LightingDesign

By reading this book, youwill develop the skills to perceive a space and its contents inlight,andbeabletodevisealayoutofluminairesthatwillprovidethatlitappearance.

WrittenbyrenownedlightingexpertChristopher(Kit)Cuttle,thebook:

explainsthedifferencebetweenvisionandperception,whichisthedistinctionbetweenprovidinglightingtomakethingsvisible,andprovidingittoinfluencetheappearanceofeverythingthatisvisible;demonstrateshowlightingpatternsgeneratedbythree-dimensionalobjectsinteractingwithdirectionallightingarestronglyinfluentialuponhowthevisualperceptionprocessenablesustorecogniseobjectattributes,suchaslightness,colourfulness,textureandgloss;revealshowadesignerwhounderstandstheroleoftheselightingpatternsintheperceptualprocessmayemploythemeithertoreveal,ortosubdue,ortoenhancetheappearanceofselectedobjectattributesbycreatingappropriatespatialdistributionsoflight;carefullyexplainscalculationaltechniquesandprovideseasy-to-usespreadsheets,sothatlayoutsoflampsandluminairesarederivedthatcanbereliedupontoachievetherequiredilluminationdistributions.

Practical lighting design involves devising three-dimensional light fields that createluminoushierarchiesrelatedtothevisualsignificanceofeachelementwithinascene.Byproviding you with everything you need to develop a design concept from theunderstandingofhowlightinginfluenceshumanperceptionsofsurroundings,throughtoengineering efficient and effective lighting solutions KitCuttle instils in his readers anew-foundconfidenceinlightingdesign.

Christopher Kit Cuttle,MA, FCIBSE, FIESANZ, FIESNA, FSLL, is visiting lecturer inAdvanced 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 forArtsSake,ButterworthHeinemann,2007).His previous positions include Head of Graduate Education in Lighting at the LightingResearchCenter,RensselaerPolytechnic Institute,Troy,NewYork;SeniorLecturerat theSchools of Architecture at the University of Auckland and the Victoria University ofWellington, both in New Zealand; Section Leader in the Daylight Advisory Service,

PilkingtonGlass;andLightingDesignerwithDerekPhillipsAssociates,bothintheUK.HisrecentawardsincludetheSocietyofLightandLightingsLeonGaster2013AwardforhisLR&T paper A New Direction for General Lighting Practice, and the LifetimeAchievement Award presented at the 2013 Professional Lighting Design Conference inCopenhagen.

PublishersNote:

Todownloadthespreadsheetsthatareusedtofacilitatethecalculationsinthisbook,gotothe e-resources link shown on the back cover of the book and click oneResource/Downloads.

LightingDesign

Aperception-basedapproachChristopherCuttle

Firstpublished2015

byRoutledge

2ParkSquare,MiltonPark,Abingdon,OxonOX144RN

andbyRoutledge

711ThirdAvenue,NewYork,NY10017

RoutledgeisanimprintoftheTaylor&FrancisGroup,aninformabusiness

2015ChristopherCuttle

TherightofChristopherCuttletobeidentifiedasauthorofthisworkhasbeenassertedbyhiminaccordancewith

sections77and78oftheCopyright,DesignsandPatentsAct1988.

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AcataloguerecordforthisbookisavailablefromtheBritishLibrary

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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,

ArchitecturalanddecorativeDesign.2.Visualperception.I.Title.

NK2115.5.L5C882015

747.92dc23

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-outsforsquaresAandB,andnowtheyappeartobeidentical

1.3Previouslythecylindricalobjectappearedtobeuniformlygreen

1.4Theobjectattributesofthisbuildingareclearlyrecognisable(ChartresCathedral,France)

1.5ChartresCathedral,Francebutavastlydifferentappearance

2.1Tostartthethoughtexperiment,imaginearoomforwhichthesumofceiling,walls,andfloorareais100m2

2.2Totheroomisaddedaluminaire

2.3Allroomsurfacesaregivenaneutralgreyfinishsothatrs=0.5

2.4Roomsurfacereflectanceisincreasedsothatrs=0.8

2.5Roomsurfacereflectanceisreducedtozero,sors=0

2.6Thefinalstageofthethoughtexperiment

2.7ReflectanceplottedagainstMunsellValue

2.8Usinganinternallyblackenedtubemountedontoalightmetertoobtainameasurementofsurfacereflectance

2.9Thevalueofthereflectance/absorptanceratioisproportionaltomeanroomsurfaceexitance,MRSE

3.1Demonstrationset-upforgainingassessmentsofnoticeable,distinct,strongandemphaticilluminationdifferences

3.2Flowchartforachievingmeanroomsurfaceexitance,MRSE,andtask/ambientillumination,TAIR,designvalues

4.1RelativesensitivityfunctionsforV(),andthethreeconetypes;long-,medium-andshort-wavelength;L(),M()andS()

4.2TheVB3()spectralsensitivityofbrightnessfunctionfordaytimelightlevels

4.3TheV()andV()relativeluminousefficiencyfunctionsrelatetophotopicandscotopicadaptationrespectively

4.4ReasproposedVC()functionfortherelativecircadianresponse

4.5Theblack-bodylocus(solidline)plottedontheCIE1931(x,y)chromaticitychart

4.6ThereciprocalmegaKelvinscale(MK1)comparedwiththeKelvin(K)scale

4.7Contoursofperceivedleveloftint

4.8Kruithofschartrelatingcorrelatedcolourtemperature(TC)andilluminance(E)tocolourappearance

4.9OutputfromCIE133W.execomputerprogramtocalculateCRIs,foraWarmWhitehalophosphatefluorescentlamp

4.10Colour-mismatchvectordataforahalophosphateCoolWhitecolour33fluorescentlamp

4.11GamutareasforsomefamiliarlightsourcesplottedontheCIE1976UCS(uniformchromaticityscale)diagram

4.12TheGretagMacbethColorCheckercolourrenditionchartbeingexaminedunderdaylight

5.1Thetripleobjectlightingpatternsdevice

5.2Forthethreelightingconditionsdescribedinthetext

5.3ThestrikingfirstviewoftheinterioroftheQELAboutique,Doha

5.4QELAThedisplaylightinginthecentralareahasstrongdownwardflow,withsharpnesscreatingcrispshadowandhighlightpatterns

5.5QELAInthisdisplayarea,whichisadjacenttothecentralarea,thelowermeanroomsurfaceexitance(MRSE)levelhastheeffectofstrengtheningtheshadingpatterns

5.6QELAInthisdisplayarea,themannequinappearsisolatedbythestrongshadingpatterngeneratedbytheselectivelighting

5.7QELAOntheupperfloor,thefireontherightmatchesthewarmwhiteilluminationusedthroughouttheboutique

5.8ThepointPislocatedattheintersectionofthex,yandzorthogonalaxes

5.9Thethree-dimensionalilluminationdistributionaboutpointP

5.10TheilluminationsolidisnowthesumofcomponentsolidsduetosourcesS1andS2

5.11Theilluminationsolidatapointinaspacewherelightarrivesfromeverydirection

5.12Themagnitudeanddirectionof(EAEB)maxdefinestheilluminationvector,whichisdepictedasanarrowactingtowardsthepoint

5.13Thisisthesymmetricsolid

5.14In(a),asmallsourceSprojectsluminousfluxofFlmontoadiscofradiusr,producingasurfaceilluminanceE=F/(.r2).In(b),thediscisreplacedbyasphereofradiusr,givingasurfaceilluminanceE=F/(4.r2)

5.15(a):VerticalsectionthroughPshowingilluminationvectoraltitudeangle,and(b):HorizontalsectionthroughPshowingazimuthangleofthehorizontalvectorcomponent

5.16ThepointPisonasurface,andisilluminatedbyadisc-shapedsourcethatisnormaltothesurfaceandofangularsubtence

5.17Thiscomparisonsurfacehastwomountedsamplesthatresponddifferentlytothediscsource

5.18Asthesubtenceofalargediscsourceisreduced,thesourceluminancerequiredtomaintainanilluminancevalueof100luxincreasesrapidlyassubtencefallsbelow30degrees

5.19Forsmallsources,theincreaseinluminancerequiredtomaintain100luxincreasesdramaticallyforsubtenceangleslessthan3degrees

5.20HighlightcontrastpotentialHLCforthreevaluesoftargetreflectance

5.21Lightsourcesofsmallersubtenceangleproducelesspenumbra,increasingthesharpnessofthelighting

6.1Measuringsurfacereflectance,usinganinternallyblackenedcardboardtubefittedoveranilluminancemeter

6.2Applicationofthepoint-to-pointformula

6.3DeterminingtheilluminanceatpointPonaverticalplane

6.4ThepointPisilluminatedbytwoalternativesources

6.5ThecorrectionfactorC(D/r)tobeappliedtopointsourceilluminationformulae

6.6TheCubicIlluminationconcept

6.7ThelocationofsourceSrelativetoathree-dimensionalobjectisdefinedintermsofX,Y,andZdimensions

6.8Assessmentoflikelyprospectsforvariousrolesforfenestrationinbuildings

6.9AsimplewayofmakinganapproximatemeasurementofMRSEusingaconventionallightmeter

6.10Asix-photocellcubicilluminationmeter

6.11Themeasurementcubeistiltedsothatalongaxisiscoincidentwiththezaxis,and

threefacetsfaceupwardsandthreedownwards

6.12Averticalsectionthroughthetiltedcubeontheuaxis,whichliesinthesameverticalplaneastheyaxis,againstwhichitistiltedthroughtheanglea

6.13Aphotocellheadmountedonaright-anglebracket,ontoaphotographictripod

6.14Thephotocelltiltedto+35degreesrelativetothehorizontalplane

7.1Alightingdesignflowchart

7.2TAIRvaluesforthehorizontalworkingplane,whenitisthetarget

7.3Theinfluenceofroomsurfacereflectionproperties.

Tables

2.1Perceivedbrightnessordimnessofambientillumination

2.2Perceiveddifferencesofexitanceorilluminance

4.1The14CIETCS(Testcoloursamples).TCS18comprisetheoriginalsetofmoderatelysaturatedcoloursrepresentingthewholehuecircle,andthesearetheonlysamplesusedfordeterminingCRI.Theothersixhavebeenaddedforadditionalinformation,andcomprisefoursaturatedcolours,TCS912,andtwosurfacesofparticularinterest.Regrettably,detailsofcolourshiftsfortheseTCSareseldommadeavailable

5.1Vector/scalarratioandtheperceivedflowoflight

7.1Valuesoftarget/ambientilluminanceratio,TAIR,againstroomindexwherethehorizontalworkingplane,HWP,isthetargetsurfaceandalldirectfluxisincidentontheHWP.Lightsurfacereflectancesareassumed

Acknowledgements

Thecontentsof thisbookhavegrown from theAdvancedLightingDesigncourse that Ihavetaughteveryyearsince2005attheQueenslandUniversityofTechnologyinBrisbane,Australia, forwhichI thanktheprogrammecoordinator,Professor IanCowling,andalsothesuccessionoflivelyandenquiringCPD(continuingprofessionaldevelopment)studentswhohavecausedmetokeepthecurriculuminastateofcontinualrevision.

