colour as sensation in visual art and in science.pdf

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Colour as Sensation in Visual Art and in Science

Author(s): Jonathan C. Fish

Source: Leonardo, Vol. 14, No. 2, (Spring, 1981), pp. 89-98

Published by: The MIT Press

Stable URL: http://www.jstor.org/stable/1574399

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Leonardo,Vol. 14, No. 2, pp. 89-98. 1981.Printed n GreatBritain. 0024-094X/81/020089-10 02.00/0? PergamonPressLtd.

OLOUR S SENS TIONN VISU LR T N D N S IEN E

Jonathan C. Fish*Abstract-Contemporary colour science and colour artare, for the most part, on separatepathsin spite of strong reasons for unity and collaboration. It is argued that this schism is not basedon any intrinsic incompatibility, but on causes that are symptomatic of a disorientated culture.These causes are recent in origin and may be classified into those that are common to art or toscience, in general, and those that are peculiar to the subject of colour. Prominent in the latterclass is the failure by artists to appreciate the importance of distinguishing colour sensationsfrom the external causes of these sensations and the inevitable priority given in science to theobjective, ratherthan to the subjective, aspects of colour.It is suggested that material for a common philosophy, deeper than that of medium ortechnique, can befound in the study of colour sensation as part of the widerproblem of relatingprivate experience to external reality. An important example is provided by a discussion ofcontemporary uncertainties about the number and nature of colour attributes and of therelevance that this problem has for the visual arts. Some of the author's kinetic artworks andcolour demonstrationspertinent to the subject are discussed.

I.Since C. P. Snow first published his 'Two Cul-tures' lecture [1], the separation of science fromthe so-called humanities and the arts in westernindustrial countries has been much discussed. Inthe words of W. D. Wright, a pioneer of colourscience, 'We have in the subject of colour a quitespecific example of the two cultures, in whichartist and scientist speak a language the otherdoes not understand. We have a responsibility toput an end to this dichotomy, and if, as I nowbelieve, the scientist's concept of colour is incom-plete, then it is high time we developed a moreadequate philosophy of colour to which bothartist and scientist can subscribe' [2]. (For theview that science is not a culture in the traditionalsense see Ref. 3.)In the first half of the 19thcentury, painters werereceptive to science and also to the idea, ridiculedtoday, that there might be precise rules in visualart similar to the laws of nature [4-6]. In thatperiod, black and white photography was in-vented, which may have caused a shift in theemphasis in painting towards colour and moti-vated some artists to study its properties. Theinterests of the painters J. M. W. Turner, Eugene

*Painter,kineticartist, eacher,biochemist,21CollegeRoad,Cneltenham, Gloucestershire,GL53 7HY, England. (Re-ceived 7 Sept. 1979)

Delacroix and Georges Seurat in the colourscience of their day have been well documented.In spite of their pioneering attempts, a reactionensued leading to the wide-spread belief that thescientific knowledge of colour is useless, if notharmful, to visual artists. Thus this early attemptby painters to bridge the gap between art andscience was premature.

II.I believe that one of the reasons for the failureof artists generally to accept colour science asessential knowledge has been confusion betweencolour as it is subjectively experienced and the

external causes of that experience, namely mate-rial substances or the light reflected by them. J. S.Mill pointed out over a century ago that theconfusion is preserved in the everyday Englishlanguage. This is still true. Mill wrote: 'We have aname for the objects which produce in us a certainsensation: the word white. We have a name forthe quality of those objects, to which we ascribethe sensation: the name whiteness. But when wespeak of the sensation itself (as we have notoccasion to do this often except in our scientificspeculations), language, which adapts itself forthe most part only to the common uses of life, hasprovided us with no single-worded or immediatedesignation; we must employ a circumlocution,and say, the sensation of white or the sensation

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of whiteness;we must denominatethe sensationeitherfrom the object, or fromthe attribute,bywhich t is excited'[7].Physiologistshave shown that light reflectedfroma dabof paintandentering he retinaof theeye results in the dischargeof electricalsignalsthat aretransmittedo certainregionsof the braincausingthe occurrenceof colour sensations[8].These sensationsare one meaning of the termcolour. But since other meaningsare commonlygivento it, two of which arediscussedbelow, it issometimes helpful to refer to the sensation ofcolour aspsychologicalcolour.A clearexpositionof this approachs givenin Ref. 9.When light falls on a dab of colouredpaint,part of it is absorbed and the rest is eitherreflectedor transmitted.Usually the wavelengthcompositionof the light fallingupon a colouredmaterialandthatof the reflected or transmitted)lightare different. Becausethe lightfalling upona materialcanbe variedbothin compositionandintensity, it follows that the composition andintensityof the lightreflected(or transmitted)byit must also vary. However, the relationshipbe-tweenincident ightand reflected ortransmitted)light remains constantand can be used to char-acterizepaintsand other materials. shallrefer othis as the colour property.A tinyproportionof the reflected(or transmit-ted) light reaches the retinaof the eye, carryingcolourinformation o it. This light is termedthecolourstimulus.Unlike colourproperties,colourstimuli from an object vary not only with itsilluminationbut also with movementsof the eyeandhead.The termcolourstimulushardlyoccursin the literatureof art, but it should, for artistscan controlcolour sensationsonly by manipulat-ing colour stimuli. However, although colourperceptiondependsdirectlyon colourstimuli,itusuallycorrelateswith colourproperties.This isdue to the capacityof the brain to utilizespatialand temporal informationprovided by colourstimuliabout the illumination 10, 11], a subjectthat I shall not discuss here. Because of thiscorrelation and because of the way in whichcolour sensations are experienced,humans havecome to associatecolour with objects and mate-rials. Indeed, the biological function of colourvision is to provide informationabout colourpropertiesrather han colour stimuli.Unlike colour stimuli and colour properties,colour sensations are wholly private. For ex-ample, two persons looking at a paintingmayagreethat one areaof the canvas s paleceruleanblue; however, given the present knowledgeofneurophysiology, t is an untestableassumptionthat their colour sensationsare identical.It is notsurprisinghat until recentlymost colour scienceamountedto the study of colour propertiesandstimuli.III.

