OWES 1023, Page 1
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`TCL 1023, Page 1
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`LOWES 1023, Page 1
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`

`

` nT
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`I318127995| 00474
`
`
`
` RANEGIE
`Ml
`
`Camegie Library of Pittsburgh
`
`ls a free public [ibrary, mulntoined by the
`Giy of Pittsburgh ond the County of Alle~
`gheny, with
`sopplemental
`oppropriations
`from the State of Pennsylvania
`
`(2)
`
`TCL 1023, Page 2
`
`Carnegie Library of Pittsburgh
`4400 Forbes Ave.
`
`Ly
`495
`| 287
`
`| 1985;-
`
`LOWES 1023, Page 2
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`

`

`eae
`=Soe
`
`
`
`HAZEL ROSSOTTI
`
`COLOUR
`
`Qe
`49.5
`R21
`\AS5x
`cog’ {
`
`Princeton University Press
`Princeton, New Jersey
`
`TCL 1023, Page 3
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`LOWES 1023, Page|3
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`LOWES 1023, Page 3
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`

`

`Published by Princeton University Press,
`4! William Street, Princeton, New Jersey 08540
`
`Copyright © 1983 by Hazel Rossotti
`All nghts reserved
`First Pelican original edition, 1983
`First Princeton Paperbackprinting, with corrections, 1985
`
`Loc 44-1451
`ISBN 0-691 -08369-X
`ISBN 0-691 - 02386 -7 (pbk.)
`
`Reprinted byarrangement with Penguin Books Ltd,
`Made and printed in Great Britain
`by Richard Clay (Tie Chaucer Press) Ltd,
`Bungay, Suffolk
`Ser in VIP Times
`
`Clothbound editions 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
`
`bao
`
`. Light Particles
`. White Light on Clear Glass
`
`Part Two: Lights and Colours
`
`Atn&
`
`22o~J
`
`. 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.
`12.
`13.
`14.
`15:
`
`Light and the Eye
`Anomalous Colour Vision
`Colour Vision in Animals
`The Eye and the Brain
`Sorting and Recording
`
`Part Five: Technology
`
`16.
`
`Colour Reproduction
`
`11
`12
`13
`
`19
`26
`
`37
`
`48
`55
`
`65
`77
`84
`91
`
`109
`122
`126
`130
`143
`
`169
`
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`