Whilemanypeoplehavecontributedtothedevelopmentoftheideascontainedinthisbook,whethertheyrealiseditatthetimeornot,threeformercolleagueswithwhomIhavemaintainedemailcontacthaverespondedtospecificissuesthatIencounteredinpreparingthe text. They are, in no particular order, Joe Lynes and ProfessorsMarkRea and PeterBoyce.Mythankstoeachofthem.

Thosewhohavegivenpermissionformetoreproducefiguresareacknowledgedinthecaptions, but Iwant tomakeparticularmentionofEdwardAdelson,ProfessorofVisionScience at the Massachusetts Institute of Technology, who not only permitted me toreproduce his Checker Shadow Illusion (Figure 1.1), but also two of my own modifiedversionsofhisbrilliantfigure.

Introduction

Theaimofthisbookistoenablepeoplewhoarefamiliarwiththefundamentalsoflightingtechnology to extend their activities into the field of lighting design.While the text isaddressed primarily to students, it is relevant to professionals working in the fields ofbuildingservices,interiordesignandarchitecture.

Thepremiseof this book is that thekey to lightingdesign is the skill tovisualise thedistribution of light within the volume of a space in terms of how it affects peoplesperceptionsofthespaceandtheobjects(includingthepeople)withinit.Theaimisnottoproduce lighting that will be noticed, but rather, to provide an envisioned balance ofbrightnessthatsetstheappearanceofindividualobjectsintoanoveralldesignconcept.

This isdifferentfromcurrentnotionsof goodlightingpractice,whichaimtoprovideforvisibility,wherebyvisualtasksmaybeperformedefficientlyandwithoutpromotingfatigueordiscomfort. It isalsoquitedifferent fromsome lightingdesignpractice,wherespectacular effects are achieved by treating the architecture as a backdrop onto whichpatternsofcolouredlight,orevenbrilliantimages,areprojected.

Several perception-based lighting concepts are introduced to enable distributions ofillumination tobedescribed in termsofhow theymay influence theappearanceofa litspace.Thesedescriptionsinvolveperceivedattributesofillumination,suchasilluminationthatbringsoutcolourfulness,orhasaperceivedflow,orperhapssharpness.Itisshownthatthethree-dimensionaldistributionsofilluminationthatunderliethisunderstandingoflighting can be analysed in quantitative terms, enabling their characteristics to bemeasured and predicted. The principles governing these distributions are explained, andspreadsheets are used to automatically perform the calculations that relate perceivedattributestophotometricquantities.

Theobjectiveis toenablea lightingdesignertodiscuss lightingwithclientsandotherprofessionals in terms of how illuminationmay influence the appearance of spaces andobjects.Whenagreementisreached,thedesigneristhenabletoapplyproceduresthatleadtolayoutsofluminairesandstrategiesfortheircontrol,andtodothiswithconfidencethattheenvisionedappearancewillbeachieved.

1TheRoleofVisualPerception

Chaptersummary

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

Theevidenceofyoureyes

Figure1.1 shows theChecker Shadow Illusion, andat first sight, thequestionhas to be,whereistheillusion?Everythinglooksquitenormal.TheanswerliesinsquaresAandB:theyareidentical.Thatistosay,theyarethesameshadeofgreyandtheyhavethesamelightness,ortobemoretechnical,theyhavethesamereflectance(andtherebyluminance)andthesamechromaticity.

Doyoufindthiscredible?Theycertainlydonotlookthesame.NowlookatFigure1.2,whichshowsawhitesheetdrawnoverthefigurewithcut-outsforthetwosquares.Seeninthiswaytheydolookthesame,andifyoutakeapieceofcardandpunchaholeinit,youcanslideitoverthepreviousfigureandconvinceyourselfthatthetwosquaresareinfactidenticalandasshowninFigure1.2.

Thisraisesaquestion:howisitthat,whentheimagesofthesetwoidenticalsquaresaresimultaneouslyfocussedontotheretina,inonecase(Figure1.2)theyappearidenticalandintheother(Figure1.1)theyappeardistinctlydifferent?

Figure1.1TheCheckerShadowIllusion.SquaresAandBareidentical.Theyarepresentedhereasrelatedcolours,thatis

tosay,theyappearrelatedtotheirsurroundings.Thelightingpatternsthatappearsuperimposedoverthesurrounding

surfacescauseaviewertoperceiveaflowoflightwithinthevolumeofthisspace,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 relatedcolours, that is to say, colours are perceived to belong to surfaces or objects seen inrelationtoothercolours,andinFigure1.2,theyareshownasunrelatedcolours,meaningtheyareseeninisolationfromothercolours(Fairchild,2005).Asunrelatedcolours(greyisacolour), theyareperceived tocomprisenothingmore thanrectangularcolouredshapesonaplainwhitebackground,butwhentheyaresetintothecontextofFigure1.1,theyareperceived as solid elements in a three-dimensional scene that have recognisable objectattributes. It is this change in the way they are perceived that causes them to appeardifferently.

So what are the components of the surrounding scene that make this illusion soeffective?Askyourself,whyisthecylindricalobjectthere?Doesitcontributesomething?Infact,itisavitalcomponentoftheillusion.So,whatcolourisit?Obviously,green.Isituniformlygreen?Well,yesbutlookmorecarefullyattheimageoftheobjectandyouwill see that both its greenness and its lightness vary hugely. The image is far fromuniform,sohowdidyousupposetheobjecttobeuniformlygreen?Theansweristhatyouperceivedadistinctive lightingpatternsuperimposedover theuniformlygreenobject. InFigure 1.3, the area enclosed by the object outline is shown as uniformly green and itappearsasnothingmorethanaformlessblob.

Thesolid,three-dimensionalobjectperceivedinFigure1.1isobservedtobeinteractingwithadirectional flowof light,which causes ashadingpattern to be generated, andthis appears superimposed over the green object surface. Note also that the cylinderssurface is not perfectly matt, and there is just a hint of a highlight pattern due to aspecularcomponentofreflectionthatisapparentattheroundedrimofthecylinderstopedge.Theselightingpatternsinformyouabouttheobjectsattributes(Cuttle,2008).

Figure1.3Previouslythecylindricalobjectappearedtobeuniformlygreen.Nowitisuniformlygreen,butitdoesnot

looklikeacylinder.Thatisbecauseitisnowlackingthelightingpatternduetointeractionwiththeflowoflight.

Nowlookatthecheckerboardsurface.Againwehaveapatternduetothelighting,butinthiscaseitisashadowpattern,whichhasadifferentappearancefromtheshadingandhighlight patterns, but nonetheless is quite consistentwith our perception of the overallflowoflightwithinthevolumeofthespace.Itwillbeobvioustoyouthatiftwosurfaceshavethesamelightness(whichalsomeanstheyhavethesamereflectance)andoneoccurswithin theshadowpatternandoneoutside it, theywillhavedifferent luminancevalues.Thecreatorofthisbrilliantillusion,EdwardH.Adelson,ProfessorofVisionScienceattheMassachusettsInstituteofTechnology,hascarefullysetitupsothatsquaresAandBhavethe same luminance value,whichmeans of course, that their images on your retina areidentical.However,thefunctionofthevisualprocessistoprovideinformationtothevisualcortexofthebrain,andhereyourperceptualprocessistellingyouthat,althoughthesetwosquaresmatchforluminance,theycannothavethesamelightness.Theoneintheshadowmustbelighter,thatistosay,itmusthavehigherreflectance,thantheoneinfulllight.Youhold this innate understanding of lighting in your brain, and you cannot apply yourconsciousmindtooverruleit.

Inthisway,itcanbeseenthattheimagefocussedontotheretinaissimplyanopticalprojection of the visual scene that corresponds directly with the luminance andchromaticityvaluesoftheelementswithintheexternalscene.Sinceitsinception,thestudyoflightinghasconcentratedonthevisualprocessandhowilluminationmaybeappliedtoprovideforvisibility,laterdefinedintermsofvisualperformance,buttheroleofvisionistoservetheprocessofperception,andthisoccursnotattheretina,butinthevisualcortex

ofthebrain.Whatweperceiveisnotapatternofbrightnessandcolour,butagestalt,thisbeingapsychologicaltermthatdescribestheholisticentitythatenablesustorecogniseallthe forms and objects that make up our surroundings (Purves and Beau Lotto, 2003).Consciously,weareawareofthree-dimensionalspacesdefinedbysurfacesandcontainingobjects,butinordertomakethismuchsenseoftheflowofinformationarrivingthroughtheopticnerve,wehavetobesubconsciouslyawareofalightfieldthatfillsthevolumeofthe space.This ishowwemake senseof squaresAandB. Seen in thisway, it becomesobvious why attempts to analyse scenes in terms of luminance and chromaticity wereboundtoleadtofrustration.

Theroleofambientillumination

Formostofthetime,weliveinaworldofrelatedcolours.Wearesurroundedbysurfacesand objects which, providing the entire scene is adequately illuminated, our perceptualfaculties reliably recognise andmake us aware of, sometimes so thatwe can copewitheverydaylife,andsometimestoelevateoursensestohigherlevelsofappreciation,aswhenwe encounter artworks or beauties of nature. Recognition involves identifying objectattributes associatedwith all of the things thatmake up our surrounding environments,and our innate skill in doing this is truly impressive. Scientists working on artificialintelligencehavetriedtoprogramsupercomputerstoperforminthisway,butsofartheirbesteffortsfallfarshortofwhathumanperceptionachieveseverymomentthroughoutourwakinghours.

Provided that ambient illumination is sufficient, we are able to enter unfamiliarenvironments, orientate ourselves, and go about our business without hesitating toquestion the reliabilityof theperceptionswe formof the surroundingenvironment. It isclearthatsubstantialprocessinghastooccur,veryrapidly,betweentheretinalimageandformation of the perception of the environment. There is no good reason why ourperceptions of elements of the scene should show in-step correspondence with theirphotometriccharacteristics.Visualperceptionmaybethoughtofastheprocessofmakingsenseoftheflowofsensoryinputthroughtheopticnervetothebrain,wherethepurposeis to recognise surfaces and objects, rather than to record their images. Colours areperceived as related to object attributes, and effects of illumination are perceived aslightingpatternssuperimposedoverthem.AswerecognisedthecylinderinFigure1.1 tobe uniformly green with a superimposed shading pattern, so we also recognised theidenticalsquarestodifferinlightnessbecauseofthesuperimposedshadowpattern.

There will, however, be situations where we are confronted with elements seen inisolation from each other, and this is particularly likely to occur in conditions of lowambient illumination.Whenwefindourselvesconfrontedbydarksurroundings,relianceuponrelatedcoloursandidentificationofobjectattributesmaygivewaytoperceptionofunrelatedcolours,andwhenthisoccurs,ourperceptionsdonotdistinguishlightnessandilluminanceseparately,andluminancepatternsdominate.Thatistosay,theappearancesofindividual objects within the scene relate to their brightness and chromaticity values,ratherthanuponrecognitionoftheirintrinsicattributes.