Sincecoloursensationsareproducedbystimuli

and stimuli are commonly a productof colourpropertiesand illumination, it is very easy toconfuse them. Three types of confusion canoccur:(1) The pigment-stimuluserror-the incor-rect identificationsof the colour prop-erties of materialswith the propertiesofthe lightthattheyreflect(ortransmit).(2) The stimulus-sensation rror-the incor-rect identificationof colour stimuli withcolour sensations.Ewald Hering, one ofthe pioneers of colour science, wrote:'The historyof physics mightalso be de-scribedas a historyof struggleagainst heprejudicesoriginating romthis linguisticidentificationof sensory qualities withtheirphysicalcauses'[12].Thisdistinctionis important to artists, because coloursensationscan be evoked and controlledonlyby manipulating olourstimuli.(3) The pigment-sensationerror-the incor-rect identificationof colourpropertiesofmaterialswithcoloursensations.Painterscommonly refer to colour sensationsbythe names of pigments-rose madder,chromegreen, raw umber,etc. But theyfrequentlyexperiencesituations n whichspatialinteractionsbetween stimuli causethe coloursensationproducedby a parti-cular pigment to be unexpected.Colourconstancy, a psychological effect thattends to maintain recognition of, say,blackpaintin different lluminationsmaybe destroyedby the influenceof stimulusinteractions n human-madeobjects (seeSection IX) [13]. The mechanismof col-our vision is thus exposed to a situationfor which the biological evolution of vi-sion did not prepare t.IV.

The effect of these sources of confusionon thevisual arts is exemplifiedby the conceptual m-broglioconcerningprimary olours,a termthat isfrequently used without specifying whether itrefers to colour properties, colour stimuli orcolour sensations.From the time when JacquesChristopheLe Blon publishedhis ideas on print-ing in colour (1756) [5], it was widely believedthat any colour could be derived from red, blueandyellow.This madesenseonlywhenapplied ocolourproperties,however,since it was thoughtthatred,blue andyellowpigmentsweresufficientto mixanycolour,a beliefsharedbymanyartists,but not borne out on theirpalettes[4, 5]. In 1831thephysicistDavidBrewsterexpoundedhisbeliefthat red, blue and yellow were the primarycol-ours for mixing light as well, which was latershown to be untrue-an exampleof the pigment-stimuluserror.Influencedby Brewster, Turnerassumedthatred,blue andyellowwere the principal olours n

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Colouras Sensation nVisualArt and in Sciencesensationand used this triad as the colourstruc-ture for some of his paintings [4]. The samepigment-sensationerror appears in Piet Mon-drian'srestricteduse of these colours 90 yearslater. Turnerand Mondriansucceededsuperblyin their use of colour, despite their incorrectassumption.

Perhaps t wasthe realization hat 19th-centurycolour science had not adequatelydifferentiatedbetween stimuli and sensations that led to itsabandonment ollowingits seemingly prematureuse in Impressionismand Neo-Impressionism.The pseudo-scientifichypothesesof colour har-mony of the chemist Michel Eugene Chevreul(1839) and of the painterSeuratmay have donemore harm than good to the cause that theyespoused. Henri Matisse, departingfrom Neo-Impressionism,wrote:'Mychoice of coloursdoesnot rest on any scientifictheory; it is based onobservation,on sensitivity,on felt experiences.Inspiredby certainpages of Delacroix, an artistlike Signac is preoccupiedwith complementarycolours, and the theoreticalknowledgeof themwill lead him to use a certaintone in a certainplace. But I simplytryto putdown colourswhichrendermysensation'[14].In 1959NaumGabo expressedan opinionthatis probablywidely sharedamongstartiststoday.He wrote: 'All textbook theories about coloursare concerned with their chemical and physicalpropertiesandhave no relationwhatever o theiraestheticnature.'An artistcanbe absolved romthe necessityofknowingall that the scientist knows about col-ours. Except for some practical informationwhich it is necessaryfor him to have about thetreatmentof pigmentsandmaterials, he scientificknowledgeof their chemicaland physicalprop-erties is apt to confuse the artistmore than toenlightenhim in his art and to distracthim fromknowingwhat his experiencesof colour are'[15].The firstsentence above is untrue.PresumablyGabo had not read EwaldHering'sOutlinesof aTheory of the Light Sense [12] or David Katz'sWorldof Colour [10]. However, to the secondsentence quoted, it is embarrassingly ardertofind more than a partialrefutation,such as pre-sented above.It must be noted that Matisse was wrong toimplythat, if colour sciencewere to be useful, itwould play a role in the making of aestheticdecisions. Before artists can make colour deci-sions, they must have a conceptualframeworkwithwhichto thinkabout colourexperienceandalso a knowledgeof the stimulus conditionsbywhich this experience s produced.

