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`TCL 1023, Page 1
`TCL 1023, Page 1
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`

`HAZEL ROSSOTTI
`COLOUR
`
`Awow ko
`HAS
`
`Princeton University Press
`Princeton, NewJersey
`
`
`
`TCL 1023, Page 3
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`TCL 1023, Page 3
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`

`

`Published by Princeton University Press,
`41 William Street, Princeton, New Jersey 08540
`
`Copyright © 1983 by Hazel Rossotti
`All rights reserved
`First Pelican originaledition, 1983
`First Princeton Paperback printing, with corrections, 1985
`
`
`
`
`LOC 84-1145]
`ISBN 0-691 -08369-X
`ISBN 0-691 -02386 —7 (pbk.)
`
`Reprinted by arrangement with Penguin BooksLtd.
`Made andprinted in GreatBritain
`by Richard Clay (The ChaucerPress) Ltd,
`Bungay, Suffolk
`Set in VIP Times
`
`Clothboundeditions ofPrinceton University Press books are printed
`on acid-free paper, and binding materials are chosen for strength and durability.
`Paperbacks, while satisfactory for personal collections, are not usually suitable
`for library rebinding.
`
`CONTENTS
`
`List of Text Figures
`Foreword
`Preface to the Princeton Edition
`Introduction
`
`Part One: Light and Dark
`
`Ne
`
`. Light Particles
`. White Light on Clear Glass
`
`Part Two: Lights and Colours
`
`AunWw
`
`owes.
`
`. Steady Colours
`. Shimmering Colours
`. Special Effects
`. Lights
`
`Part Three: The Natural World
`
`. Air and Water
`. Earth and Fire
`. Vegetable Colours
`. The Colours of Animals
`
`Part Four: Sensations of Colour
`
`11
`. Light and the Eye
`12:
`Anomalous Colour Vision
`15,
`Colour Vision in Animals
`14.
`The Eye and the Brain
`15
`. Sorting and Recording
`
`Part Five: Technology
`
`16.
`17;
`
`Colour Reproduction
`Added Colour
`
`it
`12
`
`19
`26
`
`37
`
`48
`55
`
`65
`77
`84
`91
`
`109
`122
`126
`130
`143
`
`169
`185
`
`TCL 1023, Page 4
`TCL 1023, Page 4
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`

`

`
`18. Imparting Information
`19. Communicating Feelings
`20. Colour, Music and Movement
`21. Words and Colours
`
`Index
`
`Acknowledgements
`
`
`
`
`
`
`
`203
`209
`220
`222
`
`231
`
`239
`
`LIST OF TEXT FIGURES
`
`COIAMRYN
`
`Light waves
`Prismatic colours
`A home-madeprism
`From a rose window
`How light travels through glass
`How light bounces off glass
`The dark eyes of houses
`A single atom of matter
`Glass
`A metal
`. Interference of light
`. Printer’s blue and blood red
`. Oil patches and soap bubbles
`. Diffraction by twoslits
`. Diffraction by a grating
`. Polarization of light
`. Three combs
`A highly ordered fluid
`. Light sources
`. Laserlight
`. Colours from white light
`. White light and tiny particles
`. Distant blue
`. White light and droplets
`- Reflected colour in clouds
`. The Blue Grotto
`. Sunset
`. Colours from a dewdrop
`. A primary rainbow
`. A double rainbow
`
`. The Brocken spectre
`. How a glory occurs
`. Shiny graphite
`. Fine blue, damp pink
`
`TCL 1023, Page 5
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`TCL 1023, Page 5
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`

