VOLUME 65, NUMBER 5
JOURNAL OF THE OPTICAL SOCIETY OF AMERICA
MAY 1975
Color essays* David L. MacAdam Research Laboratories, Eastman Kodak Company, Rochester, New York 14650 (Received 30 January 1975) Webster's New Collegiate Dictionary defines "essay" first as "an effort to do something; attempt; trial." Only second is it defined as "a literary composition, analytical or interpretative, dealing with its subject from a more or less limited or personal standpoint." This article is an essay (second meaning) about a few essays (first meaning) that have been made to understand color. Genesis, Aristotle, Plato, Newton, Palmer, Young, Helmholtz, Maxwell, and Frederic Ives are represented. Index Headings: Color; Vision; Colorimetry.
other. Seneca seems to agree, but Galen reverts to the aristotelian triad, which is also found in the Edda and in the Arabian literature, and in the West until the seventeenth century.
Colors have always attracted and fascinated mankind. Although Genesis Chapter 9 does not name the colors in the rainbow, they were certainly seen by the writers. There is nothing but colors to be seen in the rainbow. The uniqueness and glory of the rainbow, which made it appropriate as the token of God's first promise to mankind, results from the striking display of colors in it. Everyone knew what it looked like. Absence of a description of the colors is not surprising. There is very little description of anything in Genesis. The first color name occurs sixteen chapters later, where Esau is reported to have been born all red and hairy. Blue, purple, and scarlet are first mentioned in Exodus 26: 30.
COLOR SCIENCE
Plato4 recommended reticence concerning the study of color: "There will be no difficulty in seeing how and by what mixtures the colors are made... He however who should attempt to verify all this by experiment would forget the difference of the human and the divine nature. For God only has the knowledge and also the power that are able to combine many things into one and again to resolve the one into many. But no. man is or ever will be able to accomplish either the one or the other operation. The law of proportion, according to which the several colors are formed, even if a man
COLOR NAMES
Queen Victoria's prime minister, Gladstone, a leading authority on the Homeric poems, 1pointed out2 "that although Homer had used light in its various forms for his purposes with perhaps greater splendor and effect than any other poet, yet the color adjectives and color descriptions of the poems were not only imperfect but highly ambiguous and confused. " From the paucity of words for description of color in the language known to Homer, Gladstone concluded' that "the organ of color was but partially developed amongthe Greeks of Homer's age. " Subsequently, students of evolution and of color vision have rejected Gladstone's generalization that the ancient Greeks were color blind. On the other hand, philologers have found that, in their early stages of development, all languages have a dearth of color names. After black and white, red is usually the first color name to appear. Yellow comes later, then green and blue. Those four are the unique colors of perception; each can be perceived as uncontaminated with any of the others. Names for orange, scarlet, purple, pink, and violet, which are perceived as combinations of pairs of the perceptually unique colors, occur much later. The presence and absence of names for the various colors are taken as indications of the stage of development of each language. They are not, as Gladstone thought, indications of color blindness or incomplete development of color vision among the people who use that language, but of the sophistication of their language.
knew, he would be foolish in telling,
for he could not
give any necessary reason nor, indeed, any tolerable or probable explanation of it. " Sir Isaac Newton was first to explain the cause of the rainbow. 5 He was able to do so as a result of his study of the colors exhibited by glass prisms, which were popular novelties and decorations. When he passed white light from the sun through a prism, he obtained a beautiful display of colors; because they were not corporeal, he called them spectra (i.e., specters, ghosts), and he called the array of colors a spectrum. Newton listed seven colors that he could see in the spectrum, and for which he knew names: violet, indigo, blue, green, yellow, orange, and red remained at its out that there is a continual and smooth progression and variation of colors between the colors so named. The quality that changes continuously along with spectrum is called hue. By combining the light from the two ends of the spectrum in various proportions, for example by increasing the violet content at first, while the red remained at its maximum strength, and then, with violet at its maximum, decreasing
the red, Newton found that he could
produce many hues that are not found anywhere in the spectrum.
