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`TCL 1023, Page 1
`TCL 1023, Page 1
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` CARNEGIEIEHA
`
`(IN
`
`Carnegie library of Pittsburgh
`
`I: a free public library. mainhlnod by the
`flfyoffimbumhuflfluwrdfllo-
`OW: with
`ruppionnntul
`appropriation.
`[turn the State at Pennsylvania
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`FINES: Five cents a day an iuvenile and adult cards for
`each book kept past due date. A borrower is respon-
`
`sible for books damaged while charged on his card.
`TCL 1023, Page 2
`TCL 1023, Page 2
`Books may not be renewed.
`munn
`
`
`Carnegie Library of Pittsburgh
`4400 Forbes Ave.
`Tel. 622-3131
`
`!
`
`

`

`
`
`HAZEL “OSSOTTB
`COLOUR
`
`Princeton University Press
`Princeton, New Jersey
`
`
`
`TCL 1023, Page 3
`TCL 1023, Page 3
`
`

`

`
`
`Published by Princeton University Press.
`41 William Street. Princeton. New Jersey 08540
`
`Copyright © 1983 by Hazel Rossotti
`an rights reserved
`First Pelican original edition, l983
`First Princeton Paperback printing. with corrections. 1985
`
`lL‘C84—ll45l
`[SEND—691—08369—X
`ISBN D—69l 432336 —'r' (plain)
`
`Reprinted by amusement with Penguin Books Ltd.
`Made and printed in Great Britain
`by Richard Clay (The Chaucer Press) Ltd,
`Bungay. Suffolk
`Set in VIP Times
`
`Clothbound editions ofPrinoeton U niversity 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
`
`NH
`
`. Light Particles
`. White Light on Clear Glass
`
`Part Two: Lights and Colours
`
`. Steady Colours
`. Shimmering Colours
`. Special Effects
`. Lights
`
`GUI-FUR
`
`Parr Three: The Natural World
`
`
`
`. Air and Water
`Earth and Fire
`
`. Vegetable Colours
`. The Colours of Animals
`
`crepe-.1
`
`Parr Four: Sensations of Colour
`
`. Light and the Eye
`12.
`Anomalous Colour Vision
`13.
`Colour Vision in Animals
`14.
`15.
`
`The Eye and the Brain
`Sorting and Recording
`
`Part Five: Technology
`
`16.
`17.
`
`Colour Reproduction
`Added Colour
`
`11
`I2
`13
`
`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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`

`

`
`
`Contents
`
`Part Six: Uses and Links
`
`
`203
`18. Imparting Information
`209
`19. Communicating Feelings
`220
`20. Colour, Music and Movement
`21. Words and Colours
`222
`
`
`
`
`
`Index
`
`Acknowledgements
`
`231
`
`239
`
`
`
`LIST OF TEXT FIGURES
`
`. Light waves
`. Prismatic colours
`
`. A home-made prism
`From a rose window
`
`Hpiece-.Jouzhwn-a
`
`. 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 two slits
`. 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
`. Fine blue, damp pink
`
`WUWUWMNNNNNNMNMHHHI—IHMHHH
`
`hMNflowfl~JQMhuMHQVQm~JQMJ§wMH
`
`21
`23
`23
`24
`27
`28
`29
`31
`32
`33
`33
`41
`46
`49
`50
`51
`52
`53
`57
`59
`61
`65
`66
`67
`68
`69
`69
`71
`72
`72
`74
`75
`79
`82
`
`TCL 1023, Page 5
`TCL 1023, Page 5
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`

`

`
`
` List of Text Figures
`
`L35! of Text Figures
`
`35.
`77. Phosphor dots
`Lea! green
`87
`36.
`78. Colour negatives and prints
`Carrot orange
`88
`79. Code for resistors
`37.
`Delphinium blue
`38.
`89
`The colourful juice of pickled red cabbage
`94
`39.
`Butterfly wings
`40.
`95
`Insect iridescence
`41.
`Blue birds
`95
`42.
`97
`111
`43.
`44.
`112
`45.
`113
`114
`46.
`47.
`115
`48.
`117
`49.
`123
`50.
`128
`51.
`131
`132
`52.
`53.
`133
`54.
`134
`55.
`136
`56.
`138
`139
`57.
`145
`58.
`‘ 59.
`146
`147
`60.
`148
`61 .
`62.
`150
`63.
`151
`64.
`152
`153
`65.
`66.
`153
`67.
`155
`156
`68.
`157
`69.
`159
`70.
`71.
`161
`72.
`163
`73.
`164
`165
`74.
`75.
`166
`173
`76.
`
`Munsell‘s tree
`Comparison of saturation
`Mixing lights
`Labelling with filters
`Tops for colour measurement
`A good match?
`Maxwell's triangle
`‘Negative' colours
`ClE tongue diagram
`Purple and white
`Dominant wavelength
`Colours and complementaries
`Brightness
`Some common colours
`
`
`
`175
`178
`206
`208
`223
`225
`226
`
`80. Coloured signals
`81. Evolution of colour words
`82. Heraldic tinctures
`83. Names for colours
`
`TCL 1023, Page 6
`TCL 1023, Page 6
`
`
`
`How animals change colour
`The human eye
`A light twist
`Changes in the retina
`Brightness by night
`Brightness by day
`Cones and colour: 3. 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
`
`Colour and temperature
`The flight of colours
`Colour television
`
`