V.While contemporary artists generally havemadelittle use of pertinentnew knowledgeaboutcolour[9], it should be pointedout that therearecertaindifficultiesn coloursciencethathinder ts

use in art. Forexample,effectivecomposition npainting nvolvescarefullydeterminedcolourdif-ferences-sometimes subtle modulations,some-times strongcontrasts.In each instancepainterschoosingcolourswill, perhapsunconsciously,askthemselves,'In how manydifferentwayscantwocolour sensations be alike or be different?'Sur-prisingly, hisquestion s still a subjectof debate,evenamongstcolour scientists.Since sensationsare subjectiveand priority sgiven to objective measurement, emphasis incolour science until recentlyhas been put on thestudy of stimuli rather than on the study ofsensations. This has resulted n the sophisticatedsubjectof colorimetry,whichprovidesa way ofdescribingcolour stimuli by combiningphysicalmeasurementwith the knowledge, obtained byresearch,of a typicalhumancolourresponse.Thismethod s commonlyapplied o anisolatedcolourstimulus,that is, one that is uniform andhas verydarksurroundings.For these conditionsand for normalcolour visionat daylight evels oflight intensity, it has been determinedthat onlythree variables of the stimulus are relevant to aspecificcolour sensation.These variablescan beexpressedin differentways, but the most perti-nent triad here are the three that describelightobtainedby mixingmonochromaticight (lightofa singlewavelength)with whitelight (light, suchas sunlight, of two or more wavelengthsthatproduces an achromatic(hueless) response) tomatchthe givencolourstimulus.Thus a stimuluscan be specifiedquantitativelyby the threevari-ablesdescribing he mixturethat matches t. Thevariablesare: (1) luminance-a measure of totalintensity of the stimulus (radiation) weightedwave-length by wavelength according to thevisual sensitivityof an internationallyacceptedstandardobserver; 2) dominantwavelength-thewavelengthof the monochromaticight requiredin mixturewithwhitelight to match the stimulusand(3) colorimetric urity-the ratio of the lumi-nance of the monochromatic ight to the lumi-nance of the mixture.These three variablesare psychophysical ari-ables, and the colourthey describe s here calledpsychophysicalcolour in order to distinguish tfrom colour sensation or psychologicalcolour.The reason for this is thatphysicalmeasurementsof the light are used to calculatethree numericalvalues.The link with a typicalcolourresponse smadebyweighting he measurements sing nter-nationallyacceptedcoefficients hat are obtainedfromcolourmatchingdata for a standardobser-ver. These weighted numericalvalues are thenused to calculate he threepsychological ariables[9].

VI.I have selected for discussion here threepsychophysicalvariables (luminance, dominantwavelengthand colorimetricpurity), which are

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JonathanC. Fish

z

Fig. 1. Cylindricaloordinatesor therepresentationf colourspace. Colourspecifiedby threevariabless represented y apoint in colourspace.sometimes used as approximate indicators ofthree particularpsychologicalvariables,or attri-butes, of colour sensation, respectively: (1)brightness, he attributethat varies mainlywithluminanceon a scale fromdazzling o verydim;(2) hue, the attribute that is suggested by thewordsred, blue, turquoise, purple, etc. (3) satu-ration, the attribute that seems to refer to theamountof hue componentrelativeto the total ofthe hue and the huelesscomponents n the coloursensation.Thus a perceivedredmaybe judgedtobe bright, rom ts intensity,and to havea redhuewith an orange tinge. It may be judged to berather ow in saturation,meaningthatquitea bitof white sensationappearsto be mixed with theorange-red,producinga pinkishcolour.These three attributesof psychologicalcolourare commonly describedas three continuouslyvarying quantities that can be represented ascoordinatesin a 3-dimensionalgraph, a colourspace, such that a point located in that spacedesignates a perceived colour. A colour spacegiven by cylindricalcoordinates is indicatedinFig. 1. Here hue is commonlygiven by anangularposition1 (muchas the hour of the dayis given bythe angularpositionof the hour-handon the faceof a clock). The hues are considered to bearrangedn a continuouslyvaryingmanner romviolet to red as they are in the continuouscolourspectrum,and the circleis completedby joiningred to violet by a continuous series of non-spectral hues (the bluish reds and purples).Brightnesss representedby a verticalscale 2 andsaturationby a radial scale 3, both increasingnthe direction indicatedby the arrows. Thus thepinkishcolour mentioned above would be rep-resented n colourspaceby a pointon a radius na directiondesignatinghue and positionedat alevel designatingbrightness.The point wouldbelocated on the radius,possiblyat some point at,say,40 or 50%of thewayfrom the verticalaxisofthe cylindricalcoordinate system (where 0%saturation s represented) o the extremityof the

radius 100%saturation).A muchusedrepresen-tation in cylindrical oordinatess Munsellcolourspace (Munsellcoloursystem)where the coordi-natesaregivenas follows:MunsellHue, 1; Mun-sell Value, 2; andMunsellChroma,3.Helen Thomas has described n Leonardoheruse of the psychophysicalOstwaldcolourspace nher paintings[16]. The Ostwald colour space iscylindrical,but it is boundedby a double conewhose axis s vertical.D. Nickersonhaspresentedin Leonardoa discussionof the OpticalSocietyofAmerica (OSA) uniform colour scale samples[17]. A colour space has been selected in whichcoloursof supplied amplesare located'accordingto a space lattice in which each sample is sur-roundedby 12 other samples, each at an equaldistance from the central sample', and equaldistancesrepresentperceptually qualcolour dis-tances.VII.There are difficulties hat arise in the use ofthree stimulus-orientedpsychophysical ariablesas a conceptualbasisfor the descriptionof coloursensations. One arisesbecause,whencoloursen-sations are scaled equally, the correspondingpsychophysicalariablesmust be spacedunequal-ly. (Thisproblemhas been partiallyovercome orthe purposesof measuring olourdifferences 18;see also 9, p. 83].) Another arises because,although otal luminance s principally ssociatedwith the sensation of brightness, luminance