`

` . Leaf green
`
`175
`178
`206
`208
`223
`225
`226
`
`TCL 1023, Page 6
`TCL 1023, Page 6
`
`List of Text Figures
`
`77. Phosphor dots
`78. Colour negatives and prints
`79. Code for resistors
`80. Coloured signals
`81. Evolution of colour words
`82. Heraldic tinctures
`83. Names for colours
`
`
`
`
`List of Text Figures
`
`. Carrot orange
`. Delphinium blue
`. The colourful juice of pickled red cabbage
`. Butterfly wings
`. Insect iridescence
`. Blue birds
`. How animals change colour
`. The human eye
`. A light twist
`. Changesin the retina
`. Brightness by night
`. Brightness by day
`. Cones and colour: a possible mechanism
`. Blue-green ambiguity
`. A bee’s-eye view?
`. Colours which recede or advance
`. The corner of the eye
`. Connections for contrast
`. Stereo
`. Brain waves
`. Coloured tops
`. Sidney Harry’s top
`. Colours in space
`. Ostwald’s coloursolid
`. Munsell’s tree
`. Comparison of saturation
`. Mixing lights
`. Labelling with filters
`. Tops for colour measurement
`. A good match?
`. Maxwell’s triangle
`. ‘Negative’ colours
`. CIE tongue diagram
`. Purple and white
`. Dominant wavelength
`. Colours and complementaries
`. Brightness
`. Some commoncolours
`. Colour and temperature
`. The flight of colours
`. Colour television
`
`

`

`
`
`142
`
`Sensations of Colour
`
`him from the donkey at that particular momentthan to the average colour
`which the donkey would have if viewed by a layman in diffuse light.
`It is not surprising that the ability to look analytically can be cultivated by
`willpower and training; nor that those without training often prefer a
`naturalistic picture which represents colours as they remember them to
`those in whichtheartist tries to reproduce the actuallight coming from the
`object. In much the same way, an observerwill often select, as the most
`‘natural’ colour photograph, one in which the colours are actually brighter
`than those in the original scene. Colours selected to match memories from
`dreams, however, are often paler than those of similar objects we observe
`when awake.
`Colour sensations may often be even more dramatically altered by, or
`even produced by, factors other than light. Some forms of hysteria and
`hallucinogenic drugs enhance appreciation of colour: hallucinogens often
`provide experience of brightly coloured patterns which bearlittle relation
`to the real world, and those aboutto suffer attacks of epilepsy or migraine
`may see coloured rays and geometrical shapes before an attack. The blind,
`particularly if they are old, may have similar sensations, akin to seeing
`‘golden rain’ and coloured patterns. In childhood, a pressure on a closed
`eye was enough to produce patterns as exotic as Catherine wheels or
`peacocktails. In adult life, a firmer touch, preferably on the upperpart of
`the eye, is needed to produce an inferior but nonethe less impressive result.
`Electrical and mechanical stimulation of the optic nerve andthe visual areas
`of the brain can makeus see colours; so can some acuteillnesses, and evena
`strong magnetic field. And so, of course, can memory and dreams. But
`although we canexperience an immense variety of colour sensations pro-
`ducedin these different, non-visual, ways, we haveas yetonly a negligible
`understanding of any of the mechanismsinvolved.
`
`15
`
`SORTING AND RECORDING
`
`To what extent can we record a colour? Can we impose any order on our
`rich variety of colour sensations? If we can, would our schemebe entirely
`personal, or could we use it to communicate information about a colour?
`How can webesttell someonethe exact colour we should like the new door
`to be?
`As weshall see in Chapter 21, there are manydifficulties in trying to
`describe colours with words. A request to paint the door turquoise would be
`likely to produce a fairly bright door in the greenish-blue (or might it be
`bluish-green?) range, neither very pastel nor very murky. Perhaps we could
`get nearer to the colour we have in mind by a request to paint the wall the
`same colour as the curtains. But, though the two may seem a good match in
`one light, they may clash horribly in another (see page 152). And as they
`will have different textures, they will have different highlights; so although
`the colours of the two may ‘go’ very satisfactorily, they will certainly not
`matchall over, even if the light falling on them happensto be identical.It is
`safer to choose from samplesof the actual paint, on the manufacturer’s own
`colour card. But maybe the exact colour requiredis not available. ‘Please
`mix me...’ How does one continue? ‘Something between these two’;
`‘Something like this, only lighter’; ‘A more subtle shade of that’; ‘A bluer
`versionofthis one.’It is difficult to know howto specify, or how to produce,
`the colour required. And it may even be difficult to envisage precisely any
`colours which are not on the colour card.
`Perhapsit would help if we could arrange colours in somerational order
`and label them appropriately. We could then refer to them in much the
`same way as we can pinpoint a place by a map reference. Many child-hours
`must be passed in just this way, rearranging crayons, pastels or embroidery
`threads. It is easy enough to makea lineof the rainbow colours, and join the
`ends, through purple, to give a circle. But problems soon arise. Do black,
`grey and white count as colours? If they are to be included, where should
`they go? What should be donewith pale colours: primrose, duck-egg blue,
`salmon? And what about the browns? One can imagine a cross-roads at
`yellow, with primrose leading to white on one side, and ochre leading to
`khaki, brown and black on the other. Perhaps, however, black and white
`should be joined through the greys, to give anothercircle, perpendicular to
`
`TCL 1023, Page 7
`TCL 1023, Page 7
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`