Figures1.4and1.5showtwoviewsofthesamebuilding.InFigure1.4,weseeaviewofthismagnificentcathedralinitssetting,andwereadilyformasenseofitssubstantialmassandthematerialsfromwhichitisconstructed.Also,evenifwearenotconsciousofit,weperceivetheentirelightfieldthatgeneratesthisappearance.InFigure1.5,ourperceptionof this building is quite different. We have no notion of a natural light field, and thebuildingseemstofloat,unattachedtotheground.Itisrevealedbyaglowinglightpattern

thatdoesnotdistinguishbetweenmaterials,andactuallymakesthebuildingappearself-luminous.Thebuildingsappearanceisdominatedbybrightness,andobjectattributesarenotdiscernible.Thesetwoviewsshowclearlythedifferencebetweenrelatedcolours,inthedaylight view, and unrelated colours in the night-time view. They also give us dueappreciationoftherolethatlightingmayplayinbringingaboutfundamentaldifferencesinourperceptions.

Undernormaldaytimelighting, two-wayinteractionsoccurthatenableourperceptualprocesses tomake sense of the varied patterns of light and colour that are continuouslybeing focussed onto our retinas. Working in one direction, there is the process ofrecognisingobjectattributesthatarerevealedbythe lightingpatterns,whileat thesametime,andworkingintheoppositedirection,itistheappearanceoftheselightingpatternsthat provides for the viewers understanding of the light field that occupies the entirespace.

Figure1.4Theobjectattributesofthisbuildingareclearlyrecognisable,andtheambientilluminationprovidesamplyfor

allelementstoappearasrelatedcolours.(ChartresCathedral,France.)

Figure1.5Thesamebuilding,butavastlydifferentappearance.Lowambientilluminationprovidesadarkbackdrop

againstwhichthecathedralglowswithbrightness.Objectattributesareunrecognisableinthisexampleofunrelated

colours.

Perceptionasabasisforlightingdesign

Fromadesignpointofview,lightingpracticemaybeseentofallintotwobasiccategories.Ononehand,forilluminationconditionsrangingfromoutdoordaylighttoindoorlightingwheretheambientlevelissufficienttoavoidanyappearanceofgloom,weliveinaworldof related colours in which we distinguish readily between aspects of appearance thatrelate to the visible attributes of surfaces and objects, and aspects which relate to thelightingpatternsthatappearsuperimposeduponthem.

Ontheotherhand,inconditionsoflowambientillumination,wherewehaveasenseofdarknessorevengloom,whetherindoorsor,mostnotably,outdoorsatnight,wetypicallyexperience unrelated colours and this may lead to the appearances of objects andsurroundings dominated by brightness patterns that may offer no distinction betweenobjectlightnessandsurfaceilluminance.

Theimplicationsofthisdichotomyforlightingdesignareprofound.Outdoornight-timelighting practice, such as floodlighting and highway illumination, is based on creatingbrightnesspatterns thatmaybear littleornorelationship to surfaceorobjectproperties.Alternatively,forsituationswhereambientilluminationisatleastsufficienttomaintainanappearance of adequacy (apart from outdoor daylight, this may be taken to include allindoorspaceswheretheilluminationcomplieswithcurrentstandardsforgenerallightingpractice)wetakeinentirevisualscenesincludingobjectattributes,andinvolvinginstantrecognitionoffamiliarobjectsandscrutinyofunfamiliarorotherwiseinterestingobjects.The identification of object attributes may become a matter of keen interest, as whenadmiring an art object or seeking to detect a flaw in a manufactured product, and wedepend upon the lighting patterns to enable us to discriminate and to respond todifferencesofobjectattributes.

Betweenthesetwosetsofconditionsisarangeinwhichsomeuncertaintyprevails.Wehave, for example, all experienced tricks of the light that can occur at twilight, andgenerally, recommendations for good lighting practice aim to avoid such conditions.Perhaps surprisingly, it iswithin this range that lightingdesignersachieve someof theirmostspectaculardisplayeffects.By isolatingspecificobjects fromtheirbackgroundsandilluminating them from concealed light sources, lighting can be applied to alter theappearance of selected object attributes, such as making selected objects appear moretextured, or colourful, or glossy. All of this thinking will be developed in followingchapters.

Beforeweclosethischapter,askyourself,whydowecallFigure1.1anillusion?Ifthepage is evenly illuminated, squaresA andBwill have the same luminance and so theystimulate their corresponding areas of our retinas to the same level. The fact that theseequalstimulidonotcorrespondtoequalsensationsofbrightnessiscitedasanillusion.Thepointneedstobemadethatvisionservestheprocessofperception,andperceptionisnot

concerned with assessing or responding to luminance. Its role is to continually seek torecognise object attributes from the flow of data arriving from the eyes.When we areconfrontedwithFigure1.1inaconditionofadequateillumination,ourperceptionprocessperforms its task toperfection.A iscorrectly recognisedasadarkcheckerboardsquare,andBasalightsquare.RatherthanlabellingFigure1.1asanillusion,perhapsweshouldrefertoitasaninsightintotheworkingsofthevisualperceptionprocess.

However, therealpurposeforexaminingthis imagehasbeentoshowhowperceptiondependsuponandisinfluencedbythelightingpatternsthatobjectsandsurfacesgeneratethroughinteractionswiththeirsurroundinglightfields.Theselightingpatternsmayhavetheeffectsofrevealing,subduing,orenhancingselectedobjectattributes,anditisthroughcontroloflightfielddistributionsthatlightingdesignersinfluencepeoplesperceptionsofobject attributes. Skill in exercising this control, particularly for indoor lighting, is theessenceoflightingdesignandthecentralthemeofthisbook.

References

Cuttle,C.(2008).LightingbyDesign,SecondEdition.Oxford:ArchitecturalPress.

Fairchild,M.D.(2005).ColorAppearanceModels,SecondEdition.Chichester:Wiley.

Purves,D.andR.BeauLotto(2003).Whyweseewhatwedo:Anempiricaltheoryofvision.Sunderland,MA:SinauerAssociates.

2AmbientIllumination

Chaptersummary

Theperceptionofambientilluminationconcernswhetheraspaceappearstobebrightlylit,dimlylit,orsomethinginbetween.Atfirstthismightseemarathersuperficialobservationuntilweconsideralloftheassociationsthatwehavewithbrightlightanddimlight,atwhich point ambient illumination becomes a key lighting design concept. It provides abasis for planning lighting based on the perceived difference of illumination betweenadjacentareas,orspacesseeninsequenceaswhenpassingthroughabuilding.Athoughtexperiment is introducedwhich leads to theconclusionthatmeanroomsurfaceexitance(MRSE)providesausefulindicatorofambientillumination,whereMRSEisameasureofinter-reflectedlightfromsurroundingroomsurfaces,excludingdirectlightfromwindowsorluminaires.TheAmbientIlluminationspreadsheetfacilitatesapplicationofthisconcept.

Theamountoflight

An important decision in lighting design is, What appearance of overall brightness (ordimness) is this space to have?General lighting practice gives emphasis to the issue ofhowmuchlightmustbeprovidedtoenablepeopletoperformthevisualtasksassociatedwithwhateveractivityoccurswithinthespaceand,ofcourse,thismustalwaysbekeptinmind. In a banking hall, for example,we need to ensure that the counters are lit to anilluminance that is sufficient to enable the tellers to perform theirwork throughout theworking day without suffering strain. While that aspect of illumination must not beoverlooked, there is an overarching design decision to be made, which is whether theoverall appearanceof the space is tobe abright, lively and stimulating environment, orwhetheramoredimoverallappearanceiswanted.Theaimofadimappearancemaybetopresentasubdued,andperhapssombre,appearance,oralternatively,tocreateasettinginwhichilluminationcanbedirectedontoselectedtargetstopresenttheminhighcontrastrelative to their surroundings. Of course, the surroundings cannot be made too dim asilluminationmustalwaysbesufficientforsafemovement,butthereissubstantialscopeforadesignertochoosewhether,inaparticularsituation,theoverallimpressionistobeofabrightspace,orofadimspace,orofsomethinginbetween.Clearly,theimpressionsthatvisitorswouldformofthespacewillbesubstantiallyaffectedbythedesignersdecision.

This raises a question. Ifwe are not lighting a visual task plane for visibility, but areinstead illuminating a space for a certain appearance of overall brightness, how do wespecify the level of illumination that will achieve this objective? All around the world,lightingstandards,codes,andrecommendedpracticedocumentsspecifyilluminationlevelsfor various indoor activities in terms of illuminance (lux) and a uniformity factor. Ifsomeone states that This is a 400 lux installation, that means that illuminance valuesmeasuredonthehorizontalworkingplane,usuallyspecifiedasbeing700mmabovefloorlevelandextendingfromwalltowallwithinthespace,shouldaverageatleast400lux,andfurthermore,atnopointshouldilluminancedroptolessthan80percentofthataveragevalue.

Thereasonsforthisarehistorical.Itwasinthelatenineteenthcenturythatthepracticeof measuring illumination emerged, and for indoor lighting, the prime purpose was toenable working people to remain productive for the full duration of the working day,despite daylight fluctuations.While the recommended illuminance levels have increasedmorethantenfoldsincethosedays,themeasurementproceduresareessentiallyunchangedeven though light meters have undergone substantial development. The two specifiedmeasures, an average illuminance and the uniformity factor, are the means by whichlighting quantity is specified, andmore than that, they govern how people think aboutilluminationquantity.Perhapstheworstfeatureofthesespecificationsisthattheyhavetheeffectofinhibitingexplorationofdifferentwaysinwhichthelightmightbedistributedin

aspace,andhowlightingmaybeappliedtocreatealitappearancethatrelatestoaspaceand the objects it contains. For lighting designers, these aspects of appearance are all-important,andinfact,itmaybesaidthattheyformtheverybasisofwhatlightingdesignisallabout.Tobeobligedtoensurethatalllightingiscodecompliantisnothingshortofadenialtopursuethemostfundamentallightingdesignobjectives.

Athoughtexperiment

Weare going to conduct a thought experiment as a first step to exploring how lightingdoes more than simply make things visible, and in fact, we are going to explore howlightingaffectstheappearanceofeverythingwesee.Tostart,youneedtogetyourselfintoan experimentalmindset. The first requirement is to forget everything you know.Then,imagine an indoor spacewhere the sum total of ceiling,wall and floor areas add up to100m2,asshowninFigure2.1.

Then,intothisspaceisaddedaluminairethatemitsatotalaluminousflux,F,of5000lumens(Figure2.2).

How brightly litwill the space appear? Thismight seem to be a difficult question toanswer,whichisas itshouldbebecauseavitalpieceof informationis lacking.Until theroom surface reflectance values are specified, you have noway of knowing howmuchlightthereisinthisspace.

Figure2.1Tostartthethoughtexperiment,imaginearoomforwhichthesumofceiling,walls,andfloorareais100m2.

Figure2.2TotheroomisaddedaluminairewithatotalfluxoutputF=5000lumens.

Tokeeplifesimple,wewillspecifythatallroomsurfaceshaveareflectancevalue,rs,of0.5,thatistosay,50percentofincidentlumensareabsorbedand50percentarereflected(Figure2.3).Nowwecanworkouthowmanylumensthereareinthespace.