changesalso involve shifts in hue (the Bezold-Briicke phenomenon) [19]. Similarly, whilstchanges n purityof the stimulusmainlyaffectthesensation of saturation,they also involve smallshifts n hue.More seriousdifficultiesareencounteredwhenit is desiredto describecoloursensationswhen auniformly olouredsurface nview is not isolated.Although three variables may be sufficienttodescribe an isolated stimulus, such a stimulusdoes not have the same appearanceas the col-ouredsurfaceswithwhichmostpainterswork.Toelicit such surfaceappearancest is necessary ointroducea surrounding timulus. When this isdone, dramaticchangesoccurin the colour sen-sations that are associatedwith the stimulus hatis surrounded (termed the target stimulus) [20,21]. This is most easilyexplainedby describingasimple experimentusingtwo slideprojectors.Letone projectorproject a small disc of colouredlight upon a white screen. This is the targetstimulus.Let the otherprojectorprojecta ringofwhite light in such a way that it surroundsandtouches but does not overlapthe targetstimulus(termed the surround). If, to start with, thesurround s very dim, then the targetis isolated,and its appearance an be variedonlyin the threeways that have been described.Moreover, thetargetlooks like a patch of light, and its colourdoes not appearto belong to the surfaceof the

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Colouras Sensationn VisualArtand in Sciencescreen. Now suppose that the luminance of thewhite surround s increasedto about half that ofthe target. The targetnow begins to look like abrilliantlycolouredsurface that is emittinglight(it givesthe illusionof beingfluorescent).As theluminanceof the surround s raisedfurther,thetarget becomes less brilliant and resembles anormalcolouredsurface.As the luminanceof thesurround s made higherthan that of the target,two new attributesof the coloursensationappearthatarenot present n isolated stimulibut that arenormallypresent in the colour sensations pro-ducedby colouredareasin paintings.Therefore,a stimulus-based oncept about the attributesofcolour sensation as they are experiencedby apersonviewinga colouredareain a paintingmustbe based not on threepsycho/physicalvariablesbut at leaston six, three for the stimulus associ-ated with the coloured area and three for thesurround.(In Leonardo, Ref. 22, a method hasbeen discussed or extending he luminance angeof coloursin a paintingby selectiveilluminationof parts of it. A striking3-dimensionalappear-ancemaybe producedby so doing.)In a fascinatingand provocativebook, Evanshas suggestedthat, except when the stimulus sisolatedagainsta darkbackground, hereare notthreebut five variablesof colour sensationrelatedto the six stimulusvariables[21]. In additiontothe attributes of brightness,hue and saturationalreadydescribed,there are two additionalvari-ables, lightnessandbrilliance.Both lightnessandbrilliance are related to the luminanceof thestimulusrelativeto the luminanceof its surround,but brilliance varies with dominantwavelengthwhereas lightness does not. The term lightnesscorrespondso the artists' ermstonalityandvalueof a surfacecolour. While the attributeof light-ness may be indicated by the psychophysicalluminance actor, the relationis not a linear one(SectionIX).Brightness in related (that is, non-isolated)colours is the impressionof illuminationand isrelated to the total luminanceof the stimulusplusthat of the surround.A piece of coal in strongsunlight ooks both darkandbright,whilst whitepaper in a dark room looks light but dim. Thisdistinctionwas made long ago (1911) by DavidKatz, who demonstrated hat the visual mechan-ism is able to separateinformationaboutillumi-nationfromthatabout the reflectingpropertiesofsurfaces 10].Partof the fascinationof painting sthe wayin whichthe visiblelightnessof pigmentsmay generate paradoxical llusionsof brightness.FaberBirren in an article in Leonardohas pre-sented illustrations howingseveral different llu-sions of illumination hat may be producedwiththe use of paint [23]. On the other hand, artistswho work with coloured light rather than pig-mentsknow that it is possibleto elicitthe illusionof pigmented urfaces romthe projectionof lighton the back of a translucentscreen. In fact, allsurface colours can be produced this way, as