`

`Sensations of Colour
`
`Sorting and Recording
`
`145
`
`
`
`Yellow
`
`
`
`Pe
`
` 144
`
`brightness (or value). The so-called ‘natural’ or ‘achromatic’ colours, black,
`the first. It seems we cannot arrange our coloured objects onaflat surface,
`but need some three-dimensional scheme.
`grey and white, are of zero saturation, and differ from each other only in
`Maybeit would be better to seek a more‘scientific’ classification than
`brightness. A series obtained by adding onehue,say blue, to white differs
`any subjective arrangement of coloured materials? We might try to specify
`only in saturation, as does a series obtained by adding a blue pigmentto a
`the colour of a sample byirradiating it with the light of a large numberof
`grey one. If the same pigment were added to a white one in the same
`very narrow bands of wavelength and measuring the percentage of each
`proportions, the dusky, pale blue would differ from the clear pale blue only
`which the sample reflects. These measurements can be made extremely
`in brightness.
`easily, given the appropriate equipment. But the light which enters the eye
`There are many three-dimensional arrangements of colours, the best
`dependsonthe lighting as well as on the sample, so we would also need to
`knownbeing those devised by Munsell and by Ostwald. Both are based on
`know the composition of the illumination. Even this does nottell us the
`the colour circle formed by joining the two endsof the spectrum through
`colourof the sample unless we know howtheeye reacts to lightofdifferent
`purple (see Figure 58). So the hue changes around the circumference of the
`wavelengths. Two materials may match exactly underone type ofillumina-
`circle, much as the hours progress aroundthe face of a clock. Through the
`tion even if they sendlight oftotally different composition to the eye; we
`centre of the clock face, and perpendiculartoit, like the axle of a wheel,
`knowthat manyyellows can be matched by mixturesof red and greenlight.
`runsthe line representing the neutral colours, usually with white at the top,
`We might, however, combine, for each narrow band of wavelength,
`changing, through deepeninggreys, to black at the bottom. Radially, like
`measurementsofthe reflecting powers of the sample, and the composition
`spokes on a wheel, the saturation increases towardsthe rim, to give a space
`of the light source with our knowledge of the response of the retina.
`which can be filled in by different colours, according to which system is
`Although this procedurestill needs laboratory equipment,it relates the
`being used.
`scientific measurements of the light reaching us to our perceptions of
`colour, assuming that the observer has normalcolour vision, adapted for
`daylight viewing and uninfluenced by the effects (such as after-images,
`contrast of near-by colours, memory, expectation) which we discussed in
`the previous chapter. So we are attemptingto chart,not just the stimulusof
`the light entering the eye, but a normal observer’s response toit. The idea of
`attempting to measure colour in this way soundsattractive, if somewhat
`Vivid colour
`complex. But do its advantages always outweigh those of map references
`eco+»Saturation (chroma)
`with an ordered arrangement,albeit subjectively chosen and represented in
`three dimensions? Since both systems are usedin practice, we shall look at
`each in more detail.
`If we are to arrange a numberofcolours in any systematic order, we must
`decide whatqualities we shall use to sort them.Let usfirst recall the ways in
`which light can vary. The sensation of colour depends primarily on the
`composition of the light, and partly on the intensity; and the composition
`maybe usually described as a mixture, in a certain proportion,of white light
`with a ‘coloured’ light of a particular dominant wavelength within the
`visible spectrum. (For purplelight, the ‘coloured’ componentis itself a
`mixture.) We can describe the primary sensationof colourin terms ofhue,
`which refers to the greenness, blueness and so forth, and varies with any
`changein the dominant wavelength. The extent to which this wavelengthin
`fact dominatesthe light is knownas saturation (or chroma). As the domin-
`ant wavelength is diluted with white light, the saturation decreases. An
`increase in intensity in light of a particular composition increases the
`
`Green
`
`Blue
`
`
`
`
`Black
`
`Figure 58. Colours in space The skeleton ofa colour solid. The ‘achromatic’ colours
`formthe vertical backbonefrom which the different hues radiate: red in one direction,
`green in the opposite one, and the others in between, For any one hue, the colour
`becomes more vivid the fartherit is from the centre.
`
`Ostwald arranged his colours in a double cone, based on twenty-four
`different hues, arranged around the circumference (see Figure 59), Each
`hue is combined, in a number of fixed proportions, with each of eight
`
`TCL 1023, Page 8
`TCL 1023, Page 8
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`