Howmuchlightdowehave?

addition totalInitialflux(F) 5000 5000Firstreflection 2500 7500Secondreflection 1250 8750Thirdreflectionandsoon 625 9375

Figure2.3Allroomsurfacesaregivenaneutralgreyfinishsothatrs=0.5.

All of the initial 5000 lumens from the luminaire are incident on room surfaces thatreflect 50 per cent back into the space, so the first reflection adds 2500 lm, bringing thetotalluminousfluxinthespaceupto7500lm.Thesereflectedlumensareagainincidenton room surfaces, and the second reflection adds another 1250 lumens to the total. Theprocess repeats, so that you could go on adding reflected components of the initial fluxuntil theybecome insignificantlysmall.Alternatively, theeffectofan infinitenumberofreflectionsisgivenbydividingtheinitialfluxby(1),sothat:

An interestingpointemergeshere.Wehavesurroundedthe luminairewithsurfaces thatreflect 50 per cent of the light back into the space, and this has doubled the number oflumens.Keepthispoint inmind.Nowwedividethetotal fluxbythetotalroomsurfaceareatogettheaverageroomsurfaceilluminance:

Atlastwehaveameasurewecanunderstand.Thiswouldbeenoughlightforustoseeour

wayaroundthespace,butnotenoughtomaketheroomappearbrightlylit.Letssupposethat we want a reasonably bright appearance. Well, we could fit a bigger lamp in theluminaire, butbeforewe take that easyoption, lets thinkabitmoreabout the effectofroom surface reflectance.Wehave seen that it canhave a quite surprising effect on theoverallamountoflightinthespace.

Whatwouldbetheeffectofincreasingrsto0.8,asshowninFigure2.4?Combiningtheexpressionsweusedbefore,itfollowsthatthemeanroomsurfaceilluminance:

Thisdeserves somecarefulattention.We increasedrs from0.5 to0.8,which isa60percentincrease,andthetotalfluxincreasedtwo-and-a-halftimes!Howcanthisbeso?Thinkaboutitthisway.Itisconventionaltorefertosurfacereflectancevalues,buttrythinkinginsteadofsurfaceabsorptancevalues,where=(1).Whatwehavedonehasbeentoreducersfrom0.5to0.2,andthatiswherethe2.5factorcomesfrom.

Figure2.4Roomsurfacereflectanceisincreasedsothatrs=0.8.

Asthisisathoughtexperiment,thinkaboutwhatwouldhappenifwecouldreducersto zero.Well, the lumenswould just keep bouncing around inside the room.When youswitchedontheluminaire,thetotalfluxwouldkeeponincreasing.Ifyoudidnotswitchoffintime,theroomprobablywouldexplode!Ifyoudidswitchoffintime,thelightlevelwouldremainconstant.Youcouldcomebackamonthlateranditwouldbeundiminished,untilyouopenthedoorandinaflashallthelumenspouroutandtheroomwouldbein

darkness.Thoughtexperimentsreallycanbefun.Nowthinkaboutgoingintheoppositedirection.

What would be the effect of reducing rs to zero? How brightly lit would the roomappear? The question is of course meaningless. The only thing visible would be theluminaire,asshowninFigure2.5.Ifyouweresufficientlyadventurous,youcouldfeelyourwayaroundtheroomandyoucouldusea lightmeter toconfirmthevalueof themeanroomsurfaceilluminance:

The meter would respond to those 50 lux, but your eye would not. Here is anotherimportantpoint.Thedirectfluxfromtheluminairehasnoeffectontheappearanceoftheroom. It is not until the flux has undergone at least one reflection that it makes anycontribution towards our impression of howbrightly, or dimly, lit the roomappears. Tohaveausefulmeasureofhowtheambientilluminationaffectstheappearanceofaroom,weneedtoignoredirectlightandtakeaccountonlyofreflectedlight.

Figure2.5Roomsurfacereflectanceisreducedtozero,sors=0.

Letsthinknowaboutageneralexpressionforambientilluminationasitmayaffectourimpressionofthebrightnessofanenclosedspace.Theluminaireistobeignored,andsoinFigure2.6,itisshownblack.Admittedly,ablackluminaireemitting5000lmisrathermoredemanding of the imagination, but bear with the idea. To take account of only light

reflected fromroomsurfaces,weneedanexpression formeanroomsurfaceexitance,MRSE,whereexitanceexpressestheaveragedensityofluminousfluxexiting,oremergingfrom,asurfaceinlumenspersquaremetre,lm/m2.

Figure2.6Thefinalstageofthethoughtexperiment.AblackluminaireemitsFlminaroomofareaAanduniform

surfacereflectance,andmeanroomsurfaceexitance,MRSE,ispredictablefromFormulae2.1and2.2.

TheupperlineofFormula2.2isthefirstreflectedfluxFRF,whichistheinitialfluxafterit has undergone its first reflection. This is the energy that initiates the inter-reflectionprocess that makes the spaces we live in luminous. More descriptively, it is sometimesreferredtoasthefirstbouncelumens.

The bottom line is the room absorption, A. One square metre of perfectly blacksurfacewouldcomprise1.0m2ofroomabsorption;alternatively,itmaycomprise2.0m2ofamaterialforwhich=0.5,oragain,4.0m2if=0.25.Itisafactthatwhenyouwalkintoaroom,theambientilluminationreducesbecauseyouhaveincreasedtheroomabsorption.Youcouldminimisethateffectbywearingwhiteclothing,butthatisunlikelytocatchonamonglightingdesigners.Myownobservationisthatiflightingdesignerscanbesaidtohaveauniform,itisblack.Itseemsweaspiretobeperfectlightabsorbers!

TheMRSEconcept

Ofcourse,realroomsdonothaveuniformreflectancevalues,butthiscanbecopedwithwithoutunduecomplication.

OnthetoplineofFormula2.1,FistheFirstReflectedFlux,FRF,whichisthesumoffirstbounce lumens fromallof the roomsurfaces, suchasceiling,walls,partitionsandanyothersubstantialobjectsintheroom.Itisobtainedbysummingtheproductsof:

directilluminanceofeachsurfaceEs(d)surfaceareaAssurfacereflectances

So,inaroomhavingnsurfaceelements:

On the bottom line of Formula 2.1, A(1 ) is the RoomAbsorption, indicated by thesymbolA,anditisameasureoftheroomscapacitytoabsorblight.Asitisconventionaltodescribesurfacesintermsofreflectanceratherthanabsorptance;

Thegeneralexpressionformeanroomsurfaceexitance,Formula2.2,maybesummarisedas:

Themeanroomsurfaceexitanceequalsthefirstbouncelumensdividedbytheroomabsorption.

MRSEhasthreevaluableuses:

1TheMRSEvalueprovidesanindicationoftheperceivedbrightnessordimnessofambientillumination.Table2.1givesanapproximateguideforthetwodecadesof ambient illumination that cover the range of indoor general lighting practice.Thesevaluesarebasedonvariousstudiesconductedbytheauthorandreportedbyother researchers, and it should be noted that ambient illumination relates to aperceived effect, while MRSE is a measurable illumination quantity, likeilluminance,butnottobeconfusedwithworkingplaneilluminance.

Table2.1Perceivedbrightnessordimnessofambientillumination

Meanroomsurfaceexitance(MRSE,lm/m2)

Perceivedbrightnessordimnessofambientillumination

10 Lowestlevelforreasonablecolourdiscrimination

30 Dimappearance100 Lowestlevelforacceptablybrightappearance300 Brightappearance1000 Distinctlybrightappearance

2TheMRSEratioforadjacentspacesprovidesanindexoftheperceiveddifferenceofillumination.Table2.2givesanapproximateguideforthisperceiveddifferenceas one moves from space to space within a building, or to the appearance ofdifferently

Table2.2Perceiveddifferencesofexitanceorilluminance

Exitanceorilluminanceratio Perceiveddifference

1.5:1 Noticeable3:1 Distinct10:1 Strong40:1 Emphatic

illuminatedsurfacesorobjectswithinaspace.Thereismoreaboutthisperceiveddifferenceeffectinthefollowingchapter.

3Itmayprovideanacceptablemeasureofthetotalindirectilluminancereceivedbyan object or surface within the space, so that for a surface S, the total surfaceilluminancemaybeapproximatelyestimatedbytheformula:

where Es(d) is the direct illuminance of surface S. Procedures for predicting directilluminationareexplainedinChapter6.

BeforeweexaminehowMRSEmaybeappliedinthedesignprocess,Iamconsciousthatsomereadersmaybefindingtheexitancetermunfamiliar,asitofteniscustomarytoreferto illuminance as themetric for incident light, and luminance for reflected light. To seewhere exitance fits in, take a step back. Illuminance is a simple concept. It refers to thedensityof luminous flux incidenton a surface, either at apoint or over an area, in lux,where1luxequals1lumenpersquaremetre(lm/m2).Exitanceisalsoasimpleconcept.Itrefers to thedensityof fluxexiting,or emerging from,a surface in lm/m2. (It shouldbenotedthattheluxunitisdefinedastheunitofilluminance,andsoshouldnotbeusedforexitance.Actually,keepingtheseunitsdistinctforincidentandexitingfluxhelpstoavoidconfusion.) Now consider luminance. This is not a simple concept. As simply as I canexpressit,itistheluminousfluxduetoasmallelementinagivendirection,relativetothe

area of the element projected in that direction and the solid angle subtending the flux,measured in candelas per squaremetre (cd/m2). It needs to be recognised that there aretimeswhenit isnecessarytousethe luminancemetric,asforvisualtaskanalysiswherethecontrastofthecriticaldetailhastobedefined,buttorefertotheaverageluminanceofawalloraceilingreallyismeaninglesswithoutadefinedviewpoint.Afterall,whatistheaverage projected area of one of these elements?Readerswho are not familiarwith theexitancetermarestronglyadvisedtomakethemselvesacquaintedwithit.Notonlyisitamuchmoresimpleconceptthanluminance,butwhenweareconcernedhowilluminationaffects the appearance of room surfaces, it is the correct term to use. Seen in thisway,MRSEisthemeasureoftheoveralldensityofinter-reflectedlightwithinthevolumeofanenclosedspace.

Applyingtheambientilluminationconceptindesign

Roomsurfacereflectancesaresoinfluentialuponboththeappearanceofindoorspacesandthe distribution of illumination within them that, in an ideal world, lighting designerswould take control of them. The reality is that generally someone elsewillmake thosedecisions,butlightingdesignersmustpersistinmakingthesedecisionmakersawareoftheinfluence they exert over ambient illumination and the overall appearance of theilluminatedspace.

The creativityof a lightingdesigner is largelydeterminedby the ability toperceive aspaceand itsobjects in light, and aswehave seen, the perceived light is reflected (notdirect)light.Aroominwhichhighreflectancesurfacesfaceotherhighreflectancesurfacesisoneinwhichinter-reflectedfluxpersists,anditisthisinter-reflectedfluxthatprovidesforoursenseofhowbrightlyordimlylitthespaceappears.

To initiate this inter-reflected flux,direct light,which travels fromsource to receivingsurfacewithoutvisibleeffect,has tobeapplied.Theessential skillofa lightingdesignermay be seen as the ability to devise an invisible distribution of direct flux that willgenerateanenvisageddistributionofreflectedflux.