indicatedabove. (A demonstration f this can begiven with the use of my kinetic light box de-scribed n SectionIX.)By itself the additionof lightnessas a variableof colouris not sufficient o invalidate he use ofthree variables o describesurfacecolours, f theyare assumed o be viewed under the same illumi-nation, for in this case brightness s irrelevant.The same cannotbe saidabout brillianceor, in amore limited sense, greyness (as indicated be-low). (I think the wordblacknesswould be moreappropriate.)One can readilybe convincedthatthis is an attributeof non-isolatedcolours,if onenotes that the specificcolour sensationsneutralgrey, ochre, brown, olive green, maroon andnavy blue are not elicited by isolated stimuli.Those who insiston the use of three attributes odescribepsychologicalcolourmustexplainthesedistinctive ensationsby the assumptionhattheyaresimplydarkeror less saturatedversionsof thecorrespondingcolours, white, yellow, orange,green,red andblue. Whetheror not brownreallylooks like a mixture of orange and black orwhether t is 'unique' s anotherquestion [24].One importantpoint made by Evans is thatlightnessand saturationare perceivedseparatelyfrom greyness. Another is that separatecolourattributesare not necessarily ndependentfromone another. In the past this has caused confu-sion. Forexample,the series of coloursextendingfrom blackthroughgreyto whitehasusuallybeenconceived as the variationof a single attribute.According o Evans,however,two attributesareconcerned hatvarysimultaneously.As the seriesprogresses from black to white the coloursbecome lighter (lightness increases), but sim-ultaneously they become less grey, or morebrilliant, Fig. 2, top).The evidence that lightness is separatefromgreyness (brilliance)is based on absorbingre-search to which justice cannot be done here.Briefly, Evans discoveredthat, when the lumi-nance of a veryhighpuritystimulus s decreasedfrom a high value and the luminanceof a whitesurrounds held constant,the relative uminance(the ratioof one to the other) at whichgreynessfirstappearsconstitutesa thresholdquantity hatvariesgreatlywith the dominantwavelength andhencethe hue) of thatstimulus.Lightness,on theother hand, varies only with the relative lumi-nance. To comprehendthis, compare in 'themind'seye' the sensationsevoked by viewingachromeyellow and an ultramarinepaintsample.Both sensations can be produced without anydiscernablegrey content and of equalbrilliance,yet, when matched, the blue is darkerthan theyellow. The sensation of greynessof a colour isnot the samesensationasits darkness.Evansusedthe word brilliance to refer to two mutuallyopposite attributes,greynessand fluorence, theapparent luorescenceof verybrilliant timuli.Unfortunately,Evans died before the publi-cation of hisbook and,sincemuchof the research

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JonathanC.Fish

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WH .rE YESS >< SAA.ACANEIS ORq7.jEA YMjIS

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Fig. 2. (Top) The simultaneous change of lightness and bril-liance in the black-grey-white series of colour sensations.(Bottom) Hering's schematic representation showing the pro-gressive change in theproportions of white and black sensationsexperienced in the black-grey-white series. [Modified fromRef. 12].that he had initiated needs independentevalu-ation and continuation,his interesting deas arenot yet universallyaccepted. More recently,R.G. W. Huntand his colleagueshave reintroducedhis (Hunt's)termcolourfulness25]to referto theapparentabsolute chromatic(hue) content of acolour as opposedto its achromaticblack-white)content [18]. Hunt's definitionsdistinguishattri-butes of colour appearance his term) that haveabsolute magnitude, such as colourfulness andbrightness, from those that have only relativemagnitude,such as saturation, ightnessandper-ceived chroma.

(possibly unconscious?)of stimulusconditions,which mostpersons maynot possess, is assumed.Does a light surface appear to reflect a higherfractionof light than a darksurface to painters?This method of definingattributesof perceivedcolourusingthe verbsperceive,appearandjudge(Hunt) mayworkbetter for some attributes, uchas brightness,but it does suggestan obstacle tounderstanding etweenthe objectiveandsubjec-tive approach.Those who are not concernedwithpsychophysicalmeasurementmay also feel thatthe representationof hue in colourspace as if itwere a dimensionalattributewithangularmagni-tude fails to describe the associatedqualitativechangesof subjectivehue. Even morepuzzling omost artists must be an hypothesis of coloursensation that describes the positive colour ex-perience black as a colour with zero lightness,zero saturationor colourfulnessandzero hue.The mostconvincingalternative ramework orthe descriptionof colour sensationwassuggestedbyEwaldHeringover 100yearsago [12].As earlyas 1865, H. Aubert had stated that all coloursensationscould be conceivedof as if they weremixturesof, or related to, six fundamental en-sations[12].Thesepsychologicalprimary olours(commonly termed unique colours) are thosereferred o by the wordsblack, white, red,green,blue andyellow.Theycan be imaginedasformingthe verticesof an octahedronof which the linejoining two vertices represents the possiblepsychological (visual) mixturesbetween the twocorrespondingprimaries (Fig. 3, left) (Hofler1897, see ref. 5). Hering developed the idea ofthe six primarycolours in what he described asthe Natural ColourSystem(N.C.S.), havingde-claredhis intention to classifycolour sensationswithcompletedisregard f stimuliandproperties.He pointedout that the black-grey-white achro-matic) series should be regardednot as a singlemagnitude hange,but as a progressive hange nproportionof two distinct and equally powerfulsensations. As any grey approaches black, itappearsmoreblackish,as it approacheswhite, itappearsmore whitish(Fig. 2, bottom).

VIII.The varietyof waysin whichcolour sensationsare either described or defined in the scientificliteratureon colour is confusing or artists.Whileit is most unlikely that a colour scientist willconfusestimuliwithsensations, n myopinion t ispossiblethatthe convenienceof the colour-spaceconceptfor specifyingmathematicallyhe stimu-lus correlates of sensation may prejudice thedefinitionand choice of sensation attributes.Forexample,in the C.I.E. (1970)International ight-ing Vocabulary, ightness s defined as 'that attri-buteaccordingo which anareaappears o reflect

diffuselyor transmita greateror smaller ractionof incident light' [26]. By the use of the verbappear the task of defining lightness wholly intermsof itscorrelates s avoided,but a knowledgeFig. 3. (Left) Octahedral arrangement for visualising the mix-ture of unique colours proposed by the author. (Right) TheSwedish N. C.S. Colour Space afterA. Hdrd [Ref. 33].

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ColourasSensation n VisualArt and in Science

Auniquehueora mixture funiquehues

Constantblackcontent

UniqueblackFig. 4. A colour specified by a point (Q) in a N.C.S. constanthue triangle.