Aristotle3 named three colors, red, green, and blue, in both the primary and the secondary rainbows. Ovid, in Metamorphoses vi 65-7, wrote that the rainbow has a thousand colors, each hardly distinguishable from its neighbor but with extremes very remote from each
In that way, he found that he could complete
the circuit, from the red of the spectrum through pinks, purples, and lavenders, back to the violet with which he began naming the hues. There are no abrupt changes of hue in the progression thus produced, from red through purple to violet. Therefore, Newton represented 483
DAVID L. MacADAM
484
Vol.
65
ORANGE
0
VIOLET
a) 0V
Q, INDIGO
FIG. 1. Newton's color circle.
the entire circuit of colors, through the spectrum from violet through green to red and then through purple back to violet again, with a circular diagram. 6 (Fig. 1.) Because he could reconstitute white light by recombining all of the colors of the spectrum,
Newton represented
white at the center of the circle and placed weak or strong tints of various hues on radii, at distances from the center that corresponded to their strengths. He pointed out that this representation is analogous to a center of gravity. In this analogy, each of the colors in the spectrum may be thought of as a weight. The weight of each color is proportional to the energy of that color. Newton con-
0.9 0.8 0.7 0.6 0.5 y
0.4 0.3 0.2 0.1
FIG. 2. Spectrum locus according to CIE 1931 standard observer for colorimetry.
500 600 Wavelength (nm)
FIG. 3. Weights per watt of spectrum colors, for center-ofgravity determinations with spectrum locus in Fig. 2.
ceived each weight to be located at the point on the circle that represents that particular color in the spectrum. This idea has been a powerful tool in color science, although the spectrum locus (shown in Fig. 2), is now known not to be a circle. 5 Nowadays, we label each color of the spectrum with a number, called wavelength, such as are shown in Fig. 2. The factor of proportionality between energy and weight is different for different wavelengths. It is shown in Fig. 3, which is the sum of the three curves, known as color-mixture functions, shown in Fig. 4, which are the basic data of modern colorimetry .7 Newton suggested that the center of gravity that corresponds to weights located around the spectral locus, proportional to the energies of the several spectrum colors in any particular light, represents the color of that light. The same idea applies to the colors of reflecting or transmitting objects, in which cases the weights are proportional to the energies of the various wavelengths that are reflected or
transmitted. The center of gravity is called the chromaticity of any color and the diagram, Fig. 5, that consists of the spectrum locus and the straight line that joins its short- and long-wavelength extremities (violet and red, respectively) is called the chromaticity diagram. The chromaticities of all physically possible colors are represented by points within or on the spectrum locus and the line between its extremities..
COLOR ESSAYS
May 1975
485
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40 -
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500
600
Wavelength
FIG. 4. Color-mixture ver for colorimetry.
As a consequence
700
10
(nm)
functions for CIE 1931 standard obser-
400
of the center-of-gravity
principle,
1
I
520 0
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1
510 550 06560 0.6
700
transmit any yellow spectrum light. the spectral transmittance of a piece of yellow glass; the curve shows that no wavelength that appears yellow in the spectrum gets through the glass. The wavelengths of the portion of the spectrum that appears yellow are simply not transmitted by the glass. Yet, in white light, the glass appears yellow! Color is therefore not wavelength. Rather, it is the resultant of the combined effects of all of the wavelengths that are present, which is represented by the center of gravity of all of those wavelengths, weighted in proportion to their energies.
I
530
07
500 600 Wavelength (nm)
FIG. 6. Spectral transmittance of a yellow glass that does not
it is wrong to say, as some teachers and textbooks do, that color is wavelength. For example, Fig. 6 shows
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0.5
580
y
590 600 610 620
0.4 0.3
THREE-COLOR VISION
490
\700 0.2 480 0.1 00
Ithree-quarters
.S470 \400 0.1
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FIG. 5. Chromaticity diagram for CIE 1931 standard observer for colorimetry.