`

`146
`
`Sensations of Colour
`
`Sorting and Recording
`
`147
`
`
`
`(a)
`
`11
`
`Full saturation at edge
`
`Planes of constant dominant
`wavelength (hue)
`
`numbered on a grid system, so that any colour contained by the solid can be
`specified by a map-reference.
`In Munsell’s arrangement, saturationis increased byaseries of visually
`equal steps rather than by addinga fixed proportion of pigment; and there
`are nine neutral colours, rather than eight. As the numberof equal steps of
`saturation at a particular hue and brightness dependson the hue, the arms
`are of different length in different parts of the solid. Munsell’s solid is
`therefore much less regular than Ostwald’s, and on account ofits untidy
`appearanceis knownas a colour‘tree’. A typical vertical section throughit
`is shown in Figure 60. Each position in the tree, as in Ostwald’s cone, is
`encoded, thereby allowing colours to be specified. And the tree has one
`great advantage over Ostwald’s solid: whenever some new,dazzling pig-
`ment is made, it may be incorporated by extending an existing branch.
`
`
`
`Hue (1)
`Hue(2)
`Full
`sina
`colour/(complementary to (1))
`
`(b)
`
` Full
`
`Grey axis
`
`Figure 59. Ostwald's colour solid (a) Exterior view, (b) Vertical section.
`(Adapted, with permission, from G.J. Chamberlin and D. G. Chamberlin, Colour: Its
`Measurement, Computation and Application, Heyden, London, 1 980.)
`equally spaced neutral colours from white to black. The resulting colours
`are arranged so that brightness decreases vertically towards the bottom of
`the diagram, while saturation decreases towardsthe centre. Thus Ostwald’s
`coloursolid consists of twenty-four triangles (one for each hue), arranged
`radially so that a vertical section through it gives two such triangles, for
`complementary hues, fused at the centre, Each position within the solid is
`
` Hue (1)
`
`Black
`
`Hue (2)
`(complementary to (1))
`
`Figure 60. Munsell’s tree Vertical section (cf. Figures 59(b), page 146, and 83, page
`226),
`
`The two systems resemble each otherin that the circumference is divided
`arbitrarily into hues, and the vertical axis is graduated into visually equal
`steps, which are obtained by asking large numbers of observers to estimate
`equal differences in brightness. But the two solids are based on different
`waysof varying saturation. While Ostwald usedthe ratio of neutral pigment
`to saturated pigment, Munsell again invoked visual assessment by the
`average observer.
`When Ostwald devised his colour solid, he was an old man whose sensi-
`tivity to blue was doubtless declining, which accounts for the slight com-
`pression in the blue region of the circumference. Swedish workers have
`attempted to correct this defect in Ostwald’s system by placing each of
`the four psychological primaries at right angles to one another on the
`
`TCL 1023, Page 9 —
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`TCL 1023, Page 9
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`