Large, high reflectance surfaces enable the direct light to be applied efficiently andunobtrusively, andwhere highMRSE levels are to be provided, the availability of large,light-colouredsurfacesthatcanbewashedwithlightbecomesanimportantconsiderationforbothappearanceandenergyefficiency.Conversely,wheretheaimistokeepMRSElow,perhaps to provide high contrasts for display lighting, dark-coloured room surfacesreinforce the visual effect by absorbing both spill light (display lighting thatmisses thedisplay)andfirstbouncelumensreflectedfromthedisplays.

Figure2.7ReflectanceplottedagainstMunsellValue,whereasurfaceofMV0wouldbeassessedasaperfectblackand

MV10asaperfectwhite.PerceptuallyMV5ismid-waybetweentheseextremesandmightbeexpectedtohavea

reflectanceof0.5,butactually,ithasareflectanceofapproximately0.2.

Estimating surface reflectance values is not straightforward. TheMunsell Value (MV)scale orders surface colourson a 10-step scale according to lightness assessments,whereMV0appearstobeaperfectblack,andMV10aperfectwhite.Unlikereflectance,lightnessisasubjectivescale,andwhileitrelatestoreflectance,therelationshipisfarfromlinear.Avalue of MV5 is perceptually mid-way between black and white and so it might beexpected to have a reflectance around 0.5, but as Figure 2.7 shows, its actual value isapproximately0.2.Furthermore,itcanbeseenthatasurfacehavingareflectanceof0.5hasaMVofapproximately7.5,andthatputsitperceptuallythree-quartersofthewaytowardsperfect white. The practical implication of this pronounced non-linearity is thatinexperienced designers are inclined to substantially overestimate reflectance values. Areasonably reliable procedure is to fit an internally blackened tube over an illuminancemeterasshowninFigure2.8andtotaketworeadings,oneforthesurface,RSandoneforasheetofgoodqualitywhitepaperwhichhasbeenslidoverthesurface,RP.Itisreasonableto assume that the paper has a reflectance of 0.9, so that for a measure of surfacereflectance,S=0.9RS/RP.Patternedaswellasplainsurfacescanbedealtwithinthisway,but care needs to be taken to avoid specular reflections, particularly for glossy surfaces.Also, it shouldnotbe assumed that shiny surfaceshavehigh reflectance.These surfaces

simplyreflectwithoutdiffusion,sothatifthemeterisexposedtospecularreflection,whatisbeingmeasured isan imageofa lightsourcerather thantheoverallreflectionof lightfromthesurface.

Figure2.8Usinganinternallyblackenedtubemountedontoalightmetertoobtainameasurementofsurfacereflectance.

Twomeasurementsaremadewithoutmovingthemeter,oneofthesurfaceasshown,andacomparisonreadingwitha

sheetofwhitepaperinthemeasurementzone.

Theeffectsof this tendencytooverestimatereflectancevaluesarecompoundedbytheimpactofsurfacereflectancevaluesonMRSE.ItcanbeseenfromFormula2.1thatMRSEisproportionaltotheratioofroomsurfacereflectancetoabsorptance,/.Figure2.9plotsthevalueof this ratiorelative toreflectance,andagain itcanbeseen that the impactofroomsurfacereflectanceincreasesexponentiallywithreflectance,andcouldleadtogrosslyinflatedMRSEvaluesbeingpredictedwhere reflectancevalueshavebeenoverestimated.Wecanseeheretheeffectsofreflectancethatwereobservedinthethoughtexperiment,andwhile these effects are real, theywill not be realizedunless reflectance values havebeenaccuratelyassessed.

Theseconsiderationssuggestaninitialsequenceforapplyingtheseconcepts:

1. Decide upon the level of MRSE, taking account of design considerationsconcerning the perceived brightness or dimness of ambient illumination, andreferring toTable2.1 and the discussion in the section entitled The amount of

light.2. Calculatetheroomabsorption,A,referringtoFormula2.4.3. Determine the levelof first reflected flux, turningFormula2.2around toFRF=

MRSEA4. DetermineadistributionofdirectfluxtoprovidetheFRFvalue.Atthispoint,we

cometoacentraldesignissue:howtodistributethedirectflux,Fs(d),orinotherwords, how to choose the surfaces ontowhich fluxwill be directed.To explainthisissuewewillconsidertwocases.

Figure2.9Thevalueofthereflectance/absorptanceratioisproportionaltomeanroomsurfaceexitance,MRSE.Notehow

valuesincreaseexponentiallyathigherreflectancevalues.

Throughout thisbookwewillbemakinguseof spreadsheets to facilitate calculations,and their outputs are shown in the Boxes alongside the text. Readers are stronglyencouragedto followthe instructions fordownloadingthespreadsheetsso theycanthenfollow the applications described. Boxes 2.1 and 2.2 show two outputs of the AmbientIlluminationSpreadsheet,buttherealbenefitofdoingcalculationsinthiswayisnotthatitallhappenssoquicklyandeasily(althoughthatundoubtedlyisabenefit)butthat,onceasituation has been set up, the user is able to explore alternative solutions with instantfeedback.Readersarestronglyencouragedtofollowtheseexamples,andthentogobeyondthembyasking,Whatif?

Box2.1

AmbientIllumination

140117

Project Case1MRSE 150lm/m2

RoomDimensions

Length Width Height12 9 3m

RoomabsorptionA 160.2m2

Firstreflectedflux(FRF) 24030lmTotalluminaireflux(F) 32583lm

Key

Notes

Enterdataonlyincellsshowninredallotherdataarecalculatedautomatically.

DirectFlux(%)isthedirectfluxincidentonSasapercentageoftotalluminousflux.

Envisage an indoor space measuring 12m long, 9m wide, and 3m high. To keep lifesimple,wewill not get too specific about the function of this room. ForCase 1wewillwork on the basis that the aim is to provide a fairly bright overall appearance, whereeverythingappearsadequatelylitbutnoobjectsaretobeselectedforparticularattention,andwhatiscalledforisawell-diffused,overallillumination.Decisionshavebeenmadeforsurfacefinishes,andithasbeenagreedthatceilingreflectance,clg, istobe0.85,wall tohave a value of 0.5, and flr will be 0.25, and Box 2.1 shows the dimensions and thereflectancesenteredontheAmbientIlluminationSpreadsheet.

After giving due consideration to the points discussed in The amount of light, wedecideuponaMRSElevelof150lm/m2.Thisvalue isenteredonthespreadsheet,notingthatdataaretobeenteredonlyintocellsmarkedinred.Tofullyunderstandtheprocedure,thereaderisadvisedtocheckthecalculationonpaperusingtheaforementionedformulae.

TheFRFvalueshowninBox2.1isthenumberoflumensreflectedfromalloftheroomsurfacesrequiredtoprovidethemoderatelybrightoverallappearancethatwehavesetasour goal. Nowwe address the first really important design issue: how to distribute thedirect flux? The aim is to achieve a well-diffused illumination, and to do this withoutcreatingdistinctlybrightzonessuggestsalightinginstallationthatdistributesilluminationevenly over large surfaces. The only remaining red values are in the Direct Flux (%)column, and this is the column where the designer experiments with direct fluxdistributions.Twovalueshavebeenentered:15percentoftotalluminaireoutputistobedirectedonto thewalls,and10percentonto the floor.Asnoobjectshavebeenentered,that leaves 75 per cent onto the ceiling. The next column, Fs(d), shows the number oflumens of direct flux required on each room surface; next, the Es column shows theilluminance(includingindirectflux)oneachsurface;andinthefinalcolumn,theratiosofsurfaceilluminancetoambientilluminance,Es/MRSE.BelowthesecolumnsarethevaluesofA,FRFandthetotalluminousflux,F,tobeemittedbytheluminaires.

Ways of predicting luminaire layouts for direct light distributions are explained inChapter6,butbefore that, this spreadsheetgives thedesigneropportunity toexplore theimplicationsof fluxdistribution.To experience this, download theAmbient IlluminationspreadsheetandclicktheBox2.1 tag.Trychanging thewallsandfloor fluxpercentages,andifyoulike,youcanaddafewobjects,suchasfurnitureitems.Youwillseethateverytimeyouaddmoreroomabsorptionordirectmorefluxontosurfacesoflowerreflectance,upgoes the luminaire flux.Foroptimumenergyefficiency, set thewallsandfloordirectfluxpercentagestozerosothatthedirectceilingfluxbecomes100percent,andyouwillseetheluminairefluxdroptojustover28,000lm.ThiswouldbethemostenergyefficientsolutionforachievingtheMRSEtargetinthislocation,butwhenthishappens,thevalueofEs/MRSEclimbsto2.7,andthismaybeacauseforconcern.

If the aim is to achieve the ambient illumination without any surface appearingnoticeablymorestronglylitthananyothersurface,thenasindicatedinTable2.2,theaimshouldbetokeepvaluesofEs/MRSEbelow1.5.Avalueof2.7fortheceilingindicatesthatthissurfacewillappeardistinctlymorestronglylitthananyothersurfaceorobjectinthisspace,andinfact,forthecaseshowninBox2.1,wheresomefluxisdirectedontothewallsandfloor,theEs/MRSEvalueisonlyslightlyreducedto2.5,sotheappearanceofthedirectilluminationontotheceilingwouldcertainlybenoticeable,evenifnotdistinct.Wecouldtryadjustingthepercentagevaluesonthespreadsheettoachievealesspronouncedeffect,butwatchthevalueofthetotalluminaireflux,F.Asmoreluminairefluxisdirectedontolower reflectance surfaces, so the flux required to provide the MRSE value goes up. Itshouldnotpassnotice that this flies in the faceof conventionalpractice.Allaround theworld, lightingstandardsforilluminationsufficiencyforindooractivitiesarespecifiedintermsofilluminanceappliedontothehorizontalworkingplane,fromwhichitfollowsthatefficient lighting takes the form of a grid layout of luminaires that directs its outputdirectly onto that plane. While it is widely acknowledged that indirect ceiling lightinginstallationscanachievepleasanteffects,thewaythestandardsarespecifiedcausesthemtobeclassifiedasinefficient.Whenadesignerissatisfiedthatasatisfactorydistributionofdirect fluxhasbeenachieved,acopyof thespreadsheetwouldbesavedonto thedesignprojectfile.

NowturnattentiontoCase2,forwhichwehaveaquitedifferentaim.Again,wewillnotgettoospecificaboutthesituation,butthistimetheaimisthatafewselectedobjectsaretobepresentedfordisplay,andthesearetobecomethetargetsforthelightingwiththeintentionthattheywillcatchattentionbyappearingbrightlylitinadimsetting.TherevisedoutputfortheAmbientIlluminationspreadsheetisshowninBox2.2,anditshowsthatmostofthedirectfluxistobedirectedontothesetargets.Evenso,thisisaspacethatpeoplewouldneedtobeabletofindtheirwaythrough,soabackgroundofinkyblacknesswouldnotbeacceptable.Thisbringsusface-to-facewithatrickydesigndecision.Ononehand we aim to achieve a luminous environment that is dark enough to provide foreffectivedisplaycontrasts,whileontheotherhanditneedstobelightenoughforpeopletofindtheirwaythroughsafely,and,atleastasimportant,weneedtocreateanentrytothe space that people findwelcoming.We should keep inmind that in order to attractpeopletoenterthisdimspace,atleastpartofthedisplayedmaterialshouldbepositionedsothatitisvisibletosomeoneapproachingtheentrancetothespace.