Newton's contributions to color science were many and great, but he never stated the great simplification that makes color photography, color television, and color printing feasible. He never mentioned that, to produce any color whatever, a mixture of three suitable colors, in suitable amounts, is sufficient. That fact was stated, more or less clearly, by several people during the last quarter of the seventeenth and the first of the eighteenth centuries. 3 As a matter of fact, Aristotle recognized the primacy of red, green, and blue in color vision, and stated that all other colors (he mentioned yellow and orange specifically)
are inter-
mediate between those three, either mixtures of them or result from contrast between them. 3 Twenty centuries after Aristotle (in 1681) a French
DAVID L. MacADAM
486
physicist, Mariotte, 9 said again that three colors are sufficient for the production of all other colors by mixture. Mariotte, however, specified yellow, rather than green, as one of his primaries. He did not propose any theory of color vision. COLOR-VISIONTHEORIES
Lomonosov, a Russian manufacturer of tinted glass, who cited Mariotte and accepted Mariotte's conclusion that three primaries are sufficient and also Mariotte's choice of red, yellow, and blue as primaries, in 1756 associated red with salt, yellow with sulfur, and blue with mercury, loand asserted "their concordance with the salt, sulfur, and mercury, respectively, found in our optic nerves enables us to see these colors... other colors arise from a mixture of these primaries. "' Lomonosov did not suggest that there are different classes of nerves or receptors, each specifically sensitive to one of the primary
colors.
That concept was introduced by Palmer, an English chemist, who omitted alchemy. Like Mariotte and Lomonosov, but without mentioning them, Palmer named red, yellow, and blue as primaries. Palmer ascribed color vision to the presence in the retina of three different kinds of "particles, " each specifically "moved by" one of those colors. Palmer was the first to attribute color blindness to deficiency of sensitivity of any of the
classes of color-specific receptors. Until 1956 completely unheard of, Palmer published a fictitious dialogue"' between himself and a character named Johnson, whose manner of expression seems to mimic that of Dr. Samuel Johnson. Palmer won all of the exchanges. Despite this misrepresentation, forwhich the choleric
Doctor may have made London too hot for
Palmer, the statements concerning color and vision are exceedingly interesting, and a quarter of a century ahead of their time. For example, he wrote, "There is no color in the light... Each ray of light is composed of three rays only: one appears yellow, one red, and the other blue... These rays are present in different proportions, which they keep exactly, notwithstanding decrease or increase of the intensity of the lights... The surface of the retina is composed of particles of three different kinds, corresponding to the three rays of light; each of these particles is moved by its own ray... The complete and uniform motion of these particles produces the sensation of white... The absolute want of motion of these particles, whether by interception of the light, or by reflection from a black substance, produces the sensation of black... Differing excitations of these particles by rays from colored objects or from refracting prisms, produce the sensation of color... Motions of these particles by rays that are all decreased by the same amounts produce only sensations of more or less white; but not of colors. " Nine years later, in Paris, in a French brochure' 2 that he wrote to promote the sale of an artificial daylight lamp for use by artists, Palmer wrote (my translation), "It is quite evident that the retina must be composed of three kinds of fibers, or membranes, each susceptible of being stimulated
by only one of the three primary
Vol. 65
rays.* Equal sensibility of these three classes of fibers constitutes normal vision; any deficiency of sensibility, as well as excessive sensibility of any class, constitutes anomalous color vision.. . Even the best-organized eyes are disturbed temporarily when they look attentively at colors. If we look for a long time at a vivid red and then close our eyes, we see green... When sunlight shines directly in the eye, it causes violent motion of the fibers. For several minutes thereafter, we have sensations of various colors in succession, because the different fibers come to rest at different rates."