`

`149
`
`Sensations of Colour
`
`Sorting and Recording
`
`{b)
`
`
`
`
`
`Transparentplunger——
`
`|easetSeasesEEE
`,
`;
`.
`th
`fa
`cstancaael
`vil
`eviigine’
`Figure 61. Comparison of saturation A fixed depth ofthe sample is viewed with one
`eye and a variable length ofstandard with the other. The geometry ofthe instrumentis
`adjusted until the same depth of colour is observed by each eye.
`(a) Sample.
`(b) Variation of depth of standard by means oftransparent plunger.
`(c) Variation of depth of standard by use of wedge.
`(d) Split field, one halffrom each eyepiece.
`In the top two diagrams, the same depth used for sample and standard gives a darker
`field on the right. In the lower diagram,the length(1) ofstandard has beenadjusted so
`that the two fields are indistinguishable. In this example, d= 21, indicating that the
`Saturation of the sample is half that of the standard.
`
`
`
`Horizontalacan
`
`
`
`MovingeyepieceM
`
`\,
`
`
`
`
`
`FixedeyepieceF| Fixeddepth,d,ofsampleE
`
`{c)
`
`(d)
`
`(a)
`
`|
`
`circumference and dividing the segments between them intovisually equal
`steps, constituting a more ‘natural’ colour circle for use as the basis of a
`three-dimensional coloursolid.
`Althougha colour solid is a useful concept, and may even be constructed
`as a display objectof great visual and intellectual appeal, either swatches or
`books are more convenient for everyday use. Colour atlases, such as the
`Munsell Book ofColour, often represent vertical sections of a coloursolid,
`cut through each hue represented. For more specialist use, a restricted
`range of colours, varying by smaller gradations, may be reproducedas in
`collections for those who wish to specify the precise colour of a rose petal or
`a sample of humanskin or tooth.
`Such visual matching of colour is a stepwise process, a placing of the
`sample of unknown specifications between two standard colours whose
`specifications are known. But how can wetry to measure the colour specifi-
`cations of a material if we have no standard colour which matches it? What
`can we do to try to measure ‘colour’, to produce specifications of hue,
`saturation and brightness?
`Since colour is a sensation, thereis a lot to be said for the measurements
`being made by the eye. The humaneyeis, in fact, an excellent detector of
`differences of hue, and many people can assess the percentage of red, blue
`and yellow in a pigment with surprising precision. But humanestimates of
`saturation, and of brightness, are muchless reliable.
`The mostprecise visual methodsof attemptingto specify colours,like the
`use of a colour atlas, involve matching. The simplest are those devised
`merely to measuresaturation, as in the determination of the concentration
`of a single coloured componentin a liquid (see Figure 61).
`If the two solutions appear to be the same colour, the ratio of their
`concentrations is simply related to the ratio of the length of the two
`solutions through which the light has passed; and this may easily be found
`using a simple comparator involving either a plunger or a wedge.
`
`TCL 1023, Page 10
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`TCL 1023, Page 10
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`

