Theoryand practice of colourvision testing: A review Part 1

Brit. J. industr. Med., 1969, 26, 173-189

Theory and practice of colour vision testing:A review Part 1

R. LAKOWSKIVisual Laboratory, Department of Psychology, Edinburgh University

Lakowski, R. (1969). Brit. J. industr. Med., 26, 173-189. Theory and practice of colour visiontesting: A review Part L. It is the concern of this paper to examine not only the effectivenessof tests for detecting colour confusion but also their usefulness in assessing colour visiongenerally. In part 1, problems of administration and age and the question of the basicelements of such tests are discussed. The existing theory of colour vision and colour defect isoutlined and the principle of objective colour specification, which is believed to be valuablefor understanding these tests, is introduced.

There are many tests for detecting colour defectsand as their number increases it becomes progress-ively more important to find which are the mostuseful, valid, and reliable. This paper is an accountof the more popular and more frequently used testsbut it departs from the usual practice of dealing withcolour vision from the point of view of colour defectsalone. The main objective is to assess colour vision,and defects or the absence of good colour dis-crimination are regarded only as part of this largerproblem. This approach is considered more usefuland informative. It is more useful because theproblem in industry is not one of excluding onlycolour defectives but also one of selecting those witheither good or the very best possible discrimination,and from this point of view our approach puts thisin its proper perspective. It is more informativebecause it deals with the whole problem of colourdiscrimination and its variations. With it we arebetter able to understand what can and cannot bedone according to the type of visual physiology thatwe have inherited. And apart from dealing withcolour, in any real situation, factors such as age, andthe fact that a given task may involve cognitiveproblems outside the realm of colour vision, are allseen to play their part.Both theory and practice are taken into account

because it is felt that such a complementary approachshould in the long run yield better results. It issimply not enough to know how to administer andscore a test. It is useful to know why one test is goodand another less so, or why one is more appropriatethan another for a certain selection procedure; andknowing why a test is effective increases ourconfidence in our work. It is here that the knowledgeof the past 50 years helps to bridge the gaps in ourexperience and permits us to reach a higher degree ofcompetence in administering these tests.

There are other reasons for presenting the subject-matter in a novel way. The first is based on the firmconviction that an individual's test performance canonly be fully assessed when it is compared withresults of large population studies. This is essentiallya psychologist's approach, and in this sense we followthe tradition of Pickford who looked upon defectssimply as part of the total problem of individualdifferences in colour vision. The second is thatunderstanding the structure of a test, especiallyfrom the information provided by the science ofcolorimetry, is vital, since this is the best possibleway of assessing a test objectively. In a sense, this isthe approach of the physicist who always stressesthe importance of the stimulus. It is hoped that byusing both approaches in this paper we will achieve

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a better and more realistic understanding of colourvision testing.

There are many ways of assessing colour vision.The research worker does so in one way, the con-sultant in another. Thus, from the research point ofview, the efficiency of a subject's colour vision canbe found by measurng his wavelength discrimi-nation (LIA) and assessing his relative luminosity(or visibility) function (VA) since these two togetherwill indicate what basic type of visual system hepossesses. For such work we require opticallysophisticated instruments to provide pure spectralstimuli and this requires laboratory conditions andrather lengthy administration. It might be arguedthat such a sophisticated approach is wasteful whenall we are interested in is diagnosing extremedeficiencies and that less sophisticated methods cantherefore be used, and it is a fact that from wave-length discrimination studies it is almost impossibleto distinguish deutan from protan defects. Yet a goodanomaloscope can give results which, though unlikethose described above, are nonetheless more usefulfor classifying a defect correctly. There are evensimpler methods using a test involving surface colourwhere a reasonable assess'-en-t can be made. As thispaper is not intended for research purposes butdesigned to be of practical value to those who haveto select colour workers, the techniques we describeare mostly those used in this country, in Europe orin America by people involved in testing forvocational purposes. The actual choice depends onwhich tests were used in the visual laboratory so thatwe may have the benefit of direct personal knowledgeand experience. Lastly, they were selected to includea variety of tests based on different principles interms of both design and material. Four types arediscussed. Those based on the pseudoisochromaticprinciple (P.I.C.) are in the form of books in whichcertain designs have to be identified and the onesdiscussed here are the Ishihara and Dvorine plates,the Tokyo Medical College colour vision tests(T.M.C.), and the American Optical Company,Hardy, Rand 'and Rittler Plates (AO H-R-R).Secondly, a now well-established test of colourdiscrimination and colour confusion devised byCommander Farnsworth after the last war, the so-called Farnsworth-Munsel 100-Hue (100-Hue), willbe discussed. Thirdly, two anomaloscopes will bedescribed, the well-known Nagel type and thePickford-Nicolson anomaloscope (P-N). Lastly,some specialized tests designed to detect specificcolour discrimination abilities are included. Theseare the Burnham-Clarke-Munsell Colour MemoryTest (B.C.M.T.) and the Inter-Society ColorCouncil, Color Aptitude Test (I.S.C.C.-CAT).The first part of the paper deals with general

aspects of the theory of testi'ng, such as problemsof defining and classifying colour defects, followed

by an introduction to the science of colour measure-ment, whereas the second part will deal with specifictests.

Nature of colour vision

The concept of colour vision is rather complex andunderlies a great variety of experiences, which arepartly determined by the nature of the physicalstimulus and partly depend on the structural andfunctional peculiarities of the observer's visualmechanism.

If we are to understand 'colour vision' we must beconstantly aware that it depends on the nature of thestimulus, the state of adaptation of the sensoryorgan, and also on the type of observer. As a resulta multitude of concepts physical, psychophysical,neurophysiological, and psychological must neces-sarily be understood and utilized. Perhaps the bestway to regard colour vision is as an emergent andsubjective experience, dependent at intermediatestages on variables which can be studied and usuallyverified by objective means. Such an approach willremove some of the mystique associated with suchstudies and at the same time will allow us to quantifythese experiences. Instead of being able to classifycolour vision only as defective or normal, a richerand more subtle classification becomes possible,indicating that there is a continuum in ability fromthose completely devoid of it (achromatism) tothose whose colour discrimination is excellent. Thisconception is infinitely more useful than a dichot-omous scheme in deciding which type of discrim-ination is necessary for a given colour task.The most fundamental variable in vision is the

level of the surrounding illumination which deter-mines the level of visual adaptation and thus theappropriate visual process. Vision is said to bephotopic if the eye is adapted to daylight, mesopicif it is adapted to twilight, and scotopic when adaptedto minimal amounts of light. Scotopic vision pro-duces only variations in luminosity, whereasphotopic conditions also mediate chromatic function,and under these conditions both brightness andcolour attributes such as hue and saturation arepossible. As such attributes are interdependent itis' important in a testing situation that they can beisolated from each other. For example, if it ispossible to discriminate between items by observingbrightness differences, then even if hue or saturationdifferences are present no valid inference aboutcolour discrimination can be made. It is thereforenecessary to control such variables; but un-fortunately this' is not always done even in standard-ized test material, and so it is important for thetester to be aware of these shortcomings and toapproach colour vision testing with a criticalmind.