Sixteenyears later, or 25 years after Palmer's first publication of the theory, Thomas Young, an English medical doctor, commenting on Newton's studies of color, proposed the same theory of color vision. '3 In the first of a famous series of lectures that dealt with, and presented for the first time much of physical optics, Young included just this one paragraph: "As it is almost impossible to conceive each sensitive point of the retina to contain an infinite number of particles, each capable of vibrating in perfect unison with every possible undulation, it becomes necessary to suppose the number limited, for instance to the three principal colors, red, yellow and blue, and that each of the particles is capable of being put in motion more or less forcibly by undulations differing less or more from perfect unison. Each sensitive filament of the nerve may consist of three portions, one for each principal color." Note that Young, like Palmer in 1777, referred to the
retinal receptors as "particles. " Alsolike Palmer, Young named yellow and blue as primaries. We do not know whether Young's thinking was influenced in any way by Palmer's pamphlets. Within a year, however, he amended his theory with a brief note in the Philosophical Transactions, 14 "It becomes necessary to modify the supposition that I advanced in the last Bakerian lecture, respecting the fibers of the retina, substituting red, green, and violet for red, yellow, and blue." These two passages contain nearly all that Young ever published about color-sensitive particles, or nerve filaments, or fibers, in the retina. The idea is missing from the 1845 edition of Young's collected lectures that Maxwell quoted. 15 That edition merely states that "the perfect sensations of yellow and of blue are produced respectively by mixtures of red and green and of green and violet light, respectively. From these three simple sensations, with their combinations, we obtain seven primitive distinctions of color; the different proportions in which they may be combined afford a variety of tints beyond all calculation. The three simple sensations being red, green, and violet, the three binary combinations are yellow, consisting of red and green; crimson, of red and violet; and blue, of green and violet; the seventh is white light, composed of all three united. The blue produced by combining the whole of the green and
violet is not the blue of the spectrum. " In 1872, Maxwell wrote,
16
concerning
"the use of sen-
sation as a means of investigating the anatomical structure, " that "a remarkable instance of this is the deduction of Helmholtz's theory of the structure of the retina from that of Young with respect to the sensations of color.
I; :
. 11
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. i
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COLOR ESSAYS
May 197 5
Young asserts that there are three elementary sensations of color; Helmholtz asserts that there are three systems of nerves in the retina, each of which has for its function, when acted upon by light or other disturbing agent, to excite in us one of these three sensations. " However, Helmholtz' 7 (in his Physiological Optics (1860))wrote that "Young's theory of the color sensations, like so much else that this marvelous investigator achieved in advance of his time, remained unnoticed until Maxwell directed attention to it. " Actually, Helmholtz18 himself had in 1852 cited and accurately abstracted Young's three-receptor theory of the retina. Maxwell's only mention of Young's proposal of that theory was in a lecture 19 he gave, 3 years later,
in 1855.
After that, Maxwell seems to have forgotten Young's presentation of it. The theory is not mentioned again in Maxwell's collected works until 1872, when, as quoted, he attributed it to Helmholtz! Because Helmholtz called attention to it, 18whereas Young's editor suppressed it, and probably as a result of Maxwell's inaccurate
1872 summary16 of its history,
the theory of the three kinds of color receptors in the retina has been called the Young-Helmholtz theory. Neither Helmholtz nor Maxwell seems to have known of Palmer's
statement
of it 25 and 75 years prior to Young
and Helmholtz, respectively. Therefore, it is curious that Palmer's publications were discovered (in 1956) by Gordon Walls20 as a result of a search on which he was started by an obscure reference in Helfnholtz' s Physiological Optics. 17 Apparently, Helmholtz never saw or even heard of Palmer's pamphlets. His reference was to a second-hand reference to a case of color blindness that Palmer mentioned in his 1786 pamphlet. COLOR PHOTOGRAPHY
To illustrate the three-receptor theory of color vision, in the lecture he delivered in 1855, Maxwell'9 said "This theory of color may be illustrated by a supposed case taken from the art of photography. Let it be required to ascertain the colors of a landscape by means of impressions taken on a preparation equally sensitive to rays of every color. Let a plate of red glass be placed before the camera and an impression taken. The positive of this will be transparent wherever the red light has been abundant in the landscape, and opaque where it has beenwanting. Let it now be put in an optical projector along with the red glass. A red picture will be thrown on the screen. Let this operation be repeated with a green and a violet glass, and by means of three projectors, let the three images be superimposed on the screen. The color of any point on the screen will then depend on that of the corresponding point of the landscape. By properly adjusting the intensities of the projectors, a complete copy of the landscape, as far as color is concerned, will be thrown on the screen. The only apparent difference will be that the copy will be more subdued, or less pure in tint, than the original, because the process is performed twice-first on the screen and then on the retina. This illustration shows how the function that Young attributes to the three systems of nerves may be imitated by optical apparatus.