In the sameway, changesof saturatedhue canbe thought of as changes in proportionof twouniquehues butnot in anycombination.Red andgreen cannotbe combinedtogethernor can blueand yellow. Heringcalled these two pairs oppo-nent colours.The four possiblehue ranges(red-yellow, yellow-green, green-blue and blue-red)can be imaginedto form the sides of a squarewhose corners are occupiedby the four uniquehues red, yellow, green and blue, althoughHer-ing himself preferred to position these rangesalong the four quadrantsof the hue circle. Theadvantageof the circle s thatit makesthe hues onthe perimeter equidistant rom the centre wherean achromatic colour can be represented.Thedisadvantageof this circulararrangements thatthere is not a geometrical representationof thesharp change that occursat the positionsof theuniquehues.Researchhas confirmed hat hues can indeedbe describedconsistentlywith four colour names[19, 27]. Further,Hering described how coloursensations of a single hue may appear to be'veiled'with black(thatis, look blackish), veiled'withwhiteor withanymixtureof black andwhite(grey). For a givenhue, this meansthat a colourcan be specified by a point Q on or within atriangle,the corers of whichrepresentthe un-veiled hue C, black S and white W (Fig. 4).Crossing the triangle in three directions aredashedlines of constant blackcontent, constantwhite content and constant hue content. Whenthe positionsof the dashed ines aremovedin thedirections indicated by arrows, the contents ofblack,white and hue increases.Of the sixprimarycolour sensations, not more than four (black,white and two uniquehues) can be combined n ahomogenous colour sensation. Only relativeproportionsof these four primariesaremeaning-ful. Thus,the N.C.S. requires he specification fthree quantities and not more than two huequalities.

Tryggve Johansson (1905-1960) and laterothers, notablyA. Hard of the Swedish ColourCentreFoundation,developed Hering's sugges-tion into a quantitativemethod of colour sen-sation specification [28]. The Swedish N.C.S.colourspace (Fig. 3, right) and the new atlas ofcolour based on it embody much importantre-search nto the subjectivescalingof colour sensa-tions andareof real value to artists.However,theSwedish N.C.S. also involves the colour spaceconcept, a concept that, as far as I am aware,Heringdid not employ.It has been shown that inmanyrespects he SwedishN.C.S. colourspaceissimilarto the much-used Munsell colour space[29]. R. W. G. Hunt has concluded that 'for agiven set of viewing conditions,uniform colourspaces are useful for systematizing hose attri-butes that are independentof the absolutelumi-nance level' [18].So, wasEvansmistaken n his conviction hat atleast four variables are necessary to describesurfacecolours underconstant llumination? donot thinkso. Across the pages containingcolourchips of constant hue in the Munsell Book ofColour can be seen sloping lines of constantbrilliance,whichis a variablenot representednMunsellnotation. Similarly, he SwedishN.C.S.colourspaceaccommodatesbutdoes not describethe variable lightness. Accordingto Hard andSivik[30],'Inthe N.C.S., lines for constantvisuallightnessare straightand convergeat a point Poutside the N.C.S. color triangle,the positionofP beingdependenton hue'. In my opinion,thesetwo examplesshow that coloursensationcannotbe describedby one set of three variablesand,hence, that the colour space concept is mis-leading.In his originaldescriptionof the N.C.S., Her-ing pointed out that, in addition to those colourchangesthat are attributable o varyingpropor-tions of the primaries,colourscan be eitherlightor dark [12]. He used the word brightness n acontextthat makes it equivalentto Evans'light-ness. He stated that primarysensationshave anintrinsic ightness (brightness)that is invariant.White, yellow and red are intrinsically ighterthan black, blue and green. It is, therefore, adependentvariable,since the lightnessof a com-poundsensation would be intermediatebetweenthe lightnessesof its constituentprimaries.Toillustratethis, I have drawn an N.C.S. constanthuetriangle nsideanimaginary rism he verticalaxis of which represents lightness (Fig. 5). Thetrianglescorresponding o different hues tilt atdifferent angles, although data given by Huntsuggestthatthey mightbe curvedrather hanflatplanes [18]. It may be true that three variablessufficeto specifycoloursensationsagainsta con-stantbackground,but it does not followthattheyare sufficient o describehem.Thisdistinction, oimportant o painting,is made too rarely.Evansmaintained that the four variables of surfacecolour are independantof each other, but his

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JonathanC. Fish

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Fig. 5. The representation of a N. C.S. constant hue trianglein aprism to include lightness.grounds for making this claim are not whollyconvincing to me, at least) [21].However, I suggest that Hering's originalhypothesis is in agreementwith Evans' centralcontentionthat at least four variablesare necess-ary to describe surface colours under constantillumination.If Evans was right, then much ofwhat had been written about colour as it isactually experiencedhas been confusingor mis-leading.It is unfortunate that early attempts to usecolour as a bridgebetween scienceandthe visualarts did not result in the unqualifiedbelief bypainters that colour science is relevant. Thestimulus-orientatedoncepts that were used forthoseearlyattemptswereinadequate.Sincethen,however, there has been much new knowledgeabout colour. With the revival of interest incolour sensation among scientists,it is time foranotherattemptto applycolourscience in paint-ing. A failure to do this would be a sign thatperhapsColin Blakemorewas justifiedwhen heremarked hat 'societycould collapse,becauseitno longer comprehends ts own cultural nherit-ance'[31].Figures6 and 7 (see colour plate) show twoteachingdevicesI have designedto demonstratesomeof the colourphenomenamentionedabove.In one device, Maxwell discs (steel discs withsectorspainted n two colours)are rotatedrapidlyby independentlyactivated electric motors (Fig.6). The colours are mixed (averaged)by the eyeandthe appearanceof surface exturedisappears.Magnetic apeon the back of each discpermits tseasy attachment o a motor shaft.A collection ofsets of discs of differentcolour pairs has beenaccumulatedor a varietyof demonstrations.Forexample, the rotatingblack and white sectoreddiscs in Fig. 6, bottom, demonstrate hat, whenthe luminancefactor (luminancein relation topurewhite)is increasedby ten equal stepsfrom ablack (4% pure white) to a white (88% purewhite), the lightness ncreases n stepsof decreas-ing magnitude.The colour adaptationbox (Fig. 7, see colourplate) is used to demonstratea stimuluscontrast