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Theory andpractice of colour vision testing: A review Part 1 175

Theories of colour visionGlobal accounts of colour vision phenomena areprovided by the theories of which there are about 40,falling into two different groups, the first typebased on- observations of certain phenomena thatarise from colour mixing and the second based onour subjective awareness of colour experience. Thefirst is stimulus orientated and tries to explain colourvision from outside, whereas the other is sub-jectively orientated and tries to explain from within.In time these two approaches became crystallized inthe so-called trichromatic and opponent-pairstheories. The first approach arose from the ideas ofartists and early lithographers and especially fromthe ideas of Young (1807). They were formalizedby Helmholtz (1892) and given an experimentalbasis by Maxwell (1890). The second approachsprang mainly from the work of physiologists andpsychologists and was first formulated by Hering(1878, 1920) but received its mathematical rational-ization much later from Hurvich and Jameson (1955).In the 19th century, both approaches were naive intheir explanations of the underlying neural mechan-isms, and at present there is still no single theorywhich gives a satisfactory explanation of all colourphenomena.The essence of the trichromatic theory is the

concept of three simple sets of sensory mechanisms(peripheral) whose quantitative characteristics pro-vide a basis for the different type of colour visiondiscrimination. These are simple response mechan-isms involving red, green, and blue and are describedas the fundamentals which characterize the sensorymechanism as such. Of course it was known evenfrom Maxwell's work that colour matchings withthese three primaries cannot reproduce all colours,and thus the system is only an approximation.The opponent-pairs theory is based on Hering's

ideas and states that the visual mechanism involvesthree complex and antagonistic processes, each ofwhich functions as a pair of opposing sensations -one black-white, a second red-green, and the thirdyellow-blue, the equal stimulation of the two mem-bers of a pair always giving rise to a neutral sensation(achromatic or grey). And according to Hering theeffect of light depends not only on its physicalcharacteristics but also on the condition of the visualmechanism. For example, the actual hue shifts ofindividual hues depend on the wavelength of thestimulus, but they also vary to some extent as afunction of brightness in the so-called Bezold-Brucke effect, where increased brightness causesshifts in the subjective hues of the surroundingwavelengths towards the four primaries of Hering.Simultaneous contrast again could be explained interms of antagonistic processes triggered off in areasadjacent to the stimulated zone. This theory was notformally verified by experimentation on the same

scale as were the elements of the trichromatic theoryfor to Hering's followers it was difficult to experimentobjectively on phenomena that were essentiallyregarded as subjective, though a great deal ofexperimenting was done on contrast phenomena, andthe pseudoisochromatic test! that followed at theend of the last century Are based mainly on Hering'sidea of opposing-pairs.

Today's versions of these theories take up acompromise position (see Galifret, 1960) for neitheralone can satisfactorily explain the complex nature ofcolour vision. Their exponents are Stiles andHurvich, who both admit that neither theory isformulated to exclude all the elements of the other,and thus the 80 years or so of feud are slowly beingforgotten and differences are being approachedrationally.Modem versions came into being after the last

war and were more sophisticated. Their explanationswere subjected to more rigorous mathematicaltreatment; they were based on more experimentalwork and were expected to bear some relation to thebasic neurophysiological elements postulated by thephysiologists.Today as a result of this careful re-examination,

for example in the case of the trichromatic theory,a limited number of differently sensitive elements inthe retina are assumed to exist (the so-called threereceptor type of theory) in which the sensorymechanism is associated with cones and theirpigments. This has been strongly supported by anumber of studies on cone pigments, where bleach-ing techniques were used on normal and colourdefective observers, by Rushton (1958, 1962) andRipps and Weale (1963) while Brown and Wald(1964) and Marks, Dobelle, and MacNichol (1964)experimented on single-cones in monkeys and on thehuman eye using a microspectrophotometer. Thesestudies seem to indicate -the existence of three typesof cones with different spectral sensitivities althoughthey do not entirely conform with the theoreticalmaximum sensitivities of the three initial responsecharacteristics postulated by Young and Helmholtz.

Hering's formulation, on the other hand, hasreceived recent confirmation from the work ofSvaetichin and MacNichol (1958), De Valois (1960),and De Valois, Jacobs, and Jones (1962), who,experimenting with microelectrodes on goldfish andmonkeys respectively, recorded the responses atdifferent levels of the visual system. They haveadduced electrophysiological evidence for the exist-ence of red-green and yellow-blue response mechan-isms at least at the levels of the ganglion cells and thelateral geniculate bodies.Then there is the pioneering work of Granit

(1947, 1955), some aspects of which could beinterpreted in the context of trichromatic andopponent-pairs theories, especially the constructs of

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dominators and modulators (broadband and narrow-band threshold responses as a function of wave-length). In the case of photopic dominators these canbe thought of as corresponding to Hering's achro-matic black-white processes, while modulators mayprovide cues for wavelength discrimination sincetheir maxima are near the wavelengths that corre-spond to the primaries of the trichromatic colourvision theory.

Currently available evidence would seem to pointto some sort of stage or zone theory of colour visionof the type first propounded by von Kries in 1905and expressed in the contemporary work of Wal-raven (1966), though earlier work by Muller(1930) and Judd (1958) are all good examples ofcomplex stage theories. This type of formulationsuggests that a Young-Helmholtz theory must bevalid at the receptor level but that the signals fromthe receptors are processed in such a way thatHering's opponent pairs colour theory applies at thelater stages. For an up-to-date description of thesedifferent approaches see Galifret (1960), Hurvichand Jameson (1960), Graham (1965), Judd (1966),and Le Grand (1968).

Classification of colour defectsThe knowledge we acquire from such colour visiontheories is not directly applicable to the testingsituation, for many of the techniques developed overthe last 100 years were not formulated from a prioriprinciples but in many instances were the outcomeof a trial and error approach where a more prag-matic philosophy prevailed. If a given test dividedpeople into those with normal colour vision andthose who were defective, then the test was a goodone. However, an examination of how colourdefects are classified might increase our understand-ing of how tests were devised and why some aremore efficient than others and, again, might help toexplain certain difficulties which occasionally arisein the testing situation.

In practical situations it is never satisfactory toclassify colour defects on the basis of one theoryalone since this cannot account for all the diversitythat we find. The consultant very soon finds that tolabel accurately all the different types that hediagnoses he has to use the language of apparentlyopposing systems. He must try to combine thesesystems, and whatever synthesis he finally achieveswill depend on many factors. To help us towardssuch a synthesis different approaches to classifyingdefects will be discussed, bearing in mind thecontributions of the two principal theories.To show that this is a real problem let us try to

explain dichromatism in terms of one theory only.From the standpoint of the trichromatic theoryalone, we would say that this defect consists in aloss or absence of one of the fundamentals and

therefore we might talk about 'red blind', 'greenblind', or 'blue blind' people. However, suchterminology is meaningless for, even under lab-oratory conditions, these subjects do not talk aboutabsence of sensation in the relevant parts of thevisible spectrum. In the case of the so-called 'redblind', the visible spectrum is not seen as somethingthat normal observers see, but is seen without thered component. Such subjects divide the spectruminto two parts, where one part from about 500 nmto the longer wavelength is referred to as yellowprogressively fading into a 'dirty' dark colour, whilethe other half of the spectrum is referred to as blue.In a complex visual situation we find that Hering'sidea of opposing-pairs of colours is nearer theobservable facts of such a subject's discrimination,for he actually confuses the colours of the pair. Heconfuses red with green, or orange with yellow oryellow with green, etc. Thus, to say that such adichromat cannot see red conveys nothing useful ifwe wish to describe the sort of difficulties heencounters, but if we say that he will confuse allcolours between red and green this fits the observedfacts, though it still might be true that his particulardefect lies in the absence ofa red receptor mechanism.