`

`
`
`142
`
`Sensations of Colour
`
`him from the donkey at that particular moment than to the average colour
`which the donkey w0uld 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 which the artist tries to reproduce the actual light coming from the
`object. In much the same way, an observer will 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 bear little 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 seeing
`‘golden rain’ and coloarcd patterns. In childhood, a pressure on a closed
`eye was enough to produce patterns as exotic as Catherine wheels or
`peacock tails. In adult life, a firmer touch, preferably on the upper part of
`the eye, is needed to produce an inferior but none the less impressive result.
`Electrical and mechanical stimulation of the optic nerve and the visual areas
`of the brain can make us see colours ; so can some acute illnesses, and even a
`strong magnetic field. And so, of course, can memory and dreams. But
`although we can‘cxperience an immense variety of colour sensations pro-
`duced in these different, non-visual. ways, we have as yet only a negligible
`understanding of any of the mechanisms involved.
`
`”i as
`
`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 scheme be entirely
`personal, or could we use it to communicate information about a colour?
`How can we best tell someone the exact colour we should like 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 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
`match all over, even if the light falling on them happens to be identical. It is
`safer to choose from samples of the 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 of this 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.
`
`Perhaps it would help if we could arrange colours in some rational 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 make a 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
`should be joined through the greys, to give another circle, perpendicular to
`
`TCL 1023, Page 7
`TCL 1023, Page 7
`
`

`

`144
`
`Sensations of Colour
`
`the first. It seems we cannot arrange our coloured objects on a flat surface,
`but need some three-dimensional scheme.
`
`Maybe it would be better to seek a more ‘scientific’ classification than
`any subjective arrangement of coloured materials? We might try to specify
`the colour of a sample by irradiating it with the light of a large number of
`very narrow bands of wavelength and measuring the percentage of each
`which the sample reflects. These measurements can be made extremely
`easily, given the appropriate equipment. But the light which enters the eye
`depends on the lighting as well as on the sample, so we would also need to
`know the composition of the illumination. Even this does not tell us the
`colour of the sample unless we know how the eye reacts to light of different
`wavelengths. Two materials may match exactly under one type of illumina-
`tion even if they send light of totally different composition to the eye; we
`know that many yellows can be matched by mixtures of red and green light.
`We might, however, combine, for each narrow band of wavelength,
`measurements of the reflecting powers of the sample, and the composition
`of the light source with our knowledge of the response of the retina.
`Although this procedure still needs laboratory equipment, it relates the
`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 number of colours in any systematic order, we must
`decide what qualities we shall use to sort them. Let us first 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’ component is itself a
`mixture.) We can describe the primary sensation of colour in terms of hue,
`which refers to the greenness, blueness and so forth, and varies with any
`change in the dominant wavelength. The extent to which this wavelength in
`fact dominates the light is known as 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
`
`
`
`Sorting and Recording
`
`145
`
`brightness (or value). The so—called ‘ natural‘ or ‘achromatic‘ colours, black,
`grey and white, are of zero saturation. and differ from each other only in
`brightness. A series obtained by adding one hue, say blue, to white differs
`only in saturation, as does a series obtained by adding a blue pigment to a
`grey one. if the same pigment were added to a white one In the same
`proportions, the dusky. pale blue would differ from the clear pale blue only
`in brightness.
`There are many three-dimensional arrangements of colours, the best
`known being those devised by Munsell and by Ostwald. Both are based on
`the colour circle formed by joining the two ends of the spectrum through
`purple (see Figure 58). So the hue changes around the circumference of the
`circle, much as the hours progress around the face of a clock. Through the
`centre of the clock face, and perpendicular to it, like the axle of a wheel,
`runs the line representing the neutral colours, usually with white at the top,
`changing, through deepening greys, to black at the bottom. Radially, like
`spokes on a wheel, the saturation increases towards the rim. to give a space
`which can be filled in by different colours, according to which system is
`being used.
`
`Green
`
`Blue
`
`
`
`
`Black
`
`willow
`
`
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`Vldcol
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`smut-(aim:
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`
`Figure 58. Colours in space The skeleton oft: colour solid. The ‘achromatic' colours
`form the vertical backbone from 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 further it 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
`
`
`
`

