,whenitisthetarget
7.3Theinfluenceofroomsurfacereflectionproperties.


Tables

2.1Perceivedbrightnessordimnessofambientillumination
2.2Perceiveddifferencesofexitanceorilluminance
4.1The14CIETCS(Testcoloursamples).TCS1–8comprisetheoriginalsetofmoderately
saturatedcoloursrepresentingthewholehuecircle,andthesearetheonlysamplesused
fordeterminingCRI.Theothersixhavebeenaddedforadditionalinformation,and
comprisefoursaturatedcolours,TCS9–12,andtwosurfacesofparticularinterest.
Regrettably,detailsofcolourshiftsfortheseTCSareseldommadeavailable
5.1Vector/scalarratioandtheperceived‘flow’oflight
7.1Valuesoftarget/ambientilluminanceratio,TAIR,againstroomindexwherethe
horizontalworkingplane,HWP,isthetargetsurfaceandalldirectfluxisincidentonthe
HWP.Lightsurfacereflectancesareassumed


Acknowledgements

The contents of this book have grown from the Advanced Lighting Design course that I
havetaughteveryyearsince2005attheQueenslandUniversityofTechnologyinBrisbane,
Australia,forwhichIthanktheprogrammecoordinator,ProfessorIanCowling,andalso
thesuccessionoflivelyandenquiringCPD(continuingprofessionaldevelopment)students
whohavecausedmetokeepthecurriculuminastateofcontinualrevision.
Whilemanypeoplehavecontributedtothedevelopmentoftheideascontainedinthis
book,whethertheyrealiseditatthetimeornot,threeformercolleagueswithwhomIhave
maintainedemailcontacthaverespondedtospecificissuesthatIencounteredinpreparing
the text. They are, in no particular order, Joe Lynes and Professors Mark Rea and Peter

Boyce.Mythankstoeachofthem.
Thosewhohavegivenpermissionformetoreproducefiguresareacknowledgedinthe
captions, but I want to make particular mention of Edward Adelson, Professor of Vision
Science at the Massachusetts Institute of Technology, who not only permitted me to
reproduce his Checker Shadow Illusion (Figure 1.1), but also two of my own modified
versionsofhisbrilliantfigure.


Introduction

Theaimofthisbookistoenablepeoplewhoarefamiliarwiththefundamentalsoflighting
technology to extend their activities into the field of lighting design. While the text is
addressed primarily to students, it is relevant to professionals working in the fields of
buildingservices,interiordesignandarchitecture.
The premise of this book is that the key to lighting design is the skill to visualise the
distribution of light within the volume of a space in terms of how it affects people’s
perceptionsofthespaceandtheobjects(includingthepeople)withinit.Theaimisnotto
produce lighting that will be noticed, but rather, to provide an envisioned balance of
brightnessthatsetstheappearanceofindividualobjectsintoanoveralldesignconcept.
Thisisdifferentfromcurrentnotionsof‘goodlightingpractice’,whichaimtoprovide
forvisibility,whereby‘visualtasks’maybeperformedefficientlyandwithoutpromoting
fatigue or discomfort. It is also quite different from some lighting design practice, where
spectacular effects are achieved by treating the architecture as a backdrop onto which
patternsofcolouredlight,orevenbrilliantimages,areprojected.
Several perception-based lighting concepts are introduced to enable distributions of
illumination to be described in terms of how they may influence the appearance of a lit
space.Thesedescriptionsinvolveperceivedattributesofillumination,suchasillumination
thatbringsout‘colourfulness’,orhasaperceived‘flow’,orperhaps‘sharpness’.Itisshown
thatthethree-dimensionaldistributionsofilluminationthatunderliethisunderstandingof
lighting can be analysed in quantitative terms, enabling their characteristics to be

measured and predicted. The principles governing these distributions are explained, and
spreadsheets are used to automatically perform the calculations that relate perceived
attributestophotometricquantities.
Theobjectiveistoenablealightingdesignertodiscusslightingwithclientsandother
professionals in terms of how illumination may influence the appearance of spaces and
objects.Whenagreementisreached,thedesigneristhenabletoapplyproceduresthatlead
tolayoutsofluminairesandstrategiesfortheircontrol,andtodothiswithconfidencethat
theenvisionedappearancewillbeachieved.