It is with such a problem in mind that thisparticular section on classification is eclectic innature. Terminology is used from many theoriesand occasionally, when classifying predominantlyaccording to one theory, the terminology usedbelongs to another type. For example, when classify-ing dichromats (which is essentially a trichromaticapproach) we introduce the actual labelling of thevarious dichromats from language that does not fitthe scheme.The simplest way to speak about colour defects is

to refer to them as either congenital or acquired.With the congenital type no morbidity is implied,only deviations, whereas with the acquired type onealways looks for a pathological basis. They can beinherited, but the assumption is that at some stagecolour vision was normal. At one time it was thoughtthat colour vision tests should be able to differentiatebetween the two categories.We may also classify defects in terms of the two

major colour vision theories. If Hering's terminologyis accepted, we group defects according to the mostcharacteristic pairs of colours which such subjectsconfuse with white stimulus, thus we have red-greenand yellow-blue defects or confusions. If, on theother hand, Maxwell's findings on additive colourmixtures are followed, we can group the defectsaccording to the number of primaries required toreproduce all the colours in the colour space,observing that in this theory the irreducible numberof primaries is three for the majority of people(normal trichromats). This classification is the mostpopular in English-speaking countries and also

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provides us with the best known and most frequentlyused terminology.

If under photopic conditions only one primary isneeded to match any spectral colour we talk aboutmonochromatism or achromatopsia. Such subjectshave no colour discrimination but only the photopicluminosity function. Occasionally this results in ascotopic rather than a photopic relative luminosityfunction. To distinguish between these two types ofdefect we define them as either cone or rod mono-chromats; their frequency is minimal.

If, on the other hand, not more than two primariesare required to match the whole colour gamut wetalk about dichromatism. These are the defects mostoften referred to when we talk about colour blind-ness. Four types are distinguished and they may begrouped in pairs, two along the red-green confusionaxis, that is, protanopia and deuteranopia (literallymeaning 'first' and 'second' defect and named so byvon Kries so that no inferences about the physiolog-ical nature of the defect would be made from nomen-clature), and two along the yellow-blue axis, tritanopiaand tetartanopia. The frequency of the two red-greentypes is about 1% for each among men and is evenlower among women. Data for the incidence oftritan defects is less certain and least of all for thefourth type, tetartanopia, which some think is notcongenital in origin at all although it is thoughtto be so by Muller (1924) and by Judd (1949). Thiswill be discussed again in a later section dealingwith colour measurement.

Lastly, if three primaries are necessary, but theratios of any of these are appreciably different fromthose of normal observers, we have the so-calledanomalous trichromats, who are referred to asdeuteranomalous, protanomalous (terms coinedby Nagel (1907)) or tritanomalous. The incidence ofthese anomalies is higher than the incidence ofprevious defects, being 4% to 5% for the deutan typeand 1a% for the protan. No reliable data exist forthe third type. Their visual characteristics will bediscussed when the various tests are described. Amore detailed account of the red-green types ofdefect will be given later, especially in the section onanomaloscopes, where what is meant by anomalousdefects can most easily be understood.

Classification of colour defects can also be accord-ing to prediction of which is the impaired part of thevisual mechanism. Such a classification was pro-pounded in 1924 by von Kries, who recognizedthree systems, the so-called absorption, alteiation,and reduction systems. The essential characteristicof the absorption system is that the retinal mechanismis normal and colour deviation is due to pre-receptoral changes which are caused by changes ineither the cornea or lens. It is a non-selectiveabsorption system if the modification of the retinalillumination is identical for all monochromatic

radiations of the visible spectrum, and selective ifthis modification varies with wavelength. In thealteration and reduction systems the anomaly isattributed to a defect in the retinal mechanism. Thealteration system is characterized by the non-acceptance of certain of the normal colour matches,best seen in the anomaly quotient of anomaloustrichromats (see section on anomaloscopes). In thereduction system we find the acceptance not only ofthe colour quotients of normal subjects but also of theanomalous. Usually the reduction system is definedas a form of dichromatism. In this system there areonly two fundamental response curves instead ofthe normal three curves, and this is also sometimescalled the collapse system or the collapse of thenormal fundamentals (see section on colour measure-ment). For further reading on this topic, consultPickford (1951, 1958), Verriest (1964), and Kalmus(1965).

Changes in colour visionCongenital colour defects, frequently quoted as beingthe only form of faulty colour vision, have beenadmirably discussed in a recent book by Kalmus(1965) with particular reference to the way suchdefects are inherited, and from an excellent andreadable paper by Wald (1966) we may gain con-siderable insight into the molecular and geneticmechanism underlying this type of vision. Anextensive account of the frequencies of these defectsthroughout the world is given in a book byKherumian and Pickford (1959). But changes lead-ing to losses in discrimination, which might be asextensive as or similar to the congenital types, canbe brought about by processes such as normalageing or by pathological conditions of the eye itselfor as a by-product of some systemic diseases.Though our knowledge of the extent and frequencyof such losses is recent, it is important to stress theirexistence since ageing processes affect performanceon almost all colour vision tests, and losses due todisease are much more frequent than was previouslysuspected or admitted.

Colour vision changes due to ageIf studied within an age continuum, that is, fromchildhood to old age, it can be seen that there is agrowth of function up to the age of 20 and a declineafter about 30 years. Such age variations were ob-served in the late thirties by Smith (1943) and byTiffin and Kuhn (1942) but their significance hasbeen accepted only within the last decade, and recentwork shows that such variations can be found toapply to all types of discrimination. Gilb_rt (1957)showed this to be true for performance on the C.A.T.,Lakowski (1958, 1962) measured it by colour visiontesting on the anomaloscope and a number of

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P.I.C. tests, Verriest, Vandevyvere, and Vander-donck (1962) did so for the Farnsworth-Munsell100-Hue scores, and Ruddock (1965) for dis-crimination on Wright's colorimeter.

Because of its importance, the age variable will beincluded in the discussion of the various tests, andthe extent of losses will be properly assessed, sothat the information will be available to help us todistinguish congenital defects from the effects ofsenescence.How the age effect comes about is not properly

understood and its causes have yet- to be fullyestablished. Some of the minor changes in colourvision can quite clearly.be attributed to the ageingof the lens and/or macula (Said and Weale, 1959;Lakowski, 1962; Weale, 1963). The more extensivelosses might be attributable to the fact that after 30there must be increasing numbers of people who, aswell as showing the normal changes, have minorpathological conditions, which probably acceleratethe senile degenerative processes so that by, say, 50the use of the term 'normal senile changes' becomesof questionable value. Recent studies on specificacquired dyschromatopsias which have shown thatin many cases pathological conditions give rise tocolour vision losses similar to those of the very old,a kind of early ageing effect, tend to confirm this.The possibility of pre-clinical conditions affectingcolour discrimination, and in all probabilityaggravating the 'ageing effect', must not be ruled out.