AsshowninBox2.2,wehaveoptedforaMRSElevelof10lm/m2,andatthisstageweenterintodiscussionwiththedesignteam.Itisagreedthatbothclgandflraretobekeptdowntoalevelof0.15,althoughtoprovideaslightlylighterbackgroundtothedisplays,awall finishwith a reflectancevalueof 0.25 is chosen.Thedisplayedobjectshave a totalsurface area of 20m2 with an average reflectance of 0.35, but it would be unrealistic tosupposethatwewillbeabletodirect100percentoftheluminairefluxontothem.Ithasbeenassumed that therewillbe10percent spill light,halfof itonto thewallsandhalf

ontothe floor,andbasedonall these inputs, thespreadsheetshowsthatweneeda totalluminaire fluxof8690 lumens.That luminousflux,appropriatelydirected,willprovideadisplay illuminanceof401 lux,and, referringagain toTable2.2, thevisual effectwill beemphatic, as itwill provide a Es/MRSE value of 40. Note that in order to achieve thisdramatic effectwedidnot start by setting the target illuminance, but rather,we set theambientilluminanceandthendeterminedthefluxdistribution.Toprovideahigherleveloftarget illuminance would have the effect of raising the ambient illumination above thedesignvaluewithoutaddingtotheEs/MRSEratio.

Fromthese twocases itcanbeseen that inorder for lighting toexert itspotential forinfluencing the appearance of everythingwe see, control over room surface reflectancevaluesisasimportantasbeingabletocontroldirectfluxdistributions.Betweenthesetwoquiteextremecases,manyoptionsexistfordesignerstocontrolambientilluminationlevelto support chosen lighting design objectives. TheAmbient Illumination Spreadsheet is ausefultoolforachievingthiscontrol.

Box2.2

AmbientIlluminationSpreadsheet

140117

Project Case2MRSE 10lm/m2

RoomDimensions

Length Width Height12 9 3m

RoomabsorptionA 291.1m2

Firstreflectedflux(FRF) 2911lmTotalluminaireflux(F) 8690lm

Key

Notes

Enterdataonlyincellsshowninredallotherdataarecalculatedautomatically.DirectFlux(%)isthedirectfluxincidentonSasapercentageoftotalluminousflux.

3IlluminationHierarchies

Chaptersummary

Whereambientilluminationissufficientforilluminanceandlightness(whichisrelatedtoreflectance)tobeperceivedseparately,astypicallyoccursforconventionalindoorlightingpractice, lighting may be planned in terms of illuminance (rather than luminance)distributions. Local concentrations of illumination can be applied to direct attention, togiveemphasisandidentifyobjectsthatthedesignerdeemstobevisuallysignificant.Thenotion of ordered distributions of illumination leads to the concept of illuminationhierarchy,wherebyilluminationdistributionsarestructuredasaprincipalmeansbywhichthedesignermay express his or her design intentions. Suchdistributions are planned aschanging balances of direct and indirect illumination, and are achieved by specifyingtarget/ambient illuminance ratio (TAIR) values. The Illumination Hierarchyspreadsheetfacilitatesapplicationofthisconcept.

Orderedilluminationdistributions

Mostformsoflifeareattractedtowardslight,andhumansarenoexception.Phototropismistheprocessbywhichattentionisdrawntowardthebrightestpartofthefieldofview.Itcan be detrimental, aswhen a glare source creates a conflict between itself andwhat apersonwants tosee,and ingeneral lightingpracticemuchattention isgiventoavoidingsuch effects. However, for lighting designers it is a powerful tool, enabling us to drawattentiontowhatwewantpeopletonoticeandawayfromthingsofsecondaryortertiarysignificance.Anorderedilluminationdistributionistheunderpinningbasisforstructuringalightingdesignconcept.

Itisimportanttospendsometimelookingcarefullyathowourperceptionsofspaceandobjectsareinfluencedbyselectiveillumination.ItwasnotedinChapter1thatcoloursthatmake up an overall scene are generally perceived as related colours, and as long asillumination is sufficient to ensure photopic adaptation, we have no difficulty inrecognisingallthesurroundingsurfacesandobjectsthatmakeupourenvironments.Theprocess of recognising the multitude of things that may, at any time, comprise oursurroundingsfallswithinthetopicofperceptualpsychology,butwithoutgettinginvolvedin that fieldof learning it is sufficienthere toacknowledge that this recognitionprocessinvolves discriminating differences of object attributes such as lightness, hue andsaturation,fromwhichweformperceptionsofspaces,people,andobjects.Weachievethiswithout conscious effort throughout our waking hours over a very wide range ofadequatelightingconditions.Inthiscontext,theonsetofdimnessmaybethoughtofastheborderlineofreliablerecognitionofobjectattributes.

However,withorderedilluminationdistributionswecangobeyondsimplyprovidingforobjectrecognition.Retailerslongagoworkedoutthatifanobjectthatissmallinrelationtoitssurroundingsreceivesselectiveillumination,particularlywithoutthesourceoflightbeingevident,peoplesperceptionsofthatobjectsattributescanbesignificantlyaffected.Whether or not it appears more brightly lit, it is likely to appear more colourful, andperhapsmoretexturedormoreglossy,thanitwouldappearwithoutselectiveillumination.Lighting designers have at their disposal the means to establish hierarchies of visualsignificance in illuminatedscenes,andmeans forachieving this inanorderedmanner isthecontentofthischapter.

Illuminanceratios

Whenweplaceanattractiveobject,suchasavaseofflowers,besideawindowtocatchthe light,weare exploiting thepotential forapoolof local illumination to identify thisobjectashavingbeenselectedforspecialattention.Similarly,electriclightingcanprovidea planned gradation of illumination that expresses the designers concept of layers ofdifference. Hard-edged contrasts can give emphasis to such effects, but alternatively, adifferent but equally striking effect may be achieved by a build-up of illuminance thatleads the eye progressively towards the designers objective. High drama requires thatsurroundings are cast into gloom, but in architectural situations, safety requirementsgenerally require surroundings to remain visible, although perhaps distinctly dim, at alltimes. Planning such distributions is more than simply selecting a few objects forspotlighting. It involves devising an ordered distribution of lighting to achieve anilluminationhierarchy.

The concept of a structured illumination distributionwas pioneered by J.M.Waldram(1954).Working froma perspective sketch of the location, hewould assign an apparentbrightnessvaluetoeachsignificantelementoftheview,andthenhewouldconvertthosesubjective values into luminance values so that he could apply illumination engineeringprocedures to determine a suitable flux distribution. Waldrams notion of creating anordered brightness distribution related to luminance would seem to be valid for lowadaptation situations, such as occur in outdoor lighting, but not for situations wheresurfacelightnessisreadilyrecognised,suchasinadequatelyilluminatedindoorscenes.Ashas been noted, for these situations our perceptions distinguish illumination differencesmoreorlessindependentlyofsurfacereflectancevalues.

J.A.Lynes(1987)hasproposedadesignapproachbasedonWaldramsmethodwiththedifference that the designer develops a structured distribution of surface illuminancevalues. Lynes introduces his students to the topic through an exercise in perceiveddifferenceofillumination,andhissimpleprocedureisillustratedinFigure3.1.Hestandsinfrontofhisclasswithaspotlightshiningontoawhitescreen.Point0isthebrightestspot,andfromthispointanumberedscaleextendsacrossthescreen.Eachstudentcompletesascore card, and starts by indicating the scale value that, in his or her assessment,corresponds to the point along the scale atwhich a noticeable difference of brightnessoccurs.ThisisthestudentsNvalue,andwouldbefollowedbyaDvalueforadistinctdifference,anSvalueforastrongdifferenceandanEvalueforanemphaticdifference.Thecardsare thengathered, averagevalues calculatedandconsensusvalues forN,D,SandEaremarkedonthescreen.Afterthat,Lynesmeasurestheilluminancelevelateachpoint,fromwhichilluminanceratiosarecalculatedforeachperceiveddifference.

Theauthorhasconducted thisexercisewith studentsonnumerousoccasions.Perhapsthefirstsurpriseistofindhoweasyitistoobtainconsensus,andthesecondishowwell

theresultsarerepeatedyearafteryear.ThedatapresentedinTable2.2istypical,andwhilethissimpleproceduremaynotqualifyasgoodscience,itiswellworthgoingthroughtheprocedure.Itcallsforthoughtfulobservation,and,perhapssurprisingly,itprovidesusefulguidanceforlightingdesign.Notonlystudents,butanyoneinterestedindesigninglightingshouldgothroughtheprocessofmakingtheseilluminationdifferenceassessmentsatleastonceduringtheirlifetime.

Figure3.1Demonstrationset-upforgainingassessmentsofnoticeable,distinct,strongandemphaticillumination

differences.

WhereasinChapter2wediscussedhowinitialresponsestoaspacemaybeinfluencedbyambientillumination,nowweturnattentiontotheperceivedeffectsthatcanbecreatedbycontrollingthedistributionofilluminationwithinaspace.FromTable2.2itcanbeseenthatwheretheaimistoachieveadifferencethatissufficienttobenoticed,youcanforgetabout10or20percentdifferences.Unlessadifferenceofatleast1.5:1isprovided,peoplewill not notice the illumination to be anything different from uniform. To achievedifferences that are likely to be described as distinct or strong, it is necessary for thedesigner to be purposeful and deliberate in how they achieve such pronounced visualeffects. Illumination distributions will have to be carefully controlled and, preferably,surrounding reflectances kept low. An emphatic difference is quite difficult to achieveotherthaninatheatreorsimilarsetting,andaswasnotedtowardstheendofChapter2,raisingthetargetilluminationunavoidablyraisestheambientillumination.Wheretheaim

is to achieve high illuminance differences, target objects need to be small in relation totheir surrounding space, ormore specifically, to the roomabsorptionof the surroundingspace.

Wewill return to this lastpoint,butbeforemovingon, let itbe repeated thatmakingassessments of the appearance of illumination differences is a revealing exercise inobservation. Actually doing it, and measuring ones own assessments of perceiveddifference, is instructive. Then following up with observation and measurement in reallocationsisenormouslyvaluable.Themetertellsyounothingusefuluntilyouhaverelateditsreadingstoyourownexperience.ThedatainTable2.2istypical,butadesignerneedstobeabletovisualisetheseilluminanceratios.Itisbyhavinginmindtheperceivedeffectofilluminance ratios that adesigner is able to specifyvalues that reflectobservation-basedexperience.