`

`I46
`lal
`
`lb)
`
`Full saturation at edge
`
`wavelength (hue)
`
`
`
`
`Full
`colour
`
`Hue (2)
`(complementary to (1))
`
`
`
`Grey axis
`
`Figure 59. Ostwald's colour solid (a) Exterior view, (b) Vertical section.
`(Adapted, with permission. from G. J'. Chamberlr‘n and D. G. Chamberh‘n, Colour: lts
`Measurement, Computation and Application Hey/ten, London, 1980.)
`
`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
`colour solid 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
`
`Sensations of Colour
`
`Sorting and Recording
`
`[4?
`
`
`
`Planes oi oonnant dominant
`
`numbered on a grid system, so that any Colour contained by the solid can be
`specified by a map-reference.
`ln Munsell‘s arrangement. saturation is increased by a series of visually
`equal steps rather than by adding a fixed proportion of pigment; and there
`are nine neutral colours, rather than eight. As the number of 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 less regular than Ostwald’s, and on account of its untidy
`appearance is known as a 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.
`
`Maximal
`saturation
`
`
`
`Hue it)
`
`Black
`
`Hue 12)
`(complementary to (1 )1
`
`Figure 60. Munsell‘s tree Vertical section (cf. Figures 59(1)), page 146, and 83. page
`226).
`
`The two systems resemble each other in 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 used the 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
`TCL 1023, Page 9 ’
`
`
`
`

`

`149
`
`Sorting and Recording
`
`Sensations of Colour
`
`‘
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`circumference and dividing the segments between them into visually equal
`steps, constituting a more ‘natural' colour circle 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 ofColottr, 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 wish to specify the precise colour of 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 we try 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, there is a lot to be said for the measurements
`being made by the eye. The human eye 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 much less reliable.
`The most precise visual methods of attempting to 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 component in 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.
`
`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 instrument is
`adjusted until the some depth of colour is observed by each eye.
`(a) Sample.
`(b) Variation ofdepth ofstandard by means of transparent plunger.
`(c) Variation of depth of standard by use of wedge.
`(d) Split field, one halffrom each eyepiece.
`in the top two diagrams, the some depth used for sample and standard gives a darker
`field an the right. In the lower diagram, the length (l) ofstandard has been adjusted so
`that the two fields are indistinguishable in this example. d =2l. indicating that the
`saturation of the sample is half that of the standard.
`
`TCL 1023, Page 10
`TCL 1023, Page 10
`
`‘
`
`l
`
`