Acquired dyschromatopsias or acquired colour lossesThis is the name given to those losses which are notcongenital where it is always assumed that at somestage the subject's colour vision was normal. Theymay be hereditary or traumatic in origin or duesimply to pathological senile degenerations ofvision. It is perhaps useful to point out some of thedifferences between congenital and acquired dys-chromatopsias. For example, we know a great dealabout the heredity of congenital defects but verylittle about acquired ones. Although the visualmechanism underlying congenital defects is not fullyunderstood, with acquired dyschromatopsias we caninfer where the impairment lies from a knowledgeof pathology. It is axiomatic that in congenitaldefects no morbid conditions exist but pathologicalconditions are always assumed to exist with acquiredones. In congenital defects the results of colour visiontesting are stable and predictable, the axis of con-fusion is definite, colour naming always showsclassical confusions (e.g., red or purple and brownmistaken for grey or green by the red-green defec-tives), and the defect is bilateral. In acquireddyschromatopsias, on the other hand, results areunstable, progressively deteriorating or occasionallyimproving if the underlying pathological conditionsare reversible. The axes are not so definite, there is

usually good colour naming, and there are variationsbetween eyes or even between parts of the retina(on axes of confusion see section dealing with colourspecification).The earliest studies in this field were made in the

second half of the nineteenth century during theso-called German period, when it was shown thatacquired dyschromatopsias, though not so clear cut,could be subdivided in the same way as congenitaldyschromatopsias into abnormal trichromatism, di-chromatism, and monochromatism. It was Koellner(1912) who postulated that there are dyschrom-atopsias of the red-green and yellow-blue axes aswell as some without any apparent axis. It was alsoestablished that lesions in the outer layer of theretina lead to yellow-blue defects, whereas lesions inthe inner layers or in the optic nerve lead to red-green defects. For further information about causes,manifestation, and frequency of acquired dys-chromatopsias see Zanen (1953), Francois andVerriest (1957, 1961), Verriest (1963, 1964), Jaeger(1956), Grutzner (1962), and Cox (1961 a and b).It would be interesting to speculate on their incidencein relation to the general population. At present nosuch data exist but it would not be exaggerating tosay that about 5 % are as severely affected as theexisting 8% to 10% of congenital colour defectives.Ageing and pathology as causes of colour vision

losses have been deliberately introduced becausethey are sometimes ignored or their effects minim-ized. But in practical situations, where the questionof upgrading older workers often arises, such lossesmight be regarded as congenital and, if onlyrecognition of primary colours is involved, a workermight be unnecessarily penalized. On the other hand,these losses might go unnoticed, especially whereP.I.C. tests alone are used for detecting congenitalred-green defects. Nevertheless the discriminationlosses concomitant with these changes are usuallylarge enough to have an effect, especially where goodcolour discrimination is required or where yellow-blue discrimination is very important, as forexample in the printing trades or in textile colourmatching.

Basic elements in colour vision testsTo understand the merits and attributes of specifictests we must consider the basic elements in colourvision tests as a whole. The following classificationis an attempt to show what elements underlie manyapparently different tests. It is perhaps advisable toread this section twice, once before a more detailedaccount of the tests is given, and again after readingthe second part of the paper. The aim is to indicatesimilarities and differences in terms of colour visionand stimulus characteristics in tests that are ap-parently of the same type.

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Mode of appearanceIn an ideally constructed test 'colour' should beconfined to the non-spatial, non-temporal attributesof colour perception, to those aspects of discrimi-nation which involve least interpretation interms of past experience and are unaffected byintelligence or personality. Yet when colour appearsin tests even in its simplest form in the so-called'modes of appearance', it is always at least ina spatial or temporal context. These are the mostfundamental elements of a test and three such modesare distinguished - surface, volume, and film. Whencolour is perceived as belonging to a surface, it issaid to be perceived in the surface mode, and themajority of colour vision tests such as the Ishihara,Dvorine, 100-Hue, and C.A.T. fall into this category.When colour is perceived through the bulk of auniformly transparent substance it is said to beperceived in the volume mode. This is not importantin the realm of colour vision testing. When film modeis referred to, the implication is that the stimulusobject is not distinguishable as an object, for examplewhere colour seems to fill a space behind anaperture. This is what happens in most colorimetersand also in anomaloscopes. The film mode of appear-ance is considered the simplest of all modes, in thatit is the least removed from pure sensation, whereaswith surface mode the complex attributes of fullvisual perception such as form, size, glossiness,texture, glitter, etc. can never be estimated. Forfurther information see Judd (1958), Burnham,Hanes, and Bartleson (1963).

Complexity of visual taskSome tests are simple and some complex in relationto the cognitive and perceptual involvement required.The degree of cognitive involvement needed bymost P.I.C. tests is surprising, for subjects mustknow numerals (usually arabic) or recognize shapes(usually geometric). Then in colour discriminationtests, such as the 100-Hue, the concept of a 'colourseries', say from red through orange and yellow togreen, plays an important role in the ability toperform the test well, and this depends not only onthe physiology of vision but also on the maturationof cognitive processes.Some tests, on the other hand, present relatively

simple cognitive and perceptual tasks. They requirean average amount of intelligence and dependmostly on the neurophysiological state of thesubject's visual system. Anomaloscopes fall into thiscategory. Also in such instruments the colour matchachieved is relatively independent of the state ofadaptation. Though the colour naming will change,the ratios of the primaries in such a match willremain the same over a large range of adaptationlevels, provided that the general luminosity value ofthe matching field is above the scotopic level. Thus

such an instrument can be used at mesopic as wellas photopic levels of illumination without essentiallyaltering the relative scores. This does not apply totests based on the P.I.C. principle, where adaptationto the level of illumination and the colour tempera-ture of the illuminating source are important factorscontrolling the type of response and so the scoringon the tests.

Complexity of stimulusIn physical terms, the stimulus in P.I.C. tests iscomplex. The resultant colour depends not only onthe energy distribution of the illuminant but also onthe peculiarities of the pigments involved, which canhave 'broad' or 'narrow' band characteristics. Eachplate in such tests is composed of many differentlycoloured dots (four to 10 or more) each with differentreflectances, which make up the figure and back-ground. Anomaloscopes also vary in complexity. Inthe Nagel, monochromatic stimuli are used, whereasin anomaloscopes that use filters, rather broadertransmissions are used with consequent reductionin the purity (saturation) of the stimulus.Colour vision tests differ in another very important

aspect, that is in the degree of metamerism orisomerism that is present in the matching situation.These terms were adopted by Ostwald (1931-33)to describe the phenomenon exhibited when two ormore surface colours seem to appear alike under oneilluminant such as daylight but do not match underanother illuminant such as an incandescent lamp.Although these terms were coined only about 40years ago the phenomenon was by no meansunknown before. Studies of extreme metamericpairs, in which mixtures from two different parts ofthe spectrum are set up to match another spectralmixture in colour, have yielded the most valuableinformation about the properties of the averagenormal eye (the so-called Grassman Law). In thissense metameric pairs, as they are called, have to beseen as stimuli with the same subjective colour butdifferent spectral composition.Thus when colours look alike, whether they belong

to a surface or come from a source, they do sobecause either they are physically or spectro-photometrically identical (this is called an 'isomericmatch') or they are physically dissimilar, but,because of the peculiar nature of colour vision, theymatch, that is, they give rise to identical sensations(this is called a 'metameric match'). Another way ofstating this fact is to say that they match colori-metrically though not spectrophotometrically. Ifmembers of a pair are of very different spectralcomposition, and they are still perceived as identical,they are said to exhibit a high degree of metamerism.In anomaloscopes and colorimeters the colours tobe matched are essentially in a metameric relation-ship, that is the two fields evoke the same sensations

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180 R. Lakowski

but are spectrally different. In such tests we measurethe degree of metamerism that can be accepted byan individual, and this form of colour vision testingis one of the most efficient as it measures directlydifferences in sensitivity of the individual elementsof the visual system, for example, at the retinal level,say the property of 'red' to 'green' 'mechanisms'. Inmany tests where colour discrimination of surfaces iscalled for, the differences are essentially isomeric innature and thus the task consists of perceivingdifferences in brightness or saturation only. In someP.I.C. tests the surface colours used might also be ina metameric relationship (see Lakowski, 1965 aand b), and there is one test, Glen's Colour Rule,which measures metamerism by means of surfacecolours, where this is deliberately introduced.