Target/ambientilluminanceratios

While theperceivedadequacyof illumination (PAI) criterion is concernedwithensuringadequateinter-reflectedflux(MRSE)withinaspace,theilluminationhierarchycriterionisconcernedwithhowthedirectfluxfromtheluminairesmaybedistributedtocreateanordered pattern of illumination that supports selected lighting design objectives, whichmay range from directing attention to the functional activities of the space to creatingaesthetic or artistic effects. For all of this, we make use of the target/ambientilluminance ratio, TAIR, where target illuminance is the sum of direct and indirectcomponents, and TAIR relates target illuminance to the ambient illumination level. Thedesigner selects target surfaces anddesignatesvalues according to the level of perceiveddifference of illumination brightness to be achieved both between room surfaces, andbetweenobjectsandthesurroundingsagainstwhichtheyareseen.Asthepointhasbeenmade that illumination isnotvisibleuntil ithasundergone its first reflection, itmaybewondered why we are now dealing with incident target illumination, which comprisesboth direct and indirect illumination. The answer is that as both components undergoreflection at the same surface, it makes no difference whether we take the ratio of theincidentorreflectedvalues.

MRSEprovides themeasureofambient illuminationwithinaspace,andexceptwherethere are obvious reasons to the contrary, it is reasonable to assume that the incidentilluminationoneachtargetsurfacetgtwillbethesumofdirectilluminanceandMRSE,sothetotalilluminanceonatargetsurface:

andthetarget/ambientilluminanceratio:

The TAIR concept provides a basis for planning a distribution of direct flux from theluminaires that will achieve an envisioned illumination distribution within a space. Itfollowsthatforanychosentargetsurface,thedirectilluminance:

DesigninganilluminationhierarchyinvolvesdesignatingTAIRvaluesforselectedsurfacesor objects to signal noticeable, distinct, or strong perceived differences of illumination,againreferringbacktoTable2.2,andtherereallyisnolimittothesituationsforwhichthisproceduremaybeapplied.Adesignermaychoosetotargetasubstantialproportionofthetotal roomsurfacearea, andexamplesof thiswould include lightingamural coveringawholewall,oranarchitectural icon,ora libraryreadingarea,orperhaps, thehorizontalworkplaneofan industrialassemblyshop.Alternatively, thetargetareamaybeasingleobject that comprises a small proportion of the total surface area, such as a solitary

sculpture,ora featuredretaildisplay,or thepreacher inhispulpit;or itmaycompriseanumber of even smaller items, such as display of coins, or individually lit items ofglassware.Whateverthesituation,thedesignerfirstneedstodecideupontheMRSElevelto achieve the required ambient illumination for the space, and then to decideupon theTAIR for each target surface for the differences of illumination brightness. This enablesFormula3.3tobeappliedtodrawupthedistributionofdirecttargetilluminancevalues.

Thisputsthedesignerinthepositionofbeingabletodeterminethedistributionofdirectlighttobeappliedthroughoutthespaceinordertoachievetheenvisioneddistributionofreflected light. The total indirect flux provided by first reflections from all surfacesreceivingselectivetargetlighting:

Note that the suffix tgt indicates an individual target surface, and ts refers to all targetsurfaces within the space. This value of Fts(i) indicates the extent to which all of theselectivetargetlightingwillcontributetowardsthefirstreflectedfluxrequiredtoachievetheambient illuminationMRSE.Theusefulnessof this formulabecomesapparent in thefollowingsection.

Itmaybenotedinpassingthat,unlikeMRSE,TAIRisnotproposedasasuitablemetricfor lighting standards. TAIR is a tool that enables pursuit of chosen lighting designobjectives,whichmayrangefromverysimplethroughtodistinctlycomplexinnature,anditsapplicationinvolvesobjectivesthatarebeyondthescopeofstandards,whetheradvisoryormandatory.

Illuminationhierarchydesignprocedure

Withoutwishing togive the impression that creative lightingdesigncanbeachievedbyfollowingastep-by-stepprocedure,theconceptspreviouslydescribedimplyasequenceforlogical decisionmaking. The flowchart shown in Figure 3.2 should be referred towhilefollowingthisprocedure.

Figure3.2Flowchartforachievingmeanroomsurfaceexitance,MRSE,andtask/ambientillumination,TAIR,design

values.

1. For a design location, consider a level of MRSE that would provide for anappropriate appearance of overall brightness or dimness. Codes or standardsspecified in taskplane illuminance areunlikely tobehelpful. Should therebe apublishedMRSEvaluerelevanttothelocation,itprobablyrelatestotheperceivedadequacy of illumination (PAI) criterion and specifies the minimum value ofMRSE to be provided. Consider whether a higher level to give a brighterappearancewouldbeappropriate,referringtoTable2.1 forguidance,and takingintoaccounttheimmediatelypreviousbrightnessexperienceofapersonenteringthisspace.Considerwhetheritistoappearbrighterordimmerthanthepreviousspace,andifso,byhowmuch,thistimereferringtoTable2.2forguidance.Where

nominimumlevelsarespecified,designingforanappearanceofdimnessbecomesanoptionprovidingsafetyconcernsarekeptinmind.

2. Decide upon the design value ofMRSE, this being the overall density of inter-reflectedfluxtobeprovidedwithinthevolumeofthespace,andenterthisvalueinto the Illumination Hierarchy spreadsheet (see Box 3.1, and use your owndownloadedcopyofthespreadsheet).

3. Estimate theareaandreflectancevalue foreachsignificantsurfaceSwithin theroom,making sure to include any surfaces or objects that youmight decide tohighlight with selective lighting, and enter these onto the spreadsheet. Thespreadsheet calculates the room absorption value, A(rs), and the total firstreflectedflux,FRFrs,requiredtoprovidetheMRSEvalue.

4. Considertheilluminationhierarchythatthelightdistributionistocreateinthisspace. Think about which objects or surface areas you want to highlight withselectivelighting,andbyhowmuch.Youwillprovidedirectlightontothesetargetsurfaces, while surrounding areas will be lit mainly, or perhaps entirely, byreflectedlight.

5. EnteryourdesignvalueofTAIRforeachtargetarea,takingaccountofhowtheappearanceof theselectedobjectsorsurfaceswillbeaffectedbylocaliseddirectillumination. This listing of TAIR in Column 5 of the spreadsheet becomes therecordofyourilluminationhierarchyforthespace.

6. The spreadsheet completes the calculations, giving the first reflected flux to beprovidedbylightreflectedfromthetargets,FRFts,andthedifferencebetweenthisvalueandthetotalFRFrequiredtoprovidetheMRSEvalue,FRFrsFRFts.

Then:

If the first reflected flux from the targets is less than the total first reflected fluxrequired,thatistosay,ifFRFts FRFrs, the proposed balance of MRSE and TAIR values cannot beachieved in this situation. The reason is that if the direct target illuminance isapplied,thereflectedfluxwillraiseMRSEabovethedesignlevel,andreduceTAIRvaluesbelowthedesignlevels.Usuallythemosteffectiveremedialactionwillbetoreducethe total targetarea,suchasbyconcentratingtheobjects toreceivedirectlightintomorerestrictedareas.Otherwise,itwillbenecessarytoreduceeither,orboth,tsandrs,butunfortunately,lightingdesignersseldomhavemuchinfluenceoverreflectancevalues.Acompromisemaybeinevitable,butatleasttheoutcomewillnotcomeasanunwelcomesurprise.

Example:abankingpremises

Box3.1showsaworksheetfromtheIlluminationHierarchyspreadsheet,andagain,readersarestronglyrecommendedtoexperiencetheuseofthesedesigntools.Roomsurfacedatahavebeenenteredforabankingpremises, so takeamoment to familiariseyourselfwiththelocation.

A bright and business-like appearance is wanted, and a MRSE level of 200 lm/m2 isproposed.Thisvaluehasbeenentered,andaspreviously,datashowninredareinputbytheuserandallothervaluesare calculatedautomatically.Column4gives thecomputedroom absorption values, and the bottom line shows that 39,096 lumens of first reflectedfluxfromtheroomsurfacesisrequiredtoprovidetheMRSElevel.NextthedesignerentersaTAIRvalueforselectedtargetsurfaces.This is thevitalcomponentof thisstageof thedesign process, and Column 5 forms the statement of the designers initial intent forilluminationhierarchy.Atthebottomofthefinalcolumnitisshownthat20,899lmoftherequiredFRFwillbeprovidedfromthetargetsurfaces,sothatthedifferenceof18,197lmwillneedtobemadeupbyapplyingadditionaldirectlightontoroomsurfaces.

This is the information that thedesignerneeds todetermine thebalanceofdirectandindirectillumination.VariousoptionsforprovidingthedeficitFRFmaycometomind,butasimpleandefficientsolutionwouldbeuplighting.Therequireddirectceilingilluminanceis:

Thisdirect illuminanceaddedtotheMRSEvalueof200lm/m2wouldgiveatotalceilingilluminanceEclgof414lux,givingaTAIRvalueofjustovertwo.Table2.2 indicates thatthiswould correspond to a perceived difference thatwould appear somewhere betweennoticeableanddistinct,andsowouldcreateavisibleeffect thatmightcompetewith theplanned distribution of TAIR values. This effect could be reduced by applying lessilluminationontotheceilingandmakingupforthedeficiencybyaddingsomedirectlightontoothersurfaces,particularlythewalls.

Box3.1

IlluminationHierarchySpreadsheet

Date:140119

Symbols

It is at this point that the attraction of using the spreadsheet becomes evident. Bytreatingselectedroomsurfacesastargets,alternativestrategiesmaybereadilyexamined.Asthewallsurfaceshavelowerreflectancevaluesthantheceiling,itwilltakemoredirectlumenstobringtheFRFrsvalueup to therequired level,but the light-colouredblinds inwalls 3 and 4 could receive selective wallwashing, and this might create an attractiveappearance. However, the effectiveness of this solution would depend upon the staffpulling down the blinds during hours of darkness. It would be necessary to enquire

whether this could be relied upon, and after all, this is the way that lighting designhappens. It is part of the reason why no two designers would come up with identicalschemes.

Box3.2

IlluminationHierarchySpreadsheet

Date:140119

Symbols

Box3.2showsadesignproposal.TheTAIRvalues inColumn5havebeenadjustedtoprovidevariouslevelsofunnoticeable,noticeable,distinctandstrongperceiveddifferences,and by addingmore target surfaces in this way, the FRFrs FRFts difference has beenreducedtoanegligiblevalue.Thismeansthatthefirstreflectedfluxfromthetargetswillprovidetherequired200lm/m2ofmeanroomsurfaceexitance,andwiththeexceptionoftheblinds,thevisibleeffectofthisadditionalilluminationwillnotbebrightenoughtobenoticed. In thisway, theoriginaldesign intentwillbemaintained. It canbe seennotallsurfacesaretoreceivedirectlight.

Column6showsthedirectilluminancetobeprovidedontoeachtargetsurface.Allthatisleftnowistoapplysomestraightforwardilluminationengineering,andproceduresfordetermining luminaire layouts to distribute direct flux to achieve specific illuminancevaluesareexplainedinChapter6.

References

Lynes,J.A.(1987).Patternsoflightandshade.LightinginAustralia,7(4):1620.

Waldram, J.M. (1954). Studies in interior lighting. Transactions of the IlluminatingEngineeringSociety(London);19:95133.