1
TheRoleofVisualPerception


Chaptersummary
The Checker Shadow Illusion demonstrates a clear distinction between the processes of
vision and perception, where vision is concerned with discrimination of detail and
perceptioninvolvesrecognitionofsurfaceandobjectattributes.Theroleoflightinginthis
recognition process involves the formation of lighting patterns created by interactions
between objects and the surrounding light field. Confident recognition comprises clear
perception of both object attributes and the light field. Three types of object lighting
patterns are identified, being the shading, highlight, and shadow patterns, and it is by
creating light fields that produce controlled balances of these three-dimensional lighting
patterns that designers gain opportunities to influence how room surface and object
attributesarelikelytobeperceived.


Theevidenceofyoureyes
Figure1.1 shows the Checker Shadow Illusion, and at first sight, the question has to be,
whereistheillusion?Everythinglooksquitenormal.TheanswerliesinsquaresAandB:

theyareidentical.Thatistosay,theyarethesameshadeofgreyandtheyhavethesame
lightness,ortobemoretechnical,theyhavethesamereflectance(andtherebyluminance)
andthesamechromaticity.
Doyoufindthiscredible?Theycertainlydonotlookthesame.NowlookatFigure1.2,
whichshowsawhitesheetdrawnoverthefigurewithcut-outsforthetwosquares.Seen
inthiswaytheydolookthesame,andifyoutakeapieceofcardandpunchaholeinit,
youcanslideitoverthepreviousfigureandconvinceyourselfthatthetwosquaresarein
factidenticalandasshowninFigure1.2.
Thisraisesaquestion:howisitthat,whentheimagesofthesetwoidenticalsquaresare
simultaneouslyfocussedontotheretina,inonecase(Figure1.2)theyappearidenticaland
intheother(Figure1.1)theyappeardistinctlydifferent?

Figure1.1TheCheckerShadowIllusion.SquaresAandBareidentical.Theyarepresentedhereasrelatedcolours,thatis
tosay,theyappearrelatedtotheirsurroundings.Thelightingpatternsthatappearsuperimposedoverthesurrounding
surfacescauseaviewertoperceivea‘flow’oflightwithinthevolumeofthisspace,andwhichleadstothematching
luminancesofAandBbeingperceivedquitedifferently.
(Source:en.wikipedia.org/wiki/Checker_shadow_illusion.html,downloadedJanuary2013)


Figure1.2AwhitesheethasbeendrawnovertheCheckerShadowIllusion,withcut-outsforsquaresAandB,andnow
theyappeartobeidentical.Inthiscasetheyarepresentedasunrelatedcolours.


Relatedandunrelatedcolours
The essential difference is that in Figure 1.1 the two squares are presented as related
colours, that is to say, colours are perceived to belong to surfaces or objects seen in
relationtoothercolours,andinFigure1.2,theyareshownasunrelatedcolours,meaning
theyareseeninisolationfromothercolours(Fairchild,2005).Asunrelatedcolours(greyis
acolour),theyareperceivedtocomprisenothingmorethanrectangularcolouredshapes
onaplainwhitebackground,butwhentheyaresetintothecontextofFigure1.1,theyare

perceived as solid elements in a three-dimensional scene that have recognisable object
attributes. It is this change in the way they are perceived that causes them to appear
differently.
So what are the components of the surrounding scene that make this illusion so
effective?Askyourself,whyisthecylindricalobjectthere?Doesitcontributesomething?
Infact,itisavitalcomponentoftheillusion.So,whatcolourisit?Obviously,green.Isit
uniformlygreen?Well,yes…butlookmorecarefullyattheimageoftheobjectandyou
will see that both its greenness and its lightness vary hugely. The image is far from
uniform,sohowdidyousupposetheobjecttobeuniformlygreen?Theansweristhatyou
perceived a distinctive lighting pattern superimposed over the uniformly green object. In
Figure 1.3, the area enclosed by the object outline is shown as uniformly green and it
appearsasnothingmorethanaformlessblob.
Thesolid,three-dimensionalobjectperceivedinFigure1.1isobservedtobeinteracting
with a directional ‘flow’ of light, which causes a shadingpattern to be generated, and
this appears superimposed over the green object surface. Note also that the cylinder’s
surface is not perfectly matt, and there is just a hint of a highlight pattern due to a
specularcomponentofreflectionthatisapparentattheroundedrimofthecylinder’stop
edge.Theselightingpatternsinformyouabouttheobject’sattributes(Cuttle,2008).


Figure1.3Previouslythecylindricalobjectappearedtobeuniformlygreen.Nowitisuniformlygreen,butitdoesnot
looklikeacylinder.Thatisbecauseitisnowlackingthelightingpatternduetointeractionwiththe‘flow’oflight.