Retinal subtenseColour vision tests may vary in the size of the imagesubtended at the retina. In the Nagel anomaloscopea 30 15' subtense is used, while on the P-Nanomaloscope it is 11', and the diagnosis of theextent of a particular subject's acquired defect maynot be the same for the two instruments. Where testsinvolve surface colours, differences in subtense areeven greater. The little coloured dots constitutingthe figure or background of a P.I.C. plate, viewed at30 or 40 cm., may subtend from as little as 40' atthe cornea for the T.M.C. test to as much as 1li forsome of the dots in the Farnsworth F2 tritan plate.In the 100-Hue each individual cap subtends 20 10',and in the C.A.T. the size of each plastic chip isapproximately 35 x 45 mm., giving a subtense of3 to 41' at normal viewing distance.

Considerations of subtense are important,becausecolour vision up to 2° is essentially different fromcolour vision at a larger subtense. When assessingresults of a test it is vital to know how much of theretina was stimulated since this is an importantfactor in a comparison of results of different tests oreven between different P.I.C. tests. The aetiology ofdeterioration due to age or disease might be differentfor the parafoveal as distinct from the foveal regions,and results of tests designed to detect the sameanomaly might be contradictory if the subtensevaried to any extent.Enough has been said for us to realize that a critical

evaluation of basic elements of each test is vitalespecially when more than one test is used, say fordifficult cases. The apparent discrepancies betweentest results might be due to the fact that the basicelements in tests are different rather than to theunreliability of a subject's responses (see Lakowski,1968 d).

Administration

The secret of good testing procedure lies in being

able to elicit valid and reliable results. Whether thisis successful will depend to a large extent on whetherthe instructions for administering the particular testhave been followed, and on whether the relevantinformation has been extracted. In most situationssubjects are quite helpful but, when they know theyare defective, when they are suspicious of testing, orwhen the subjects are either young children orolder people, it is necessary to establish properrapport or the results may be invalid even using thebest tests.Now let us look at the elements unaffected by the

human factor. Valid results depend not only on thetest being 'correctly' constructed (this will be dis-cussed later) but also on whether the specified testingconditions have been followed. The user can do littleto improve the test but he can render his resultsinvalid if he alters the administration practice. Mosttests give explicit instructions which must befollowed. Reliable results can be obtained only ifthe testing situation remains constant from sessionto session, and testing procedure must be similar forall test users if the results are to be comparable. Ifconditions differ to any extent, valid and reliableresults can still be obtained for each individualtesting situation, but no direct comparisons can bemade. Discrepancies in administration are mostlikely to arise over the timing of the test, the viewinggeometry, and the type of illumination used.

If timing is critical it is specified. In the 100-Hue,for example, subjects are told that the test is timed,and it is found in practice that, unless such instruc-tions are given, some subjects with good dis-crimination look upon it not as a test of sensorydiscrimination but as one of aesthetics and are pre-pared to take a very long time over it with noappreciable improvement in scores. Timing is alsovery critical in the Colour Memory, but not in theColour Aptitude Test. No such instructions aregiven for most P.I.C. tests, but if comparisons ofmisreadings are to be made between large-scalestudies, most research workers limit presentationtime for each plate to 5 seconds, and it is thereforegood practice to do this for vocational testing also.Longer presentation only encourages the use of

extra cues, the most important of which are foundby changing the angle of viewing. As many P.I.C.tests are printed on glossy paper it is vital to see thatthe viewing geometry is correct if we are to eliminatethe specular component which usually enhances anycontrast between figure and background. The effectof this can be observed in the very natural habitthat colour defectives have of slanting the individualcards, so that the specular component appears. Inall testing, therefore, the viewing conditions shouldbe those recommended by the International Com-mission on Illumination (C.I.E. 1931), where thesurface of a test is illuminated at an angle of 450 and

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Theory and practice of colour vision testing: A review Part 1 181

viewing is perpendicular to the surface (shortnotation for this viewing is 450/00). The illuminantand viewing conditions can be interchanged withoutany effect (Fig. 1).

Nearly all tests employing surface colour requirestandard illumination in relation to both the relativeenergy distribution and intensity. The energy con-tent of the standard light source should resemble thatof northern light, but as this varies even during day-time from about 6,000°K to 15,000°K or more (asmeasured by the colour temperature criterion), anagreed colour temperature such as the C.I.E.standard illuminant 'C' of 6,700°K or the newlyproposed C.I.E. illuminant 'D' with colour tempera-ture of 6,500°K should be the rule. In the past it wasalways difficult to obtain an illuminant 'C' and there-fore approximations to such a standard source wereacceptable; for example, the Macbeth Lamp 40 BK6,500°K in the United States of America or modifiedtungsten lamps run at a colour temperature of2,850°K (called C.I.E. illuminant 'A') corrected tothe temperature of northern light by filters such asChance Pilkington OB 10 or Corning Daylightfilters (Judd, 1961). Recently, however, artificialdaylight sources have become easily available,some of which are manufactured in this country byLeslie Hubble Ltd., of Leicester. These have colourtemperature of 6,500°K and comply with theB.S. 950 (1966) or the C.I.E. (1966) illuminant'D' 6,5000. On no account should ordinary tungstenor fluorescent light of the 'warm tone' type be usedin place of a standard illuminant since these are runat a temperature of 3,000°K or less and wouldalter the intended colour differences (see section oncolour measurement). In other words, the intendedcolour contrast, say, between figure and backgroundin P.I.C. plates would be exaggerated, the recog-nition of correct numerals would be easier, and evencongenital red-green defectives might be able to readthem. Again such low colour temperatures wouldappreciably alter the spacing of the blue caps inthe 100-Hue test.

Lastly, it is very important that the intensity ofillumination should be correct. If it is too low thetest might become more difficult and the result would

S.

(a) 450/0° (b) 00/450

FIG. 1. C.I.E. viewing geometry; the illuminant (S)and viewing conditions are interchangeable.

be poorer scores. If it is too high, then unnecessaryexpenditure is incurred, as sources run at highintensity levels are costly to buy, to run, and tomaintain. Every test has its optimum level ofillumination, but the general rule is that the levelof adaptation should be above the upper limit ofmesopic vision, that is, the luminance level of theviewing field must be above 10 candelas per squaremeter (cd/M2) with the natural pupil. As this is theminimum requirement, a more useful level is reachedwhen luminance is twice or three times higher.As the amount of light arriving at the receptors is

not strictly a constant fraction of the intensity ofa source, it is essential to calculate the retinalillumination in all visual testing. This must be atleast 70 to 100 effective trolands when testing withcolorimeters, and when cognitive tasks are involvedit is desirable that it should be between 150 and 300trolands. The following example illustrates how thesevalues have been arrived at (see also Fig. 2). Thephotometric units used are metric, but Britishequivalents are given where relevant.To reach, say, the 200 effective trolands level of

retinal illuminance, the initial illuminance providedby the source should be about 300 lux (lumens persquare meter), i.e., 28 foot-candles (lumens persquare foot) at the test surface. The resultantluminance level of the viewing field will be about93 cd/M2 if a perfect white diffuser is being viewed.Note that this is about one third of the originalilluminance because the perfect diffuser nowbecomes a secondary source with the property ofsending back all incident light, but equally in alldirections (according to Lamberts Law) so that theamount of luminance of its surface is only 1/ir of theoriginal illuminance. In addition, as all surfaces

Retinal illumination(effective trolands)

&7XSiltes-CrawfordeffectPupildiometer

Illumination\ /(lumens per sq.m.) \ /

\/\ /

>4a/' Luminance(l)(candelas per sq.m.)

10-40% reflectancefrom C.V. tests

FIG. 2. Stages determining final retinal illumination.