`

`
`
`Sorting and Recording
`151
`magenta, one yellow and one blue-green (see Figure 63). Sets of suchfilters
`are available commercially for use in an instrument equipped with a stan-
`dard light source and knownasthe Lovibond Tintometer. Thefull range of
`250filters of different depth for each of the three hues allows nearly nine
`million different colours to be obtained,
`including the full range of
`achromatic colours from white to black. The colourof the sampleis readily
`specified in terms of the three filters used to match it.
`MBG Y
`
`150
`
`Sensations of Colour
`
`For many purposes, however, we need to know the hue, as well as the
`saturation. We again compare our sample with a colour which we can
`specify. But how can wevary this colour whilststill being able to specify it?
`One way is to mix coloured lights of known wavelength in known pro-
`portions. A simple arrangementis shownin Figure 62. The amountofred,
`blue and green light can be varied by horizontaland vertical movement of
`the filter assembly over the source of light, and the three lights are then
`mixed by diffusion and multiple reflections. More sophisticated devices,
`used mainly for research on colour,
`involve six lights of varying
`wavelengths. In each case the mixture of coloured lights, shone on to a
`white background,is matched with the unknown sample, illuminated from
`a standard source of white light.
`Alternatively, the sample may be matched by a patch of colouredlight
`which has been obtained by passing white light through threefilters, one
`
`
`
`Ae2773N
`
`
`
` Red filter
`
`
`N
`
`Greenfilter
`
`Figure 62. Mixing lights The required mixture of red, blue and green light may be
`— by adjusting that area ofeach filter which lies overtheaperture to the mixing
`OX.
`
`Standard
`
`
`white lightNN 1—
`
`Sample
`Figure 63. Labelling with filters The colour ofthe sample can be matched with that of
`light which has passed through three Lovibondfilters, magenta (M), blue-green (BG)
`and yellow (Y) ofspecified strength. A purplish blue sample, for example, would need
`a deep magenta (to absorb mostofthe green), a medium blue-green (to absorb some,
`but notall, ofthe red), and yellow ofappropriate depth to reduce the intensity ofthe
`colour to that of the sample.
`Instead of matchingthe light which reaches us from a sample with that
`from an unknown,we can exploit the phenomenonof persistence ofvision
`and match only the sensations.Split discs, coloured in saturated blue, green
`and red, are placed on a revolving platform in such a way that the pro-
`portions of the three colours can be varied (see Figure 64). When the
`platform is spun at high speed, the sensations merge, just as if three
`coloured lights were superimposed. The areas of the three colours are
`adjusted until the colour of the spinning disc exactly matches that of the
`sample, which can then be expressed in terms of the proportionsof primary
`colours used.
`Nowadays, there is an increasing tendency to use photoelectric instru-
`ments for colour matching, instead of the eye of one or twoindividual
`observers. Such instruments monitor narrow bandsover the whole visible
`spectrum,first detecting how muchlight in each band reaches them from
`TCL 1023, Page 11
`TCL 1023, Page 11
`
`