`

`
`
`Contents
`
`Part Six; Uses and Links
`
`. Imparting Information
`. Communicating Feelings
`. Colour, Music and Movement
`. Words and Colours
`
`Index
`
`Acknowledgements
`
`203
`209
`220
`222
`
`231
`
`239
`
`LIST OF TEXT FIGURES
`
`Light waves
`Prismatic colours
`
`REECESy,aeFe)bo)me
`
`A home-made prism
`From 2 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
`. Laser light
`. 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
`
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`LOWES 1023, Page 5
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`
`
`List of Text Figures
`
`. Leaf green
`. Carrot orange
`- Delphinium biue
`. The colourful juice of pickled red cabbage
`. Butterfly wings
`. Insect iridescence
`. Blue birds
`. How animals change colour
`3. The human eye
`. A light twist
`. Changes in 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 colour solid
`. 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 common colours
`. Colour and temperature
`. The flight of colours
`
`133
`134
`136
`138
`139
`145
`146
`147
`148
`150
`
`152
`153
`153
`155
`
`157
`159
`161
`163
`
`165
`166
`
`List of Text Figures
`
`. Phosphor dots
`. Colour negatives and prints
`. Code for resistors
`. Coloured signals
`. Evolution of colour words
`. Heraldic tinctures
`. Names for colours
`
`175
`178
`206
`208
`223
`225
`226
`
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`
`
`142
`
`Sensations of Colour
`
`him from the donkey at that particular moment than to the average colour
`which the donkey would have if viewed by a laymanin diffuse light.
`It is nol 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 which theartist tries to reproduce the actual light coming from the
`object. In much the same way, an observer wil) 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 about to 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 secing
`‘golden rain’ and coloured patterns. In childhood, a pressure on a closed
`eye was énough to produce patterns as exotic as Catherine wheels or
`peacocktails. In adult life, a firmer touch, preferably on the upper part of
`the eye, is needed to produce an inferior but none the less impressiveresult.
`Electrical and mechanical stimulation of the optic nerve andthe visual areas
`of the brain can make us see colours; socan some acuteillnesses, andevena
`strong magnetic field. And so, of course, can memory and dreams. But
`although we can experience an immense variety of colour sensations pro-
`ducedin these different, non-visual, ways, we haveas yet only 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 vanety 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 shouldlike the new door
`to be?
`As we shall see in Chapter 21, there are many difficulties 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 asthe 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 happens to be identical. It is
`safer to choose from samples ofthe actual paint, on the manufacturer’s own
`colour card. But maybe the exact colour required is 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
`version ofthis one.’It is difficult to know how to 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 some rational oruer
`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 passedin just this way, rearranging crayons, pastels or embroidery
`threads. Itis easy enough to makea line of 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 done with 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
`
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`144
`
`Sensations of Colour
`
`Sorting and Recording
`
`145
`
`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 one hue,say blue, to white differs
`any subjective arrangementof 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 by irradiating it with the light of a large number of
`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
`depends on the lighting as well as on the sample, so we would also need to
`known being those devised by Munsell and by Ostwald. Both are based on
`know the composition of the illumination. Even this does not tell us the
`the colourcircle formed by joining the two ends of the spectrum through
`colour of the sample unless we know howthe eye reacts to light of different
`purple (see Figure 58). So the hue changes aroundthe circumference of the
`wavelengths. Two materials may match exactly underone typeof illumina-
`circle, much as the hours progress around the face of a clock. Through the
`tion even if they sendlight of totally different composition to the eye; we
`centre of the clock face, and perpendiculartoit, like the axle of a wheel,
`know that manyyellows can be matched by mixtures of 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
`measurementsof the 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 befilled in by different colours, according to which system is
`Although this procedure still 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 normal colour 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 attempting to chart, not just the stimulus of
`the light entering the eye, but a normal observer's response to it. The idea of
`attempting to measure colour in this way sounds attractive, if somewhat
`complex. But do its advantages always outweigh those of map references
`with an ordered arrangement,albeit subjectively chosen and represented in
`three dimensions? Since both systems are used in practice, we shall look at
`each in more detail.
`If we are to arrange a numberof coloursin any systematic order, we must
`decide what qualities 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
`may be 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 purple light, the ‘coloured’ componentis itself a
`mixture.) We can describe the primary sensation ofcolourin termsofhue,
`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 known as saturation (or chroma), As the domin-
`ant wavelength is diluted with white light, the saturation decreases. An
`
`aS
`a
`Black
`
`Figure 58. Colours in space The skeleton ofa colour solid. The ‘achromatic’ colours
`form the vertical backbone fram 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 farther it is from the centre.
`
`TCL 1023, Page 8
`
`Ostwald arranged his colours in a double cone, based on twenty-four
`different hues, arranged around the circumference (see Figure 39). Each
`
`i
`
`)
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`LOWES 1023, Page 8
`
`
`
`cot
`Pale
`ecolour
`Murky colour
`
`Vivid colour
`
`
`aa ~
`
`Green
`
`dlue
`
`7
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`LOWES 1023, Page 8
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`