Fig. 6. Maxwell discs for demonstrating colour phenomena,metal discs, electric motors, 30 x 120 cm, 1970. The discs areshown at rest (top) and in rotation (bottom).phenomenondescribedby Rayona Scina (1847)[3] (I have modified he design slightly).A blackandwhitepainteddesignis arranged o that halfof it (showingblack squares)is seen throughasheetof bluePerspex,whilstthe otherhalf(show-ing larger black square bands) is seen by itsreflectionon the blue Perspex surface. In theexplanations,attention s confined o what is seenthroughand on the blue Perspexsheet.The stimulusreferring o the backgrounds amixtureof two stimuli:blue light, producedbydaylightpassing hrough he bluePerspex,reflect-ingfromthe whitebackground f the coveredhalfof the pictureandpassingbackthrough he bluePerspexagain, and white ight,producedby day-light reflectingfrom the backgroundof the un-covered half of the pictureto the blue Perspexwhere it is reflected.The backgrounds seen aslightblue.The stimulusreferring o the squares s a mix-ture of white with a traceof blue light;the bluelight component s reducedby absorptionby theblackpaint n the covered half of the picture.Thesquaresareseen asyellow.The stimulus referringto the black squarebands is a mixture of blue with a traceof whitelight; the white light component is reducedbyabsorptionby the black paint in the uncoveredhalf of the picture.The squarebands are seen asdarkblue.

Whyare the squaresyellow?The humanvisualsystem behaves in such a way that informationaboutcolour aspropertys retainedand utilized nspite of naturally occurring changes in thewavelengthcompositionof the illumination; ol-ours of surfaces in view are assignedmentallyafter compensationsare made for the colour ofthe illumination.Thus, in effect, a pictureshow-ing black squares surroundedby black squarebands is beingviewed in blue illumination.Sub-tractingblue from the blue and white stimulusmixture(background)eaves white andfromtheblue with the negligible trace of white (squarebands) leaves blue. Since white light (daylight)minus blue light normally produces yellow (thecomplementary olour), the white light with thenegligibletrace of blue (squares)leaves yellow

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ColourasSensationn VisualArt andin Science

Fig.8. Untitled,kineticartwork,a seriesof boxes in whichcolours are produced by additive mixture.Perspexscreens(15x 45cm), electronic ontrol ystem,1978.when the blue is subtracted. The yellow colouroccurs as the result of misappliedcompensationby the visual process; it does not cope withsurfacereflectionas a separatephenomenon,onethat is seldom of consequence to humans innature but frequently encountered in viewingobjects made by humans. Similarexplanationscan be applied to many colour illusionsexperi-enced in paintingandsculpture.Figures8 and 9 illustratemy interest n kineticart, a likelymediumfor the applicationof colourscience. Equipmentis shown for the displayofkinetic colour compositionsthat can be writtendown with the use of a notation or recordedontape. Figure8 shows eight (there can be more)colour mixingboxes in which red, green, blue-green and blue light are mixed and the resultdisplayed.The colour is controlled n each boxbyan electronicsystemthat can be operatedmanu-allywhilstcomposing.The coloursof a total of 36channels can be recorded simultaneouslyon achannelof an audio cassette player. Each dim-ming circuit is controlledby a single audio fre-quency that is separated from the mixture offrequencieson the tape by an electronic filter.Iam indebted to Robin Pyewell for technicalassistance with the electronics.Recently I haverealized that a more powerful method of pro-grammings availableusingmicroprocessors,withwhichI am now experimenting.Figure9 shows an earlierkineticdisplaypanelthat is viewed in moderate daylight.Black linesmade on a long band of draughting ilm that isdrawnby a rollerpastlight-sensitivewitches hatcontrol the illuminationof the panel. The paneldisplaysa 10x 10arrayof smalltranslucent irclesin a turquoisefield. Althoughthe colours of thecircles (green, blue or blue-green)are producedby projected light (a green and a blue electriclightis enclosed in a tube below each circle),theappearanceof the colour produced is almost

Fig.9. Kineticdisplaypanel. Movingdiscsof colourshavingtheappearance f surfacecoloursareproducedon translucentscreens.Perspex creen,draughtingilm, electric ights,electricmotor,120x 120cm. 1969.identical o that of a surfacecolour.Theturquoisefield consistsof a sheet of transparenturquoisePerspexbackedby plywoodpaintedwhite. Theambient ightreflected rom the turquoise ield isof high enoughluminance o producea targetandsurround onditiondiscussedabove.I have used the aboveartworksand devicesfordemonstration urposes n a courseon scienceforartists at the GloucestershireCollegeof Art andDesign. I shallbe pleasedto supplyto interestedpersonsmoredetailed nformation n theirdesignand operation. I wish to call attentionto otherdevices used in teachingcolour to art studentsdiscussedn an earlierarticle n Leonardo 32].

REFERENCES1.2.3.4.5.6.7.8.