`

`
`
`Sorting and Recording
`
`153
`
`152
`
`Sensations of Colour
`
`the sample, and then converting this information into the size of the
`stimulus which bombardseach of the three cone systemsin a normal human
`eye. Finally, the responses to these three stimuli are combinedto give the
`colour experienced by the ‘standard observer’. Inescapably, these instru-
`ments give results based on sensations experienced as a consequence of
`humanvision. But the vision is that, not of a few individuals, but of the
`“standard observer’ built up from observations made bya large numberof
`individuals selected for their normality of vision. But even when the match-
`ing is done, rather approximately, by eye, it is seldom left for a single
`individual: more often two, or even three, observers are used.
`
`(a)
`
`(b) oa
`
`
`
`Figure 64. Tops for colour measurement(a) Split circular disc ofpaper ofstandard
`colour. (b) Standard green, blue and red interlocked, exposing known areas ofeach for
`colour mixing when the disc is rotated.
`
`Matching, whether by eye or machine, often gives different results with
`different sourcesof light; and as matching implies identity only ofresponse,
`this is no surprise. Two extreme examples ofidentical colours produced by
`light of very different composition were given in Table 3 (page 119).
`Imagine a pair of yellow pigments,one reflecting light of only 580 nm and
`one reflecting only light of 540 nm and 630 nm, eachat half the intensity of
`the first pigment. If both were illuminated with light containing equal
`intensities of the three wavelengths, the two pigments would match exactly.
`But if they were illuminated with light containing a slightly higher pro-
`portion of longer wavelengths, the second pigment would look redder than
`the first. Such ‘metameric’ pigments, which match only under onelight
`source, are the bane of those who try to match clothes and accessories, or
`paint and fabrics (see Figure 65). Metamerism accounts for much of the
`popularity of ‘colour coordinated’ goods sold by the same manufacturer
`
`whouses dyessuch that any changein illumination causes almost the same
`change in colour for the different materials.
`Colour solids and atlases can give us no information about the compost-
`tion of the light which causesa particular colour. For charts relating colour
` 016
`
`Relative
`
`O12
`
`reflectance
`
`400
`
`500
`
`600
`
`700
`
`Figure 65. A good match? Thereflectance spectra oftwo fabrics which match perfectly
`in daylight. (Reproduced, with permission, from W. D. Wright, The Measurementof
`Colour, 4th edition, Adam Hilger, London, 1969.)
`Green
`
`Wavelength (nm)
`
`
` Unsaturated greenish-yellow
` Blue-green
`
`Blue-violet
`Magenta
`Orange-red
`Figure 66. Maxwell's triangle Shows how many, but notall, colours can be represented
`as a mixture of three primarycoloured lights. The nearer a point ts to an apex of the
`triangle the higheris the proportionoflight ofthe colour represented by that apex. The
`point X (50 per cent green, 35 per cent orange-red and 15 per cent blue-violet)
`represents an unsaturated greenish yellow.
`
`TCL 1023, Page 12
`TCL 1023, Page 12
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`154
`
`Sensations of Colour
`
`Sorting and Recording
`
`155
`
`+25
`
`+20
`
`+15
`
`
`
`Green (546)
`
`
`
`These ‘positive’ colours
`when mixed together
`
`to composition, we turn to the second, more‘scientific’ approach. Just as
`
` Blue (436)
`manycolour solids are based onaring ofspectral colours, joined through
`purple, a triangle usually forms the basis of attempts to chart the colours
`Red (700)
`produced by the mixing of lights. As early as 1855, Maxwell found that a
`great numberof colours could be produced by mixing lights of only the
`three ‘primary’ colours: orange-red, green and blue-violet. The colour
`resulting from a particular mixture can be represented by a point on a
`triangular grid (see Figure 66). Many colours can be specified in this way:
`but not all. Whichever three primary sources we choose, there are always
`somecolours (including many pure spectral ones) which cannot be rep-
`resented by a point in, or on, the triangle; which confirms that we cannot
`always match one colour by a mixture of three others unless we allow
`ourselves the option of mixing one of the primary colours with the sample
`and then matching the result with a mixture of the two otherlights. Figure
`67 gives the recipe for obtaining a matchfor every visible wavelength with
`three primaries. Thus vivid yellow (570 nm) (cf. page 119) can never be
`exactly matched by red (700 nm) and green (546 nm); but if a little blue
`(436 nm) is added to the yellow, a perfect match can be made. We can
`express this algebraically by stating that vivid yellow can be matchedbyred,
`green and a small negative amount of blue. But since there is no scope for
`plotting negative contributions on a Maxwell triangle, colours such asvivid
`yellow cannot be represented onit.
`It is too bad that we cannot choose any three wavelengths which, when
`themselves mixed together, will produce all visible colours. But there is
`nothing to stop us imagining that such ideal primary colours might exist;