`

`
`
`Sensations of Colour
`
`Planes of constant dominant
`
`146
`{a}
`
`(b}
`
`Full saturation at edge
`
`wavelength {hue}
`
`
`Sorting and Recording
`
`147
`
`numbered ona grid system, so that any colour contained by thesolid can be
`specified by a map-reference.
`In Munseli’s arrangement, saturation is increased by a series ofvisually
`equal steps rather than by adding a 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 depends on the hue, the arms
`are of different length in different parts of the solid. Munsell’s solid is
`therefore much fess regular than Ostwaid’s, and on accountofits untidy
`appearance is known asa colour ‘tree’, A typical vertical section through it
`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 (t}
`
`Blach
`
`Hue (25
`{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
`ways of varying saturation. While Ostwald usedthe ratio of neutral pigment
`to saturated pigment, Munsell again invoked visual assessment by the
`averape 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
`
`TCL 1023, Page 9
`
`
`
`Hue(2)
`Full
`colour¢(CcOMpiementary to {1)}
`
`
`
`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:lts
`Measurement, Computation and Application, Heyden, London, £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 towards the 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
`
`LOWES 1023, Page 9
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`

`

`149
`
`Sorting and Recording
`
`Sensations of Colour
`
`
`
`MovingeyepieceM
`
`Horizontal
`
`ic}
`
`circumference and dividing the segments between them into visually equal
`steps, constituting a more ‘natural’ colourcircle for use as the basis of a
`three-dimensional colour solid.
`Although a colour solid is a useful concept, and may even be constructed
`as a display object of great visual and intellectual appeal, either swatches or
`books are more convenient for everyday use. Colour atlases, such as the
`Munsell Book of Colour, often represent vertical sections of a colour solid,
`cut through each hue represented. For more specialist use, a restricted
`range of colours, varying by smaller gradations, may be reproduced as in
`collections for those who wishto specify the precise colourof a rose petal or
`a sample of human skin 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 colourspecifi-
`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, there is a lot to be said for the measurements
`being made by the eye. The humaneye is, 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 human estimates of
`saturation, and of brightness, are muchless reliable.
`The most precise visual methodsof attemptingto specify colours,like the
`use of a colour atlas, involve matching. The simplest are those devised
`merely to measure saturation, 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.
`
`ib)
`
`
`Transparentplungar——
`
`FM
`
`FM
`
`{d)
`
`
`
`FixedeyepieceF|
`
`£2
`3 5
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`»
`
`(a)
`
`Figure 61. Comparisonof 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 of transparent plunger.
`fc) Variation of depth ofstandard 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 been adjusted so
`
`TCL 1023, Page 10
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`LOWES 1023, Page 10
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`

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`
`
`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 we vary this colour whilststill being able to specify it?
`One way is to mix coloured lights of known wavelength in known pro-
`portions. A simple arrangement is shown in Figure 62. The amountofred,
`blue and green light can be varied by horizontal and 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 three filters, one
`
`Blue filter
`
`Red filter
`
`
`Green hiter
`
`Figure 62. Mixing lights The required mixture of red, blue and green light may be
`
`Sorting and Recording
`i51
`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 huesallows nearly nine
`million different colours to be obtained,
`including the full range of
`achromatic colours from white to black. The colourof the sample is readily
`specified in terms of the three filters used to matchit.
`MBG Y¥
`
`Standard
`
`whiteight
`
`
`iil i
`NS Lo—_
`
`—a®
`
`Sample
`Figure 63. Labelling with filters The colour ofthe sample can be matched with that of
`light which has passed through three Lovibond filters, magenta (M), blue-green (BG)
`and yellow (Y} ofspecified strength. A purplish blue sample, for example, would need
`a deep magenta (to absorb most ofthe green), 4 medium blue-green (to absorb some,
`but not all, ofthe red), and yellow ofappropriate depth to reduce the intensity ofthe
`colour to that of the sample.
`Instead of matching the light which reaches us from a sample with that
`from an unknown, we can exploit the phenomenonofpersistence of vision
`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
`TCL 1023, Page 11
`adjusted until the colour of the spinning disc exactly matches that of the
`sample, which can then be expressed in terms of the proportions of primary
`colours used.
`Nowadays, there is an increasing tendency to use photoelectric instru-
`ments for colour matching, instead of the eye of one or two individual
`observers. Such instruments monitor narrow bandsover the whole visible
`
`LOWES 1023, Page 11
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`