Nowlookatthecheckerboardsurface.Againwehaveapatternduetothelighting,but
inthiscaseitisashadowpattern,whichhasadifferentappearancefromtheshadingand
highlight patterns, but nonetheless is quite consistent with our perception of the overall
‘flow’oflightwithinthevolumeofthespace.Itwillbeobvioustoyouthatiftwosurfaces
havethesamelightness(whichalsomeanstheyhavethesamereflectance)andoneoccurs
withintheshadowpatternandoneoutsideit,theywillhavedifferentluminancevalues.
Thecreatorofthisbrilliantillusion,EdwardH.Adelson,ProfessorofVisionScienceatthe

MassachusettsInstituteofTechnology,hascarefullysetitupsothatsquaresAandBhave
the same luminance value, which means of course, that their images on your retina are
identical.However,thefunctionofthevisualprocessistoprovideinformationtothevisual
cortexofthebrain,andhereyourperceptualprocessistellingyouthat,althoughthesetwo
squaresmatchforluminance,theycannothavethesamelightness.Theoneintheshadow
mustbelighter,thatistosay,itmusthavehigherreflectance,thantheoneinfulllight.You
hold this innate understanding of lighting in your brain, and you cannot apply your
consciousmindtooverruleit.
Inthisway,itcanbeseenthattheimagefocussedontotheretinaissimplyanoptical
projection of the visual scene that corresponds directly with the luminance and
chromaticityvaluesoftheelementswithintheexternalscene.Sinceitsinception,thestudy
oflightinghasconcentratedonthevisualprocessandhowilluminationmaybeappliedto
provideforvisibility,laterdefinedintermsofvisualperformance,buttheroleofvisionis
toservetheprocessofperception,andthisoccursnotattheretina,butinthevisualcortex


ofthebrain.Whatweperceiveisnotapatternofbrightnessandcolour,butagestalt,this
beingapsychologicaltermthatdescribestheholisticentitythatenablesustorecogniseall
the forms and objects that make up our surroundings (Purves and Beau Lotto, 2003).
Consciously,weareawareofthree-dimensionalspacesdefinedbysurfacesandcontaining
objects,butinordertomakethismuchsenseoftheflowofinformationarrivingthrough
theopticnerve,wehavetobesubconsciouslyawareofalightfieldthatfillsthevolumeof
the space. This is how we make sense of squares A and B. Seen in this way, it becomes
obvious why attempts to analyse scenes in terms of luminance and chromaticity were
boundtoleadtofrustration.


Theroleofambientillumination
Formostofthetime,weliveinaworldofrelatedcolours.Wearesurroundedbysurfaces
and objects which, providing the entire scene is adequately illuminated, our perceptual

faculties reliably recognise and make us aware of, sometimes so that we can cope with
everydaylife,andsometimestoelevateoursensestohigherlevelsofappreciation,aswhen
we encounter artworks or beauties of nature. Recognition involves identifying object
attributes associated with all of the things that make up our surrounding environments,
and our innate skill in doing this is truly impressive. Scientists working on artificial
intelligencehavetriedtoprogramsupercomputerstoperforminthisway,butsofartheir
besteffortsfallfarshortofwhathumanperceptionachieveseverymomentthroughoutour
wakinghours.
Provided that ambient illumination is sufficient, we are able to enter unfamiliar
environments, orientate ourselves, and go about our business without hesitating to
question the reliability of the perceptions we form of the surrounding environment. It is
clearthatsubstantialprocessinghastooccur,veryrapidly,betweentheretinalimageand
formation of the perception of the environment. There is no good reason why our
perceptions of elements of the scene should show in-step correspondence with their
photometriccharacteristics.Visualperceptionmaybethoughtofastheprocessofmaking
senseoftheflowofsensoryinputthroughtheopticnervetothebrain,wherethepurpose
is to recognise surfaces and objects, rather than to record their images. Colours are
perceived as related to object attributes, and effects of illumination are perceived as
lightingpatternssuperimposedoverthem.AswerecognisedthecylinderinFigure1.1to
be uniformly green with a superimposed shading pattern, so we also recognised the
identicalsquarestodifferinlightnessbecauseofthesuperimposedshadowpattern.
There will, however, be situations where we are confronted with elements seen in
isolation from each other, and this is particularly likely to occur in conditions of low
ambientillumination.Whenwefindourselvesconfrontedbydarksurroundings,reliance
uponrelatedcoloursandidentificationofobjectattributesmaygivewaytoperceptionof
unrelatedcolours,andwhenthisoccurs,ourperceptionsdonotdistinguishlightnessand
illuminanceseparately,andluminancepatternsdominate.Thatistosay,theappearancesof
individual objects within the scene relate to their brightness and chromaticity values,
ratherthanuponrecognitionoftheirintrinsicattributes.
Figures1.4and1.5showtwoviewsofthesamebuilding.InFigure1.4,weseeaviewof