0ll.-4..0

.0.1

e.1

.0.1

.1.1

.0

7q787lA

IIIIIIII

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182 R. Lakowski

absorb some incidental light, and pigments employedin colour tests absorb between 60% and 90 %, onlya fraction of the luminance, about one quarter - saybetween 20 and 30 cd/m2-is effective in producingretinal illuminance.The actual level will be determined by another

factor besides the luminance of the surface viewed,namely the pupil aperture. In any given subject thesize of this will depend on age, state of health,excitation, etc., but in our case it is dependent onlyon the general luminance level of the field which givesrise to a pupil diameter of 3-4 mm. Thus retinalilluminance is the product of pupil area (S) in mm.2and the given luminance (L) in cd/m.2 In this caseL.S. is 20 or 30 cd/m.2 x 11-5 mm.2 (the approx.pupil area), or 230 to 245 trolands (so one trolandcorresponds to a luminance of 1 cd/m.2 viewedthrough a pupil area of 1 mm.2). If in a givensituation pupil size and luminance are known it iseasy to calculate the retinal illuminance.However, because of the Stiles-Crawford phenom-

enon the actual retinal illuminance will be slightlyless than the simple product of L.S. This is due toa rather curious property, the relative directionalefficiency of the retina, which is more sensitive torays striking it perpendicularly than to those whichare slightly oblique (Stiles and Crawford, 1933).This means that rays entering the pupil centrallyare more efficient than those entering near the pupilmargin, so that the effective pupil area must bereduced in this case to 8 mm.2 In this case then theeffective retinal illuminance is between 160 and 240effective trolands. For further discussion on thistopic see Le Grand (1968, Chpt. 5 with particularreference to the table of pupil diameter and areas atvarious luminances), Grossman (1967, Chpt. 3),Sidowski (1966, Chpt. 7), Graham (1965, Chpt. 1),and Crawford (1968).

Colour specificationNormally when talking about the coloured world

our descriptions are subjective, which leads to arichness in linguistic expression seen in the namesfound on designer's cards such as 'subdued persiangolden brown', etc. Though doubtless meaningfulto the individual who coined them, such colournames are not readily identifiable by others. Becauseit is difficult to know what a specified colour reallylooks like, about 30 years ago a system of descrip-tion in terms of the physical attributes of thestimulus was accepted, which has transformed ourmethods of describing colour. This can no longerbe subjective or man-orientated only but must takeaccount of the precise nature of the stimulus andmust accept a Standard Observer as a reference pointin colour perception. The system allows us to measurecolour independent of the colour vision deviations

found among normal observers. When specifyingcolour we must include not only the spectralcomposition of the radiant energy reaching theobserver's eye but also the visual properties of theobserver himself.Such specification is becoming common practice

in the industries that deal with colour, and there areseveral reasons for including a description of them.First, a great deal of our understanding of testdesign depends on being able to give exact speci-fications of the colours used. Secondly, it is best toevaluate the degree of difficulty involved in a testfrom the colour differences in it, and these in turncan be calculated from the objective data suppliedby colour measurement. Lastly, certain aspects ofdefective colour vision such as colour confusioncan best be illustrated by reference to this colourspace.

In the colour theory section it was pointed outthat a normal observer requires at most threeprimaries to match a given colout: red, green, andblue if matching is done with lights; red, yellow, andblue for paints; and magenta, yellow, and cyan inlithography or photography. Three primaries are theirreducible minimum, and it has been concludedfrom this and other studies that normal colourvision is trivariant, that is, that not more than threevariables are required to determine it. It is thereforepossible in a mathematical system to express colourby three numbers. The modern science that dealswith this type of specification is called colorimetryand the particular system of specifying coloursdescribed here is the one adopted by the Inter-national Commission on Illumination (or the C.I.E.)in 1931. In this system three imaginary primariesX Y Z, embodying the characteristics of the realR G B (red, green, blue) primaries, completelyspecify the various mathematical functions in thesystem, and specification of a given colour involvesfinding the relative amounts of the three primariesthat are required to match that colour under astandard illuminant. It would be a mistake to inferfrom the use of the number 'three' that this tends toconfirm the Young-Helmholtz theory of colourvision. The C.I.E. system shows only that a three-dimensional method of specification is both workableand useful. At most it gives one way of interpretingthe fact of trivariance (Le Grand, 1968).

Figure 3, illustrating the geometrical relationshipbetween the XYZ and RGB systems, shows themathematical primaries as the three apices of a right-angled triangle within which the 'real' colour spaceis inscribed in the form of the so-called spectrumlocus. The numbers on this line denote the positionsof certain wavelengths in nanometres (nm) and thepositions of the three real colour primaries (RGB)are also shown, as are the loci for the C.I.E.illuminants 'C' and 'A'.

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0-8- G 54

0b6- 560

O z B80 , ,X580ND ~~~A

04 - o 6 00 8C

0-240

0 0.2 0-4 0-6 0.8 I*0

FIG. 3. Chromaticity diagram for the XYZ system (afterLe Grand, 1968). RGB denotes positions of the realprimaries, while letters A and C mark positions of C.I.E.standard illuminants. The co-ordinates x and y aremarked at intervals of 0-2 unit.

Colour measurementThe mathematics involved in transforming measure-ments obtained from colour mixing with 'real'primaries into the XYZ system is complex (seeLe Grand (1968) or Graham (1965) ) but at presentall that is needed is a knowledge of how colour isspecified in the C.I.E. system, for here the tristimulusvalues (XYZ) for any colour are simply the productof the physical or spectrophotometric quantities ofthe stimulus and the colour matching quantitiesrequired for it by the 'average eye'.To make this system meaningful without involving

too much theory, an example of colour measurement(Fig. 4) is worked out for the blue cover of thisjournal (specifically for the July 1968 issue). Thisis in the form of a flow diagram showing how wearrive at the tristimulus values for the cover. Thefollowing explanation should be read in con-junction with the six diagrams in this illustration.The tristimulus quantities XYZ for this blue cover

as matched by the 'average eye' under illuminant'C' are indicated in Fig. 4 a-f and they were derivedas follows:

First, the initial values of the distribution co-efficients of the Standard Observer (as shown inFig. 4a) were modified by the energy distribution ofilluminant 'C' (EcA) shown in Figure 4b. Thesecoefficients simply represent the amounts of theC.I.E. primaries required by the Standard Observer(mean of 17 measurements by actual observers

obtained by Guild (1925-6) and Wright (1928-9)and accepted by the C.I.E. in 1931 as the visualstandard), to match narrow band wavelengths (ofequal energy) throughout the visible spectrum. Thenthese so modified distribution coefficients aremultiplied by the spectral reflectance factor of thesurface (pA) (Fig. 4c). If the areas under the threecurves in Fig. 4 d-f are s.mmated, we obtainnumerical values for these. Table 1 shows such valuesobtained for the April and July 1968 issues of theJournal under two C.I.E. illuminants 'C' and 'A'and indicates the amounts of XYZ required to

TABLE 1AMOUNTS OF TRISTIMULUS VALUES XYZ

REQUIRED TO MATCH BLUE COVERS OF APRIL ANDJULY 1968 ISSUES OF Brit. J. industr. Med. UNDER

ILLUMINANTS 'C' AND 'A'

Tristimulus Values

Issue Illuminant 'C' JIluminant 'A'

x y z x* y z

April .. 304-0 3215 427-0 1950 190-2 83*9July .. 3213 337 5 446 0 203 7 197 8 87 3

match these covers under standard northern andtungsten filament lights. Note that under the morebluish illuminant relatively more units of Z arerequired for matching both covers than for matchingthem under artificial light. Note also that under bothilluminants there are slight differences in tristimulusvalues for the April and July issues which will becommented on later.