`

`
`
`Sorting and Recording
`
`153
`
`152
`
`Sensations of Colour
`
`the sample, and then converting this information into the size of the
`stimulus which bombards each ofthe three cone systems in 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 madeby a large numberof
`individuals selected for their normality ofvision. But even when the match-
`ing is done, rather approximately, by cye, it is seldom left for a single
`individual: more often two, or even three, observers are used.
`
`(a)
`
`(b) ae
`
`
`
`Figure 64. Tops for colour measurement (a) Split circular disc ofpaper ofstandard
`colour. (b} Stardardgreen, blue and red interlocked, exposing known areas ofeachfor
`colour mixing when the disc is rotated.
`
`Matching, whether by eye or machine, often gives different results with
`different sources of light; and as matching implies identity only of response,
`this is no surprise. Two extreme examplesof identical 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 580nm and
`onereflecting only light of 540 nm and 630 nm,each at half the intensity of
`the first pigment. If both were illuminated with light containing equal
`intensities of the three wavelengths, the two pigments would matchexactly.
`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 one light
`source, are the bane of those whotry to match clothes and accessories, or
`
`whousesdyes such that any change in illumination causes almost the same
`change in colour for the different materials.
`Coloursolids and atlases can give us no information about the compost-
`tion of the light which causesa particular colour, For charts relating colour
`
`016
`
`Ralatlve
`
`012}-
`
`reflectance ale ae
`
`400
`
`:
`
`500
`
`600
`
`700
`
`Figure 65. A good match? The reflectance spectra oftwofabrics which match perfectly
`in daylight. (Reproduced, with permission, from W. D. Wright, The Measurement of
`Colour, 4th edition, Adam Hilger, Landon, 1969.}
`Green
`
`Wavelength (nam)
`
`
` Unsaturated greenish-yellow
`Blue-green
`
`Blue-violet
`Magenta
`Crange-red
`Figure 66. Maxwell's triangle Shows how many, butnorall, colours can be represented
`as a mixture of three primarycoloured lights. The nearer a pointis to an apex ofthe
`triangle the higheris the propartionoflight ofthe colour represented by that apex. The
`point X (50 per cent green, 35 per cent orange-red and 15 per cent blue-violet)
`
`TCL 1023, Page 12
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`
`
`154
`
`Sensations of Colour
`
`lo composition, we turn to the second, more ‘scientific’ approach. Just as
`many colour solids are based on a ring of spectral colours, joined through
`purple, a triangle usually forms the basis of attempts to chart the colours
`produced by the mixing of lights. As early as 1855, Maxwell found thata
`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 (sce Figure 66). Many colours can be specified in this way:
`but not all. Whichever three primary sources we choose, there are always
`some colours (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 maiching the result with a mixture of the two otherlights. Figure
`67 gives the recipe for obtaining a match for 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 (546nm); but if a little blue
`(436 nm) is added to the yetlow, a perfect match can be made. We can
`express this algebraically by stating that vivid yellow can be matched by red,
`green and a small negative amount of blue. But since there is no scope for
`plotting negative contributions on a Maxwell triangle, colours such as vivid
`yellow cannot be represented onit.
`It is too bad that we cannot choose any three wavelengths which, when
`themselves mixed together, will produce aff 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 heal primaries, such as Maxwell used. So we could draw a
`mathematical modification of Maxwell's triangle, with our three imaginary
`primariesat 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
`gtaphs and the ruled paper may not be easy to obtain. Could we not use
`squared paper instead? How many variables 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 of one, P units of the second and L of
`the third —it might look as though we need the three variables,I, P, and L, to
`specify R. But we could also say that the brightness of thelight is the sum S
`of the three primaries (so S=1+P+L), mixed in the ratio of I/S:P/S:L/S.
`And if we know I/S and P/S we already know L/S, because
`
`42-5
`
`Sorting and Recording
`
`
`
`a
`Blue (436)
`
`+40
`
`+05
`
`0
`
`155
`
`These ‘positive’ colours
`when mixed together
`
`MATCH
`
`mixture of pure patch and
`these ‘negative colours
`
`Red (700) 415
`
`
`
`700
`400
`500
`600
`Wavelength of pure patch of fight(nm}
`Figure 67. ‘Negative’ colours Many pure spectral colours can be exactly matched with
`three primaries only by use of‘negative’ colours. When oneprimary ts mixed with the
`‘pure’ patch oflight, this mixture can be matched by some mixture ofthe 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, Punciples of Color Technology, inierscience,
`New Yark, 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 calculate it. Andif we
`decided that, after all, we wanted to specify the brightness, we could
`representit on an axisrising vertically, out of the paper. Thefinal 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 whichalso
`showsheights, or isobars (or any third variable) superimposed on the plan
`aS a series of contours.
`—
`So all that we need in order to specify the colour of an object is a
`
`TCL 1023, Page 13
`
`LOWES 1023, Page 13
`
`