`‘and if they did, they could be mixed in such a way as to produce three
`convenient teal primaries, such as Maxwell used. So we could draw a
`mathematical modification of Maxwell's triangle, with our three imaginary
`primaries at the corners. Points for the real ones, and for all other colours,
`would then be within it. Three such imaginary primaries have indeed been
`devised, such that any real colours can be represented as a mixture of the
`appropriate amounts of the three of them, and plotted as an idealized
`version of the Maxwell triangle. But not everyone is used to triangular
`graphs and the ruled paper may not be easy to obtain. Could we not use
`squared paper instead? How manyvariables do we need? Since we have
`decreed that a real colour R can be matched by a mixture of our three
`Imaginary Primary Lights—say, I units ofone, P units of the second and L of
`the third —it might look as though we need thethreevariables, I, P, and L, to
`specify R. But we could also say that the brightness ofthelight is the sum S
`of the three primaries (so S=1+P+L), mixed in theratio of 1/S:P/S:L/S.
`And if we know I/S and P/S we already know L/S, because
`1/S+P/S+L/S=1. Since we are often more interested in colour than in
`
`
`
`+1-0
`
`+05
`
`Pos
`0 a
`
`MATCH
`
`-05
`
`mixture of pure patch and
`these ‘negative’ colours
`
`500
`
`400
`
`700
`600
`Wavelength of pure patchoflight (nm)
`Figure 67. ‘Negative’ colours Many pure spectral colours can be exactly matched with
`three primaries only by use of‘negative’ colours. When oneprimary is mixed with the
`‘pure’ patch oflight, this mixture can be matched by some mixture 0fithe other two. For
`theprimary lights used here, the only colour which can be matched directly by the three
`primaries is a greenish yellow in the region of550.nm. (Reproduced, with permission,
`from F. W.Billmeyer and M.Saltzman, Principles of Color Technology,Interscience,
`New York, 1966, p. 33.)
`brightness, we could concentrate on the two quantities I/S and P/S which
`specify the colour. We could then use ordinary squared graph paper. If we
`ever needed to know L/S,it would be easy enoughto calculateit. And if we
`decided that, after all, we wanted to specify the brightness, we could
`representit on an axisrising vertically, out of the paper. The final diagram
`would be muchlike a plan, with the two specifications of colour running
`north-south and east-west; or,if brightness is added,like a map which also
`showsheights, or isobars (or any third variable) superimposed onthe plan
`as a series of contours.
`ae
`So all that we need in order to specify the colour of an object is a
`knowledge of the composition of the lightfalling on it, of the modification
`
`TCL 1023, Page 13
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`TCL 1023, Page 13
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`
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`156
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`Sensations of Colour
`
`Sorting and Recording
`
`157
`
`of the light by it, of the response of the normal humaneye, andof the
`quantities of three imaginary primary lights which would produce the same
`response; and a piece of ordinary graph paper. The Commission Inter-
`nationale de l’Eclairage (CIE), in 1931, defined the standard observer and
`three possible standard sources; and they produced tables showing the
`relationship between the observers’ response and the quantities of the
`imaginary primary lights which wouldin theory be needed to producethem.
`
`?
`
`The tongue-shaped curve(see Figure 68) in the graph showsthe specifica- 100% primary P
`
`‘ideal green’
`
`p
`
`08
`
`0-6
`
`0-4
`
`02
`
`0
`100%primaryL
`—_
`:
`Figure 68. CIE tongue diagram The tongue enclosesall visible colours, with the pure
`spectral ones lying alongits curved edge. The inner triangle encloses those colours
`obtainable by mixing real primaries 436 nm, 546nm, 700 nm (i.e. those enclosed in
`Maxwell's triangle of Figure 66, page 153).
`
`0-8
`100%primary | Siete?
`
`0
`
`:
`
`:
`
`08
`
`Figure 69, Purple and white The point Efor p =0-33, i =0-33 (and sol =4=0-33)is
`‘equal energy’ white. Purples lie along the baseline (see text).
`
`tions of the pure spectral colours as their values of i=I/S and p=P/S.
`(Modified definitions of the standard observer and the standard sources,
`introduced by the CIE in 1967, change only-details on the graph.) The
`diagram, known as a CIE chromaticity curve, has been used to depict
`colours, and the relationship between them, in a wide variety ofsituations.
`ee
`s usefulness of the CIE diagrams arises from the fact that
`The enormoususefulness ¢
`8
`a
`mi
`:
`i
`i
`e
`line joining
`the
`ce
`mecan representa mixture of twolights as a point on cation :
`points which specify them. So pu rples, formed by HAINTed and
`Clue, Ne On
`the base-line of the ‘tongue’ (see Figure 69). A mixture of two parts red
`
`|
`
`TCL 1023, Page 14
`TCL 1023, Page 14
`
`

`

`
`
`
`
`0-4
`
`o2
`
`0
`
`02
`
`04
`
`06
`
`08
`
`Pre
`en
`dominant wavelength D with white light of composition represented by E.
`Tofind which is the dominant wavelength, we rememberthat X mustlie on
`a line joining E and D.So we drawaline from E to X and continueit untilit
`meets the curve, at the dominant