Using these results we can also find the relativebrightness of the blue covers from the Y tristimulusvalue by comparison with the standard white. Letus look again at Fig. 4a, paying particular attentionto the shape of the y distribution coefficient. In theC.I.E. system this function has been deliberatelychosen to be identical to the characteristics of therelative luminosity efficiency curve of the humaneye (VA photopic - see section on Nature of ColourVision), and thus the value of Y when expressed asa percentage of the Y of the comparison standardused in reflectance measurements magnesium oxide(MgO) gives the brightness or luminosity factor for

Y samplethis surface Y% = *In our example,

Y MgO)this is as shown in Table 2.Thus under both illuminants the covers will have

almost equal luminosity factors, looking slightlybrighter under illuminant 'C' than under illuminant'A.

In the C.I.E. system, therefore, the tristimulus

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184 R. Lakowski

-0

2E._-

1.5

I0

05

0

1s-

1.0

1<uLJ

0

nmFIG. 4. Flow diagram showing how the tristimulusvalues for the cover of the April (1968) issue of Brit. J.industr. Med. are calculated:

(a) distribution co-efficients Cx, y, z) in the C.I.E.system; (b) energy distribution (EA) for C.I.E. illuminant'C'; (c) reflectance curve (pA) of cover; (d)'X' tristimulus

760value I EcA X pA aA;

380760

(e) 'Y' tristimulus value : EcA Y PA aA;380760

(f) 'Z' tristimulus value 7 ECA z PA JAl380

(c)

0-5-

0

.2

I-E-

(e)

70

z

._I-

2

so-.

05

0-

E2

I-

nm

y

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TABLE 2LuMINOSITY FACTORS OR Y% FOR THE

COVERS OF APRIL AND JULY ISSUES OF THISJOURNAL

Tristimulus Value 'Y'

Issue Illuminant 'C' Illuminant 'A'

MgO Blue Y% MgO Blue Y%cover cover

April .. 7130 321-5 45 09 434 0 190-2 43-82July .. 712-0 337-5 47-43 434 0 197-8 45 56

values XYZ and the Y% arethe stimulus colorimetrically.

sufficient to describe

The C.I.E. chromaticity diagramIn practice, however, the absolute values of XYZare not usually given (as these vary from instrumentto instrument). What really matters is the relativequantities of each that are necessary for a givencolour match. It is more usual to express eachtristimulus value as a proportion of the sum of thethree values, and these ratios are then termed theC.I.E. co-ordinates, symbolized in the letters x y z.They are derived as follows:

xCo-ordinate x =

X + Y + Z

yY=y =X + Y +Z

zz =

X + Y + Z

The reader may confirm the co-ordinates of theblue covers from the tristimulus values given in

TABLE 3COLOUR SPECIFICATIONS FOR THE 'BLUE'

COVERS OF THE APRIL AND JULY 1968 ISSUESOF Brit. J. industr. Med., IN TERMS OF C.I.E.

CO-ORDINATES X AND Y AND THE Y %;SPECTROMAT FS2. (0/45' GEOMETRY,

MgO STANDARD)

C.l.E. co-ordinates

Issue Illuminant 'C' Illuminant 'A'

x y Y% x y Y%

April .. 0-289 0-305 45-1 0-416 0-405 43-8JulY .. 0291 0 306 47 4 0 417 0-405 45 6

Table 1. These co-ordinates are shown in Table 3.The co-ordinates indicate very simply the chro-

maticity of a given surface colour in the C.I.E.chromaticity diagram (or the mixture diagram).Note that in Table 3 only values for two chro-maticity co-ordinates are quoted for each colour.Since, by definition, the sum of the chromaticityco-ordinates is unity (x + y + z = 1), the value ofthe third is simply arrived at. Thus colours are mostfrequently specified by giving any two of the co-ordinates (x and y) plus the luminosity value Y%.In all subsequent diagrams showing positions ofcolours used in the various tests the actual co-ordinate values are not specified but approximatereadings correct to the second decimal place can beread off from the scale, for co-ordinate x (i.e., theratio of the tristimulus value X to the sum of all thethree tristimulus values X + Y + Z) on the abscissa,and for co-ordinate y (ratio ofY to this sum) on theordinate.

It should be remembered that each completediagram, such as is shown in Figs 3, 5, and 6,represents only one plane of the colour solid takenat right angles to the vertical white and blackaxis, where the spectrum locus marks the boundaryof all real colours in that plane. Sometimes only apart of this diagram is shown but in such cases theco-ordinates x and y allow us to place this sectionin relation to the total C.I.E. colour space. Inaddition, the position of the white illuminant andany part of the spectrum locus can be used asreference points. The C.I.E. diagram does notindicate brightness values (Y %).

Other properties of the C.I.E. diagram The diagramcan be looked on as a map of colour space, andbecause of this it can be used as a convenient short-hand for explaining certain aspects of colourrelationships. The most obvious comparison isbetween the colours used in one particular colourvision test and those used in other tests. Once theposition of a colour is specified in the diagram,inferences about its hue or saturation can easily bemade.

In the C.I.E. system, the concept of dominantwavelength (AD) corresponds most closely to thepsychological attribute of colour called hue. TheAD of any colour is determined by drawing a straightline between the position of the reference standardwhite and the position of that colour and extendingthis line until it intersects the spectrum locus. Thispoint of intersection will indicate which part of thespectrum will match the given colour when mixedwith a neutral standard (e.g., white). In Fig. 3, forexample, notice the loci of the blue covers (marked as.) in relation to illuminant 'C' and 'A'. The linesdrawn through them to the spectrum locus show thatAD for the April cover is 483 5 nm when seen under

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186 R. Lakowski

illuminant 'C' and 497 nm under illuminant 'A'.In other words, the colour of this particular issueof the journal will look more green/blue under thetungsten source than when illuminated by northernlight.On the other hand, the concept of excitation

purity within the C.I.E. system is most closelyrelated to the psychological attribute of colour calledsaturation, which subjectively specifies the extent towhich a colour differs from white. This concept canmost readily be understood with reference to thechromaticity diagram, where the 'purity' of a colourrefers to its proximity to the spectrum locus. If allmonochromatic stimuli have a purity equal to unity,then under both illuminants the purity Of AD for theblue covers is only a very small fraction. But we canbe even more specific than this if we express thepurity as a ratio of the distance between the positionof the reference source and the sample to the distancebetween the position of the source and the positionof AD on the spectrum locus. In this case the purityfor the samples under illuminants 'C' and 'A' isapproximately 7% and 10% respectively.

Thus, by placing the chromaticity of a colour inthe C.I.E. diagram, we can easily and validly makeinferences about its hue (from AD) or saturation(from purity ratio), noting that colours near thewhite reference standard are least saturated whereasthose next to the spectrum locus are most highlysaturated.

Colour confusion and discriminationThe word 'sensation' is used to emphasize that weare describing only the simplest perceptual processes.Otherwise, in all descriptions of colour perception,expressions such as 'hue discrimination' are used inpreference. The choice is deliberate, for the termsensation has rather a complex meaning inpsychology and always involves introspection anddiscussion about whether such mental acts are simpleor complicated, peripheral or central, etc. By usingthe term 'discrimination' in referring to colourattributes, we bypass the problem of 'subjectiveawareness of different sensations' and emphasizeinstead that what is perceived is the observabledifference within a given energy relationship, andthat such differences can be objectively measured.Normal observers are capable of distinguishing

a large number of colours whereas, according tosome authorities, the dichromat's colour world islimited to less than 30 discriminable hues. Thesesubjects confuse colours that are easily distinguishedby the man in the street. If someone mistakes oneprimary colour for another we use the term colourconfusion to indicate the gross nature of his mistake,but for those whose losses are less extreme we talkabout poor colour discrimination, recognizing thatthere are wide variations in this ability.