thismagnificentcathedralinitssetting,andwereadilyformasenseofitssubstantialmass
andthematerialsfromwhichitisconstructed.Also,evenifwearenotconsciousofit,we
perceivetheentirelightfieldthatgeneratesthisappearance.InFigure1.5,ourperception
of this building is quite different. We have no notion of a natural light field, and the
buildingseemstofloat,unattachedtotheground.Itisrevealedbyaglowinglightpattern


thatdoesnotdistinguishbetweenmaterials,andactuallymakesthebuildingappearselfluminous.Thebuilding’sappearanceisdominatedbybrightness,andobjectattributesare
notdiscernible.Thesetwoviewsshowclearlythedifferencebetweenrelatedcolours,inthe
daylight view, and unrelated colours in the night-time view. They also give us due
appreciationoftherolethatlightingmayplayinbringingaboutfundamentaldifferences
inourperceptions.
Undernormaldaytimelighting,two-wayinteractionsoccurthatenableourperceptual
processes to make sense of the varied patterns of light and colour that are continuously
being focussed onto our retinas. Working in one direction, there is the process of
recognisingobjectattributesthatarerevealedbythelightingpatterns,whileatthesame
time,andworkingintheoppositedirection,itistheappearanceoftheselightingpatterns
that provides for the viewer’s understanding of the light field that occupies the entire
space.

Figure1.4Theobjectattributesofthisbuildingareclearlyrecognisable,andtheambientilluminationprovidesamplyfor
allelementstoappearasrelatedcolours.(ChartresCathedral,France.)


Figure1.5Thesamebuilding,butavastlydifferentappearance.Lowambientilluminationprovidesadarkbackdrop
againstwhichthecathedralglowswithbrightness.Objectattributesareunrecognisableinthisexampleofunrelated
colours.


Perceptionasabasisforlightingdesign

Fromadesignpointofview,lightingpracticemaybeseentofallintotwobasiccategories.
Ononehand,forilluminationconditionsrangingfromoutdoordaylighttoindoorlighting
wheretheambientlevelissufficienttoavoidanyappearanceofgloom,weliveinaworld
of related colours in which we distinguish readily between aspects of appearance that
relate to the visible attributes of surfaces and objects, and aspects which relate to the
lightingpatternsthatappearsuperimposeduponthem.
Ontheotherhand,inconditionsoflowambientillumination,wherewehaveasenseof
darknessorevengloom,whetherindoorsor,mostnotably,outdoorsatnight,wetypically
experience unrelated colours and this may lead to the appearances of objects and
surroundings dominated by brightness patterns that may offer no distinction between
objectlightnessandsurfaceilluminance.
Theimplicationsofthisdichotomyforlightingdesignareprofound.Outdoornight-time
lighting practice, such as floodlighting and highway illumination, is based on creating
brightness patterns that may bear little or no relationship to surface or object properties.
Alternatively,forsituationswhereambientilluminationisatleastsufficienttomaintainan
appearance of adequacy (apart from outdoor daylight, this may be taken to include all
indoorspaceswheretheilluminationcomplieswithcurrentstandardsforgenerallighting
practice)wetakeinentirevisualscenesincludingobjectattributes,andinvolvinginstant
recognitionoffamiliarobjectsandscrutinyofunfamiliarorotherwiseinterestingobjects.
The identification of object attributes may become a matter of keen interest, as when
admiring an art object or seeking to detect a flaw in a manufactured product, and we
depend upon the lighting patterns to enable us to discriminate and to respond to
differencesofobjectattributes.
Betweenthesetwosetsofconditionsisarangeinwhichsomeuncertaintyprevails.We
have, for example, all experienced ‘tricks of the light’ that can occur at twilight, and
generally, recommendations for good lighting practice aim to avoid such conditions.
Perhaps surprisingly, it is within this range that lighting designers achieve some of their
mostspectaculardisplayeffects.Byisolatingspecificobjectsfromtheirbackgroundsand
illuminating them from concealed light sources, lighting can be applied to alter the
appearance of selected object attributes, such as making selected objects appear more

textured, or colourful, or glossy. All of this thinking will be developed in following
chapters.
Beforeweclosethischapter,askyourself,whydowecallFigure1.1anillusion?Ifthe
page is evenly illuminated, squares A and B will have the same luminance and so they
stimulate their corresponding areas of our retinas to the same level. The fact that these
equalstimulidonotcorrespondtoequalsensationsofbrightnessiscitedasanillusion.The
pointneedstobemadethatvisionservestheprocessofperception,andperceptionisnot