`

`
`
`Sorting and Recording
`
`151
`
`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 whilst still 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 amount of red,
`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 coloured light
`which has been obtained by passing white light through three filters, one
`
`Blue filter
`
`
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`
`Figure 62. Mixing lights The reqttlred mixture of red, blue and green ltght may be
`:btarned by adjusting that area ofeach filter which lies overthe apertttre to the mining
`or.
`
`magenta, one yellow and one blue-green (see Figure 63). Sets of such filters
`are available commercially for use in an instrument equipped with a stan-
`dard light source and known as the Lovibond Tintometer. The full range of
`250 filters 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 colour of the sample is readily
`specified in terms of the three filters used to match it.
`NIB-G Y
`
`Standard
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`whitelight
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`
`Figure 63. Labelling with filters The colour ofthe sample can be matched with that of
`light which has passed through three Lovlbond filters. magenta (M), blue—green (BC)
`and yellow 1' Y) ofspecified strength. A purplish blue sample, for example, would need
`a deep magenta (to absorb most ofthe green), a 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 phenomenon of persistence 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
`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 hands over the whole visible
`spectrum, first detecting how much light in each band teaches them from
`TCL 1023, Page 11
`TCL 1023, Page 11
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`152
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`Sensations of Colour
`
`Sorting and Recarding
`
`l 53
`
`the sample, and then converting this information into the size of the
`stimulus which bombards each of the three cone systems in a normal human
`eye. Finally, the responses to these three stimuli are combined to give the
`colour experienced by the ‘standard observer’. Inescapably, these instru-
`ments give results based on sensations experienced as a consequence of
`human vision. But the vision is that, not of a few individuals. but of the
`‘standard observer’ built up from observations made by a large number of
`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)
`
`
`
`/
`
`Figure 64. Tops for colour measurement (a) Split circular disc ofpaper afstandard
`colour. (b) Standard green, blue and red imerlocked, exposing known areas ofeach for
`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 examples of 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 580 nm and
`one reflecting 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 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 ‘metamen'e’ pigments, which match only under one light
`source, are the bane of those who try to match clothes and accessories, or
`paint and fabrics (see Figure 65). Metametism accounts for much of the
`popularity of ‘colour coordinated‘ goods sold by the same manufacturer
`
`who uses dyes such that any change in 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 causes a particular colour. For charts relating colour
` 016
`
`012
`
`Relative
`
`reflectance
`
`400
`
`500
`
`600
`
`7'00
`
`Wavelength [nml
`
`F1gure 65. A good match? The reflectance spectra ofrwo fabrics which match perfectly
`in daylight. (Reproduced, with permission. from W. D. Wright. The Measurement of
`Colour, 4th edition, Adam Hilger. London. 1969.)
`Green
`
`Unsaturated greenish-yellow
`
`
`
`
`
`Blue-green
`
`Blue-violet
`
`Magenta
`
`Orange-red
`
`Figure 66. Maxwell's triangle Shows how many, but not all. colours can be represented
`as a mixture of three primary coloured lights. The nearer a point is to an apex ofrhe
`triangle the higher is the proportion oflight ofrhe colour represented by that apex. The
`point X (50 per cent green. 35 per rem orange-red and l5 per cent blue-violet)
`represents an unsaturated greenish yellow.
`
`TCL 1023, Page 12
`TCL 1023, Page 12
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`154
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`Samaritans of Colour
`
`Sorting and Recording
`
`155
`
`to 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 that a
`great number of 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
`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 matching the result with a mixture of the two other lights. Figure
`67 gives the recipe for obtaining a match for every visible wavelength with
`three primaries. Thus vivid yellow (570nm) (cf. page 119) can never be
`exactly matched by red (700 nm) and green (546 nm); but if a little blue
`(436nm) is added to the yellow, 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 on it.
`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 heal primaries, such as Maxwell used. So we could draw a
`mathematical modification of Maxwell‘s triangle, with our three imaginary
`primaries at the comers. 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 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, 1 units ofone, 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 the light is the sum S
`of the three primaries (so S=l+P+L), mixed in the ratio of l/SzP/SilS.
`And if we know US and PIS we already know LIS, because
`liS+PfS+LIS=L Since we are often more interested in colour than in
`
`
`
`Blue 1436l
`
`+20
`
`+ t . 5
`
`+trO
`
`+0 5
`
`0
`
`These ‘posltlve‘ colours
`when mixed together
`
`MATCH
`
`mixture otpure catch and
`hose ‘negativd colours
`
`
`400
`500
`600
`700
`Wavelength of pure patch ottight tnm)
`Figure 67. ‘Negative‘ colours Many pure spectral colours can he exactly matched with
`three primaries only by use of‘negan've' colours. When one-primary is muted With the
`'pure' patch aflight, this mixture can be matched by some mature ofrhe other two. For
`the primary lights used here, the only colour which can be matched directly by the three
`primaries is a greenish yellow in the region of550nm. (Reproduced, With pen-hission,
`from F. W. Billmeyer and M. Saltzmart, Principles of Color Technology, Interscrence,
`New York, 1966. p. 33.)
`'
`brightness, we could concentrate on the two quantities IIS and PIS wthh
`specify the colour. We could then use ordinary squared graph paper. If we
`ever needed to know us, it would be easy enough to calculate it. And if we
`decided that, after all, we wanted to specify the brightness, we. could
`represent it on an axis rising vertically, out of the paper. The final diagram
`Would be much like a plan, with the two specifications of colour running
`north-south and east‘wcst; or, if brightness is added, like a map which also
`shows heights, 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 obyeict is a
`knowledge of the composition of the light falling on it, of the modification
`
`TCL 1023, Page 13
`TCL 1023, Page 13
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`156
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`Sensations of Colour
`
`Sorting and Recording
`
`157
`
` 0
`
`,
`
`08
`
`,
`
`Figure 69. Purple and white Thepoint Eforp = 0-33, i =0-33 (andsol =15}: 0-33) is
`'equal energy' white. Purple: lie along the base line (see text).
`
`tions of the pure spectral colours as their values of i=l/S and ptPlS.
`(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 ClE 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 diagrams arises from the fact that
`we can represent a mixture of two lights as a point on the line joining the
`points which specify them. So purples. formed by mixing red and blue. lie on
`the base-line of the ‘tongue‘ (see Figure 69). A mixture of two parts red
`
`TCL 1023, Page 14
`TCL 1023, Page 14
`
`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 (Cl E). in 1931, defined the standard observer and
`three possible standard sources; and they produce