489
It is therefore unnecessary to search for any direct connection between the wavelengths of the various kinds of light and the sensations produced by them. The threefold partition of the properties of light maybe performed by physical means. The remarkable correspondence between the results of the experiments on different individuals indicates some anatomical arrangementidentical
in all. " The quality of the picture that Maxwell ultimately (in 1861) showed to illustrate his example suffered because he did not have what he prescribed, " a preparation equally sensitive to rays of every color. " The photographic plates available in 1861 were not at all sensitive to red, yellow, or green. How he could have gotten results as good as he did is an interesting puzzle, which is the subject of a very interesting article by Ralph
Evans. 2'
COLORIMETRY
Having thus casually invented the principle of modern color photography, Maxwell went on to perform experiments that elucidated the properties of color vision. He
determined three spectral sensitivities that are typical of normal human color vision, which he had suggested should be mimicked by the three photographs. From his work, 22 modern colorimetry has developed. He learned that the colors of the spectrum should not be represented by a circle, but by a horseshoe-shaped curve, such as is the basis of the modern chromaticity diagram. The chromaticity diagram shown in Fig. 5 was recommended7 in 1931, for general technical use, by the International
Commission
on Illumination
(CIE).
In 1960,
the CIE adopted a modified form of the chromaticity diagram, 23for use "whenever a diagram yielding color spacing more nearly uniform than the (x, y) diagram is desired. " The color plate (opposite) illustrates the arrangement of colors on that diagram. The printed colors are merely schematic. For instance, colors of sufficient saturation to be represented by points near the spectrum locus cannot be printed. For qualitative, suggestive illustration, however, less-saturated colors have been printed all the way out to the spectrum locus; they indicate the appropriate hues and suggest the outward increase of saturation. The distribution of colors is appropriate only when the plate is illuminated by daylight. The abrupt changes of color that are evident in the color plate are intentional deviations from the everywhere smooth and continuous changes of color that are represented by locations in the diagram. A painting that had smooth, continual, gradual changes of color would be too dull and featureless to be worth reproducing. The abrupt, although unrealistic, changes that are evident in the color plate (opposite) are much more effective than would be the correct, gradual changes of colors, in conveying the correct impressions of the changes of hue and saturation from one area to the next. However, for these reasons, the colors printed in the plate are not accurate and should not be used for any quantitative work. The diagram is, however, illustrative of the general idea and trends of distributions of colors represented in the 1960 chromaticity diagram.