Colour confusion The C.I.E. chromaticity diagramis the most convenient way of showing the colourconfusions of dichromats. On this diagram twochromaticities may be distinguished by a normalobserver as long as the distance between them isgreater than a given small amount (Fig. 5), butprovided their luminosities are equal, the dichromatmay not be able to distinguish between them evenwhen the interval is greater. Distance for suchobservers is of no consequence. What really mattersis whether these chromaticities are aligned along theso-called isochromatic lines for the given dichromat.Note that these straight lines are his confusion loci(Fig. 6). Although the directions of these lines andpositions of their loci vary slightly from one subjectto another, the type of dichromat can always bedetermined and differences between types of defectare greater than any differences within one type.Thus, though the confusions for such defects arespecific, they are nonetheless systematic anddirectional. Figure 6 shows a slightly idealizedpicture of the confusion loci for deuteranopes,protanopes, tritanopes, and tetartanopes. For amore accurate presentation see the work of Konig(1903), Pitt (1935), Wright (1935, 1952), and Judd(1944).The set ofconfusion loci for each of the dichromats

have a common origin - the so-called centre ofconfusion or point of convergence. The co-ordinatesof each confusion centre are shown in the diagrams

I8-

0*4

02-

00 02 0.4 0-6.z

FIG. 5. Illustration of colour confusion, discriminationand saturation with reference to the C.I.E. diagram.

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Theory and practice of colour vision testing: A review Part1 187

c.0-'746y -0.253

= 04175y a 0-004

x

FIG. 6. Confusion loci, centre of confusion, and neutral axes for dichromats.

y0-8

0*7

0-6

0.5

0*4*

0-3

0-2

0*1*

0

0-8

0-7

0-6

0-5-

0*4-

0-3

0 2

0.1

0

DC.01*01=O*O

ox=0*0y=-0.

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188 R. Lakowski

and they may be looked on as theoretically rep-resenting the positions of the missing fundamentals.For each type of dichromat the confusion locuswhich passes through the neutral part of the colourspace (grey or the standard illuminant) and inter-sects the spectrum locus is called his neutral axis.This shows very simply what colours in the C.I.E.space a given dichromat will be unable to dis-tinguish from neutral and which part of thespectrum he will match with the standard white.Dichromats have no difficulties in finding such'neutral points' in the spectrum, whereas theanomalous and normal trichromats cannot do so.Each type of dichromat is characterized by a specificneutral band several nanometres in width (the meanvalues for the four dichromats are shown in Figure6). Note that there are two such neutral points in thevisible spectrum for the tetartanope but only oneeach for the other three dichromats.The position of the neutral axis also denotes the

broad axes of confusion, which for protanopes anddeuteranopes is red-green, while it is yellow-blue forthe tritanope and tetartanope. From the chromaticitydiagram we can see that what distinguishes protansfrom deutans is not only the slight difference in thewavelength of their neutral points (493 nm forprotanope and 497 nm for the deuteranope) but alsothe fact that protanopes confuse blue-green with redwhile the deuteranopes confuse blue-green withpurple. It is such subtle differences, for example inreds, that must be employed in P.I.C. test plates ifthey are to distinguish between deutan and protandefects (that is, to be qualitatively diagnostic).

In the case of dichromats, the confusions arecomplete but this is not so for the extreme anomaloustrichromats (E.P.A. or E.D.A.). In acquireddyschromatopsias involving the red-green axis,confusion loci are most often of the deutan type,while those for the yellow-blue axis resembletritanopes more closely. If a line is traced perpendic-ular to the neutral axis for each defect, we will seewhich colours each dichromat is still likely toexperience.Combining this knowledge with colorimetric

data obtained from colour vision tests, it is nowpossible to assess colour confusion theoretically fromthe chromaticity diagram, to predict whether it isintentional, and, if so, whether it is adequatelyrealized. In tests implicitly designed to detect anddiagnose dichromats, it is therefore possible toestimate whether the given design falls within thetheoretical boundaries of maximal colour con-fusion according to the tenets of colour confusiontheory.

Colour discrimination Perception of small colourdifferences in all parts of the colour space impliesgood discrimination. but even among normal

trichromatic observers there are individual differ-ences, some possessing very 'acute' discriminationwhile others are so lacking in this quality that itamounts to partial confusion. The method ofdetermining whether test elements lie on thedichromat's colour confusion loci does not provideadequate information about how difficult a test willbe for those whose colour losses do not amount tooutright dichromatism. A test situation that isdifficult for dichromats is not necessarily so for thosewith minor colour defects, and an overall index ofthe colour differences found in the task provides uswith a better measure. It is now possible to cal-culate these and also to scale the discriminationtask in order of difficulty - the smaller the colourdifference, the more difficult the discrimination task.Such predictions from distances on the C.I.E.

diagram cannot be made directly with any degree ofaccuracy or validity because the 1931 C.I.E.chromaticity space is not visually uniform andreferences to distances would not correspond withactual colour experience. It should also be remem-bered that no brightness differences can be inferredfrom such a diagram. However, when differencesexist only because of hue or saturation, colourdifference can be read off directly from a modifiedcolour space, the so-called uniform chromaticityscale (or U.C.S. diagrams) first devised by Judd in1935. But when there are also brightness differences,as is generally the case in most colour vision testtasks, colour differences have to be calculated fromthe original XYZ tristimulus values for the surfacesinvolved. There are a number of colour differenceformulae, but the one used for calculations here isthe Nickerson-Stultz formula (1942) in which themagnitude of colour difference, JIc, is expressed asa unit bearing the name of the American NationalBureau of Standards - the N.B.S. unit. One suchN.B.S. unit is equivalent to a colour difference ofabout 5 just noticeable differences (j.n.d.) as per-ceived by a normal observer under ideal viewingconditions. To give an idea of what such a unit lookslike, Jac between the colours of the April and Julyissues of this journal has been calculated (for theactual tristimulus values refer back to Table 1).These differences are very small indeed but stillmeasurable and zc between the two blue coversviewed under illuminant 'A' was found to be 1-5N.B.S. units, whereas under illuminant 'C' it wasslightly smaller, being 1-1 N.B.S. units. Althoughthe colour differences quoted are minute, they helpto illustrate the type of discrimination that would beexpected of colour workers in industry. In the plasticsindustry, the tolerances for colour matches are evenmore critical and only differences of 05 of anN.B.S. unit or less are acceptable (that is differencesinvolving only 2 to 3 j.n.d.). Such a task is very closeto the best colour resolution that can be expected of

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young people with perfect discrimination. In woollentextile industries higher tolerances are acceptablebut even here the largest acceptable Ac is onlybetween 3 to 6 N.B.S. units. Thus, tests capable ofselecting such colour workers must present an orderof difficulty far greater than is usually found in testsfor detecting colour confusions.

In the past, where individual stimuli might be only5 to 10 mm. in diameter, it was difficult to makespectrophotometric and colorimetric measurementsof surface colour. With certain modem instruments,however, reliable valid measurements can be made

(see Lakowski, 1964, 1965b, 1966) and thereforeobjective analysis is possible, and, by finding outwhat the colour differences in a test situation are, weare better able to choose the appropriate testcapable of presenting close steps in discrimination.For further readings on the topic of colour specifi-cations consult Le Grand (1968), Wright (1964),Stiles and Wyszecki (1967), and Judd and Wyszecki(1963).

The references for this paper will appear at the end ofPart 2 in the October issue of the Journal.Received for publication November 16, 1968.

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Documents

Theoryand practice of colourvision testing: A review Part 1