4SpectralIlluminationDistributions

Chaptersummary

Variouswaysinwhichhumanperceptionofalitspaceisinfluencedbythespectralpowerdistribution(SPD)ofilluminationarereviewed.Distinctionismadebetweenassessmentoflightforvisibilityandforbrightness,andalternativeresponsefunctionsforindoorspacesare examined. The effects of SPD upon the perception of illumination colour (colourappearance) and colouredmaterials (colour rendering) are examined, alongwith variousproposalsforidentifyinghowbothSPDandilluminationlevelinfluencetheappearanceoflit spaces. These include perceived attributes of illumination, such as the whiteness,naturalness and colourfulness of illumination, as well as some non-visual effects. It isconcluded thatpeoplehavedifferentdaytimeandnight timeexpectationsandneeds forlighting.

Luminoussensitivityfunctions

Before 1924, the onlyway ofmeasuring lightwas tomake comparisonswith a familiarlightsource,whichledtometricssuchasthecandlepowerandthefootcandle,butinthatyear the CIE (International Commission on Illumination) introduced the V() luminoussensitivity function which defines the relative visual response, V, as a function of thewavelength of radiant power, , as shown in Figure 4.1. This was a significantbreakthrough that required innovative research, and it enabled luminous flux, F, to bedefinedintermsoflumensfromameasurementofspectralpowerdistribution:

where:

P()=spectralpower,inwatts,ofthesourceatthewavelengthV()=photopicluminousefficiencyfunctionvalueat=intervaloverwhichthevaluesofspectralpowerweremeasured

ItcanbeseenfromFigure4.1thatV()hasitsmaximumvalueof1.0at555nm,andsotheluminousefficiencyofradiantfluxatthiswavelengthisequaltothevalueoftheconstantin Formula 4.1, 683 lm/W.At 610nm,where the value of V() is approximately 0.5, theluminousefficiencyreducestohalfthatvalue.

SobydefiningtheV()function,theCIEmadeitpossiblefortheoutputofalightsourceto be specified in terms of the lumen,while at the same time enabling light itself to bedefinedintermsofradiantpowerwithinthewaveband380780nanometres(nm).Tothisday,lightingstandardsandrecommendedpracticedocuments,aswellasthecalibrationofall lightmeters, are based on V(), and in fact, it continues to be quite appropriate formeasuringilluminationinsituationswherephotopically-adaptedviewersarefixatinguponvisual tasks.Examplesrangefroma libraryreadingroomtoahospitaloperatingtheatre,and for these, as well as for most task-based applications in between, this luminoussensitivityfunctioncontinuestoserveuswell.Thereis,however,moretohumanresponsetolightthanthis,andfordesignerstobeabletoapplylightingknowinglyandeffectivelyintherangeofsituationsencounteredingeneral lightingpractice,wecouldbenefitfrommetricsthattakeaccountofawiderrangeofhumaninteractionswithradiantflux.

Figure4.1RelativesensitivityfunctionsforV(),andthethreeconetypes;long-,medium-andshort-wavelength;L(),

M()andS().ItcanbeseenhowcloselyV()representstheresponsesoftheLandMcones,andignorestheScone

response.

Formula4.1assumesahumanobserveroperatingwithin the rangeofphotopicvision,and thismeans that error is incurredwhenever V() is applied formesopic or scotopicconditions. Also, the researchers who established the V() function had their subjectsobservingaquitesmallluminouspatchthatsubtendedjust2degreesattheeye,sothatitwas illuminating only the foveal regions of the subjects retinas. The photoreceptors inthesecentralregionsareonlylong-andmedium-wavelengthresponsivecones,whichareoften (but inaccurately) referred to as the red and green cones, and their luminoussensitivity functions are shown in Figure4.1 as L() andM() respectively. It should benotedhowsimilarare the responsesof these twocones,particularlywhen it isborne inmindthatitisthedifferenceinresponseofthispairoftwoconetypesthatenablescolourdiscriminationontheredgreenaxis,andalso,howcloselysimilar theyaretoV().Theresponsesoftheshort-wavelength(blue)cones,shownastheS()function,aswellasalloftherods,aresimplynottakenintoaccountbytheV()function.

Foraphotopically-adaptedviewer,theS()functiondoesnotaffectacuityforafixatedtask, but it does affect assessments of the brightness of the surrounding field, and thisoccurs to an extent that changes with field luminance. The Bezold-Brcke hue shiftdescribes the effect of perceived colour differences on the blueyellow axis increasingrelative to those on the redgreen axis with increasing luminance, and this affectsbrightnessassessments.Reaetal.(2011)haveproposedaluminoussensitivityfunctionforbrightness:

wherethevalueofgisrelatedtofieldluminance.Inthisway,avariableallowancefortheresponseoftheshort-wavelengthconescanbeaddedtothelong-andmedium-wavelengthcones dominated V(), and Mark Rea has tentatively suggested that for the range ofluminousenvironmentsdiscussedinthisbook,forwhich10

Figure4.2TheVB3()spectralsensitivityofbrightnessfunctionfordaytimelightlevels,wherethecontributionoftheS

conesrelativetoV()ishigh(g=3).AfterRea(2013).

Figure4.3TheV()andV()relativeluminousefficiencyfunctionsrelatetophotopicandscotopicadaptation

respectively.

In this way, while the photopic luminous flux, F, for a given source is determined byapplicationofFormula4.1, itsscotopic lumens,F,couldbedeterminedbyapplicationofFormula4.3.Note the increasedvalueof the constant in this formula to reflect thehighsensitivityofdark-adaptedrods. It followsthat if thevalueofF/F,referredtoastheS/P

(scotopic/photopic) ratio, is high, then at low light levels,where the rods are active, thevisual response will be underrated. Sources rich at shorter wavelengths, such as metalhalidelamps,will,forthesamelumens,generatestrongervisualresponsesthanlampsrichatlongerwavelengths,suchassodiumlamps.

Someothervisualandnon-visualresponses

While it would seem quite straightforward that F should be used as the measure forluminous flux for scotopic conditions, these conditions are in fact so dim that nobodyactuallyprovidesilluminationtoachievethem.Lightingpracticeforoutdoorspaces,suchas car parks, roadways and airport runways, aims to provide conditions in themesopicrange,whichextends from0.001cd/m2 up to the lower limit of thephotopic range, at 3cd/m2.Within this substantialadaptation luminance range, spectral sensitivityundergoestransition between scotopic and photopic adaptation, andwherewe are concernedwithbrightnessassessments,thismeanstransitionbetweentheverydissimilarV()andVB3()functions,whichmakesaccurateassessmentofthelikelyvisualresponseproblematic(Rea,2013).Thisisarealissueforprovidingilluminationatoutdoorlightinglevels.

For indoor lighting at photopic levels, there are some different issues that concernresearchers.Ithasbeenestablishedthat,atthesameluminancelevels,pupilsizeissmallerforhigherS/Pillumination,andthisledtotheassumptionthatpupilsizeisdeterminedbythe response of the rod photoreceptors, even at photopic levels. Berman et al. (1993)conductedaseriesoflaboratorystudiesfortasksclosetothevisualthreshold(thepointatwhichthereisa50/50probabilityofaccuratedetection)andshowedthatperformancewasbetterforhigherS/Psources.Itmightseemoddthatreducedpupilsize,whichmustreducethe amount of light reaching the retina, should give increased performance, but theexplanationofferedwasthatreducingthelensaperturewouldimprovethequalityoftheretinalimage.Aswithacamera,smallerlensaperturegivesincreaseddepthoffield,whichisanadvantageforanyonewhoserefractivecorrectionis lessthanperfect. Italsooccursthat rays passing through the peripheral zones of the eyes lens tend to undergoaberrations,asthelensoftheeyeis,infact,ofnomorethanmoderateopticalquality,sothatreducingobserverspupilsizesis likelytocausethemtoexperienceimprovedimageresolution. It was claimed that these advantages would more than compensate for thereducedretinalilluminance.

ApplicationoftheS/Pfindingstolightingpracticehasrecentlybeenthesubjectofbothresearch and debate. The notion that visual performance could be maintained at lowerilluminancelevelsoffersopportunitiesforsignificantenergysavings,andthiscertainlyhasarousedinterest,butithasbeenpointedoutthatthehigherperformancedemonstratedforthreshold visual taskswould be unlikely to apply for themuchmore usual condition ofsuprathreshold tasks. General lighting practice aims to ensure that tasks are performedwithhigh ratesof accuracy,meaning that theyare tobe illuminated towell above theirthresholdlevels,sothatadvantagesthatmayoccurinanexperimentwheretheprobabilityof error is high probablywouldnot occur in practical situations (Boyce, 2003).A recentfield study byWei et al. (2014) of office workers found not only that any advantagesattributabletohighS/Psourcesweretoosmalltobeworthwhile,butalsothatthepeople

workinginthoseconditionsdislikedthehighS/Plighting.Amongtheresearchcommunitytherenowseems tobea lackof interest inpursuing this topic,but thathasnot stoppedsome unscrupulous suppliers from making claims that are exaggerated, and evendownright false, for high S/P lamps. It may be noted in passing that since the originalinvestigations, researchers have become aware that pupil size response ismore complexthan simply responding to the level of rod cells stimulation, and seems to involve therecentlydiscoveredipRGCresponse(seefollowingparagraph).

Humansexhibitvariousnon-visualresponsestolight,andthemost important,at leastfrom our point of view, is the circadian response, being the 24-hour cycle that weexperiencealongwithmostlivingthingsonthisplanet.Withtheonsetofcircadiannight,ahormonenamedmelatonin isreleasedfromthepinealgland into thebloodstream,andthis isassociatedwiththesleep/wakecycle that issaidtoberegulatedbyahypotheticalbiological clock that each one of us carries inside us. Researchers had noted that themelatonin response to light exposure displays a spectral sensitivity that does notmatchthatofanyof the retinalphotoreceptors,but itwasnotuntil 2002 that themysterywassolved.Theanswer lies in thecomplexpatternofconnectionswithintheretinathat linkthephotoreceptorstotheopticnerveforcommunicationtothebrain.Retinalganglioncellswereknowntoplaymajorrolesinthisprocess,butwhathadnotbeensuspectedwasthatsomeof thesecellsactuallycontainaphotopigment,whichhasbeennamedmelanopsin,andthelightresponseoftheseintrinsicallyphotosensitiveretinalganglioncells(ipRGCs)connectsnottothevisualcortex,buttotheendocrinegland,andontothepinealgland.Thepeaksensitivityofthesecellsduetothemelanopsinphotopigmentoccursat460nm,whichissubstantiallyshorterthanthepeakresponsesofanyoftheretinalphotocells.

Figure4.4ReasproposedVC()functionfortherelativecircadianresponse(AfterRea,2013).

Rea(2013)hasproposedaspectralsensitivityfunction,VC(), for thehumancircadianresponse,which is shown inFigure4.4.This is rather different from the other functionsdiscussedsofarinthatitisnottheresponseofacell,butofasystem.Alargepartoftheresponse is additive, meaning that light at these wavelengths will have the effect ofdispersing melatonin from the blood, and part is subadditive, which means that for abroad-spectrumsource,energyatthesewavelengthswillhaveanegativeeffect,butifthetotalsumforthewholespectrumisnegative,theresponseshouldbeassumedtobezero.

Taking account of this function calls for a quite different way of thinking about theimpac