DAVID L. MacADAM
490
Small samples of colored materials are often fastened at the locations of points on chromaticity diagrams that represent those materials in some specified quality of illumination. Those samples are, of course, correctly located on the diagram only when they are illuminated by that light, but visual adaptation to moderate changes of illumination is so rapid and complete that the appearance of an array of samples so mounted on the chromaticity diagram changes very little. Therefore, a statement that such an array of samples on the chromaticity diagram represents their appearance in the prescribed illumination, regardless of the quality of light actually used to examine the chart, is substantially correct and didactically useful. For the same reason and those mentioned in the preceding paragraph, the illustration facing page 489 is instructive. Despite the severe restrictions and limitations of validity of the distribution of colors on it, I hope readers will find the illustration helpful. Because printed or painted colors absorb every wavelength to some extent, they necessarily reflect less than 100%of the light with which they are illuminated. Because their colors result from absorption distributed over more or less of the spectrum, there is an upper limit of the percentage of light that is reflected by samples that correspond to any particular point in the chromaticity diagram. That limit depends on the quality of the illumination. The limits for the entire chromaticity diagram, for illumination with artificial daylight (CIE Illuminant C) are shown by the contours in Fig. 7. The limits of reflectance of light from an incandescent tungsten lamp (CIE Illuminant A, 2854 K) are shown in Fig.
'x-
FIG. 7. Maximum possible luminous reflectance (or transmittance) of nonfluorescent objects in Illuminant C (artificial daylight).
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FIG. 8. Maximum possible luminous reflectance (or transmittance) of nonfluorescent objects in Illuminant A (incandescenttungsten lamp, 2854 K).
VISUAL SENSITIVITIES TO COLOR DIFFERENCES
The ability of human observers to see small color differences varies throughout the chromaticity diagram; it is different in different directions in any region of the diagram. Those variations, for an observer who had normal color vision, are shown in Fig.' 9. Under the most favorable viewing conditions, he could barely distinguish two equally light colors that are represented by points separated in the chromaticity diagram by distances equal to one-third of any radius of any of the ellipses in Fig. 9. For intermediate positions in the chromaticity diagram, he was barely able to notice color differences intermediate between one third of the lengths of the radii in the same direction in the most nearly neighboring ellipses. The ellipses in Fig. 9 transform to those shown in Fig. 10 in the 1960 CIE chromaticity diagram. Because the goal of the CIE, when it recommended the 1960 diagram, was to provide a diagram on which all of such ellipses would be transformed to equal-size circles, as nearly as possible, Fig. 10 indicates that the goal was only crudely attained. Ellipsoids whose radii in all directions represent color differences involving lightness differences (represented by distances perpendicular to the chromaticity diagram) have also been obtained. 24 Such ellipsoids are so difficult to use that many attempts have been made to develop algebraic formulas with which the noticeability of color difference can be computed, 25 rather than determined graphically. As a result of such a study, the Optical Society Committee on Uniform Color Scales is preparing a set of some four or five hundred colors, each of which shall be, as nearly as possible, equally
COLOR ESSAYS
May 1975
y0.4
491
590
610 0.3 -
490
U
620 FIG. 10.
of Fig. 9 transformed
to CIE 1960 chromatic-
ity diagram.
&
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success by the pioneers of color television. Possibly, they were steered towards Maxwell's principle and spectral sensitivities by Ives's son, Herbert, of the Bell Telephone Laboratories. Herbert Ives was President of the Optical Society of America in 1924 and 1925. He
FIG. 9. Equally noticeable color differences around 25 chromaticities,
on CIE 1931 chromaticity
diagram.
Each point on any
ellipse represents a color about three times a just-noticeable difference from an equally light color having the chromaticity represented by the center of the ellipse,
noticeably different in color and lightness (combined) from each of twelve other nearest neighbors in that set of colors.