C. P. Snow, The Two Culturesand a Second Look(Cambridge:CambridgeUniv. Press,1964).W. D. Wright, The Rays Are Not Coloured(Bristol,.ngland:AdamHilger,1967)Lecture2.P. Auger,Contemporary nthropocentrism: nScienceand TraditionalCultures,Leonardo13, 223(1980).J. Gage,Colour n TurnerLondon:StudioVista,1969).F. Birren,A Historyof Colour n Painting NewYork:Reinhold,1965).C. E. Gauss, The Aesthetic Theoriesof FrenchArtists(Baltimore: ohnsHopkinsUniv. Press,1949).J. S. Mill,A Systemof Logic(London:Longman,1843)Bk. I, Ch.3.D. Jameson and L. M. Hurvich, From ContrasttoAssimilation: n Art and in the Eye, Leonardo8, 125(1975).

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JonathanC.Fish9. G. A. Agoston,ColorTheoryandItsApplicationn ArtandDesign(NewYork:Springer-Verlag, 979).10. D. Katz,The Worldof Colour trans.byR. B. MacleodandC. W. Fox) (London:KeganPaul,Trench,Trubner,1935).11. J. Beck, SurfaceColourPerception Ithaca,NY: CornellUniv. Press,1972).12. E. Hering,Outlines f a Theory f theLightSense trans.by L. M. Hurvichand D. Jameson)(Cambridge,MA:

HarvardUniv.Press,1964).13. H. von Helmholtz, Treatiseon Physiological Optics(1866),J. P. C. Southall,ed. (NewYork:Dover, 1962).14. J. Flam,Matisse n Art (Oxford:Phaidon,1973).15. N. Gabo, Of Divers Arts (A. W. Mellon Lectures)(London:FaberandFaber,1962).16. H. Thomas,Applicationof the OstwaldColorSystem nMyPainting,Leonardo13, 11(1980)17. D. Nickerson,OpticalSocietyof America(OSA) Uni-form Color ScaleSamples,Leonardo12, 206(1979).18. R. W. G. Hunt, The Specification f ColourAppear-ance. I. Concepts and Terms, Color: ResearchandApplication , 55 (1977).19. R. M. BoyntonandJ. Gordon,Bezold-BriickeHueShiftMeasuredby Color-NamingTechnique,J. Opt. Soc.Am. 55, 654(1965).20. H. Wallach,BrightnessConstancyand the NatureofAchromaticColours,J. Exp.Psychol.38, 310(1948).21. R. M. Evans,ThePerception f Color(NewYork:JohnWiley, 1974).22. A. A. Mills,Intensity-Modulatedllumination f Paint-ingsandanUnexpected3-DimensionalEffect,Leonardo11,213(1978).23. F. Birren,ColorPerceptionn Art:Beyond heEyeinto

the Brain,Leonardo9, 105(1976).24. C. J. Bartleson,Brown,Color:Research&Application1, 181(1976).25. M. R. Pointer,Colourfulness: NewConcept, n Color77, Proc. ThirdCong.Int. ColourAssoc. (AIC), F. W.Billmeyer,Jr. andG. Wyszecki,eds. (Bristol,England:AdamHilger, 1978).26. InternationalLighting Vocabulary.PublicationNo. 17(Paris:Commissionnternationale e l'Eclairage.1970).27. R. M. Boynton, W. Schoferand M. E. Neum, Hue-Wavelength Relation Measured by Color-NamingMethod for Three RetinalLocations,Science146, 666(1964).28. A. Hard,QualitativeAttributesof ColorPerception, nColor69, Proc. FirstCong. Int. ColourAssoc. (AIC),(Gottingen:Musterschmidt, 970).29. D. B. JuddandD. Nickerson,RelationbetweenMunselland SwedishNaturalColorSystemScales,J. Opt.Soc.Am. 65, 85(1975).30. A. HardandL. Sivik,Distinctness f BorderlineRelatedto the NaturalColor System,in Color77, Proc. ThirdCong. Int. ColourAssoc. (AIC), F. W. Billmeyer,Jr.andG. Wyszecki,eds; (Bristol,England:AdamHilger,1978).31. C. Blackmore,Mechanics f Mind(B.B.C. Reith Lec-tures, 1976)(Cambridge:CambridgeUniv. Press,1977)Lecture4.32. M. L. Meixner. nstruction n LightandColor n Art atthe Iowa StateUniversity,Leonardo9, 52 (1976).33. A. Hard,The NaturalColourSystemand Its UniversalApplication n the Studyof EnvironmentalDesign, inColourfor Architecture,T. Porterand B. Mikellides(London:StudioVista,1976)p. 112.

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Top left: Jonathan Fish. Colour adaptation box, perspex, wood, 30 x 20 x 20 cm, 1970. (Fig. 7, cf. page 96)Top right: Pavel Kuznetsov. 'Kirghizian Woman',oil on canvas, 85 x 59 cm, 1919. (Fig. 3, cf. page 141)

Center: Paul Re. III-9: 'Front and Back', acrylic paint on Masonite 34.5 x 60.5 cm, 1975. (Photo: K. Cornyn,San Francisco, CA, U.S.A.) (Fig 7, cf. page 107)Bottom left: Roberto Donnini. 'Immagina in Progress', collage, paper, wood (mahogany), 65 x 135 cm, 1974 (unfinished).(Fig. 2, cf. page 122)

Bottom right: Tariffe Raslain. Untitled, acrylic on canvas. 114 x 162 cm, 1979. (Fig. 5, cf. page 132)













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