`

`Sensations of Colour
`
`‘ideal grear’
`
`p
`
`157
`
`Sorting and Recording
`
`
`
`Figure 69. Purple and white The point & for p =0-33, i = 0-33 (and sol == 0-33) is
`‘equal energy’ white. Purples lie along the base line (see text).
`
`650
`770nm
`
`of the light by it, of the response of the normal human eye, and of 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 (C1 E), 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 would in theory be needed to produce them.
`
` 156
`The tongue-shaped curve (see Figure 68) in the graph showsthespecifica- 100% primary P
`
`
`
`tions of the pure spectral colours as their values of i=1I/S and p=P/S.
`(Modified definitions of the standard observer and the standard sources,
`tlds
`TCL 1023, Page 14
`intreduced by the CIE in 1967, change only-details on the graph.) The
`I
`O38
`}
`100%primary 100% primary|‘ideal red’
`diagram, known as a CIE chromaticity curve, has been used to depict
`colours, and the relationship between them, in a wide variety of situations.
`The enormous usefulness of the CIE diagramsarises from the fact that
`we can represent a mixture of twolights as a point on the line joining the
`points which specify them. So purples, formed by mixing red and bluc, lie on
`
`Figure 68. CIE tongue diagram The tougue encloses all visible colours, with the pure
`spectral ones lying along its curved edge. The inner triangle encloses those colours
`obtainable by mixing real primaries 436.nm, 546am, 700-nm (i.e. those enclosed in
`
`LOWES 1023, Page 14
`
`

`

`
`
`158
`
`Sensations of Colour
`
`Sorting and Recording
`
`159
`
`(770 nm) and one part violet (380 nm) would lie at point X, twice as near to
`the red point asto the violet point. Since all the colours formed by mixing
`real lights lie inside the area enclosed by the tongue, it is only those colours
`which are represented by points inside the curve which are visible. As the
`area outside the curve represents only imaginary stimuli, we need not
`consider it further.
`Wecan also use C1Ediagramsto specify a colour in termsof its dominant
`wavelength and its saturation, as well as in terms of the contributions of
`responses to imaginary primaries. The point E on Figure 69 represents an
`equal mixture ofthe three primaries and hence its position is p=0-3, i=0-3
`(and so !=0-3). It is known as equal energy white. Suppose now that we
`wantto find the dominant wavelength of the colour represented by point X
`in Figure 70a. We can think of this colour as being some mixture of a
`
`Pp
`
`520
`
`o8 O6
`
`Dominant wavelength 570nm o4
`
`08
`06
`4
`02
`Q
`Figure 70. Dominant wavelength Specification ofcolour as a mixture of white light
`with:
`(a) One spectral wavelength.
`(b) A non-spectral mixture.
`(See text.)
`
`550
`7700m
`
`oz
`
`aut
`TCL 1023, Page 15
`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 wavelength D. So X is an unsaturated
`version of the pure spectral colour D. How unsaturated? We know this
`from th