26
MAXWELL-IVESCRITERION FOR SPECTRAL SENSITIVITIES FOR COLOR REPRODUCTION
Thirty years after Maxwell invented three-color photography,
dye sensitization
had been found to make pho-
tographic plates sensitive in the green, yellow, and red portions of the spectrum. Frederic Ives, whom the Ives Medal commemorates, applied that discovery to color photography.27 Ives had previously invented the halftone screen, the forerunner of those with which pictures are still printed in newspapers, magazines, books, and much of the graphic arts. Ives wanted to extend the use of halftone screens to printing color pictures. He adopted Maxwell's idea, including the principle that had been neglected by color photographers for three decades, that the spectral sensitivities for color photography should be quantitatively related to the spectral sensitivities that Maxwell had determined for color vision. In his attempts to follow that principle, Ives was probably the first American to dye sensitize photographic plates for color photography. Although the foremost contemporary English investigator of color, Willam Abney, agreed with Ives concerning the appropriateness of simulating the spectral sensitivities of the eye for color photography, and admired color photographs that Ives made with such sensitivities and exhibited in London, color photographers rejected the idea. The idea languished for fifty more years, until it was taken up with enthusiasm and
was donor of the Ives medal and the fifth recipient of it. He was a leading investigator of color vision, colorimetry, and heterochromatic photometry. He invented and demonstrated one of the first television systems. Even today, the spectral sensitivities of most films for color photography do not conform to the MaxwellIves prescription. 28 One of the most frequently noticed consequences is that blue flowers, e.g., heavenly blue morning glories, ageratums, and orchids, appear reddish in color photographs. The bottom and middle pairs of pictures on the color plate that faces p. 492 are reproductions of pairs of photographs taken with commercially available (left) color film and with specially prepared (right) film that approximated the MaxwellIves type of color sensitizations. The upper-left section of the color plate facing p. 492 is a reproduction of the color photograph of a tartan bow that Maxwell originally projected by additive combination in his 1861 demonstration, which was the first demonstration of color photography. The upper-right section of the color plate is a reproduction of one of the sets of additive color (Chromoscope) photographs made by Frederic Ives, who used spectral sensitivities that were as nearly as he could produce in accordance with the Maxwell principle.
*Ives Medal Address at Houston Meeting of Optical Society of America,
J. Opt. Soc. Am. 64, 1371A (1974).
'William Ewart Gladstone, Studies on Homer and the Homeric Age (Oxford University 2
Press,
1858), Vol. 3, Sec. 4, p. 457,
"Homer's Perceptions and Use of Colour. " William Ewart Gladstone,
The Nineteenth Century 2, 366 (1877);
in this article, Gladstone also mentions that "the fragrance of flowers is nowhere noticed in the Old Testament until we reach the Song of Solomon. "
DAVID L. MacADAM 3
Aristotle, Meteorologica 111, 2 translation by E. W. Webster (Clarendon Press, Oxford, 1923); also in D. L. MacAdam,
'OM. Lomonosov, Oratio de origine lucis ...
(This book will be referred
to by title only, sub-
Plato,
G. Palmer, Theory of Colors and Vision, (Leacroft, London, 1777); relevant passages in Sources of Color Science, p. 40.
1971;
by Benjamin Jowett,
Tinzaeus 68, Translation
A microfiche copy of Refs. 11 and 12, complete, combined on one film, can be obtained from Microfiche Publications, 440 Park Ave., South, New York, N. Y. 10016. Order ASIS/
also in Sources of Color Science, p. 1. 5Isaac Newton, Philos.
Trans.
R. Soc. (Lond.) 80, 3075 (1671);
also in Sources of Color Science, p. 14. 6
Isaac Newton, Opticks (SmithandWalford,
London, 1704); rele-
vant passage in Sources of Color Science, p. 33. Figure 1
NAPS 00701. Price $1.50, with order.
12G. Palmer,
greenness)
and that by "blue" he referred
to the greenish
blues for which many workers in color photography and color printing have adopted the name " cyan. "s See Ralph M.
Evans, The Perception of Color (Wiley-Interscience, New York, 1974), p. 65.
If Newton's usage of "blue", had been
retained throughout the subsequent three centuries,
we
would not have had the long-continued dispute as to whether green paint or dye mixtures can or cannot be made by mixing yellow and blue pigments or dyes. As Newton used and most of his the name "blue," and as his predecessors during the 18th century seem to have used it, to successors
refer to greenish short-wave hues, that statement is correct. But, to use "blue" in that sense we should have retained and used "indigo" to refer to the hues of the short wavelengths
that appear neither greenish nor reddish, as Newton did. Instead of this, Newton',s successors came to use "blue", for the hues that Newton called "indigo. " We still do so. This has had two troublesome consequences. First, students of color for the past two centuries have conjectured that Newton had some kind of anomalous color vision that enabled him to see a distinctly different hue in the spectrum between violet (the
slightly reddish, shortest, visible wavelengths) and the greenfree blues.
Second, the shift of the meaning of "blue" from
Newton's greenish blues to the green-free blues left unnamed a broad spectral band that is of great importance for color photography and all of the graphic arts, including painting. That lacuna had to be filled-and
has been, by the color-photog-
raphers' term "cyan." The extremely difficult, and so far futile, task of re-education of artists, teachers and studentsto get them to say "yellow and cyan paints make green"-would
not have been necessary if we had preserved Newton's meaning for "blue" and had kept his name "indigo" for green-free blues. 7 Commission Internationale de l'Eclairage (CIE) Proceedings of the 8th Session, Cambridge 1931; also Deane B. Judd, J. Opt. Soc. Am. 28, 359 (1933); also A. C. Hardy et al., Handbook 8 9
of Colorimetry (MIT Press, Cambridge, Mass., 1936), pp. 61-85. R. A. Weale, Nature 179, 648 (March 30, 1957). E. Mariotte, Oeuvres (Van de Aa, Leyden, 1717).
Mariotte,
seems to have been the first to use "blue" to refer to the color that Newton called "indigo. " All subsequent color-vision theorists until (and including) Thomas Youngfollowed Mariotte's lead in this. Newton's "blue", (the color-photographers' "cyan") would not have the properties assigned to "blue" by Mariotte, Lomonosov (Ref. 10), Palmer (Refs. 11 and 12), 1 of his statement of or Young (Ref. 13). Young's amendment
his theory seems to have resulted from a partial recognition of the problem. But, by shifting "blue" back to the greenish hues in accord with Newton, without recognizing the corresponding need for Newton's term "indigo, " Young was forced to overcorrect and specified "violet" as his primary.
Theory of Light (in French) (Hardouin and Gattey,
Paris, 1786); see note under Ref. 11 for availability of this otherwise nearly inaccessible item; translations of relevant passages in Sources of Color Science, p. 48.
and study of Newton's many uses of the names "indigo" and
"blue" suggest that Newton meant by "indigo" the hues that we now call "blue" (i. e., hues with no appreciable redness or
translation from
11
sequent to this.)
4
65
Russian to Latin by G. Kositzki (Acad. Sci., Petersburgh, 1757); translations of relevant sections to English in Ref. 8.
Sources of Color Science (MIT Press, Cambridge, Mass., 1970), p. 8.
Vol.
13
Thomas Young, Philos. Trans. R. Soc. Lond. 92, 20 (1802). Thomas Young, Philos. Trans. R. Soc. Lond. 92, 395 (1802). i 5Thomas Young, in A Course of Lectures on Natural Philosophy and the Mechanical Arts, edited by P. Kelland (Taylor and Watson, London, 1845), p. 344. Young's return to Newton's concept of "blue" is brought out clearly in the quoted passages of this edition of Young's Lectures. Note, particularly, Young's characterization of blue as one of the three binary combinations (along with "yellow" and 14
"crimson"), "blue" being the combination "of green and violet." Young puts the matter entirely beyond doubt by his statement "The blue produced by combining the whole of the green and violet is not the blue of the spectrum." The crucial point is not
the predicate of Young's sentence, which is certainly truecyan is two thirds of the spectrum, not a pure spectrum colorbut the defining clause. Young's blue, produced by combining the whole of the green and violet, is the color-photographer's cyan. Young was left with no color name by which to refer to
the red-free and green-free short wavelengths. He reassigned "blue" and did not recognize
the significance
of Newton's "in-
digo. " 16
J. C. Maxwell, Proc.
17
R. Inst. G. B. 6, 260 (1972).
Herman von Helmholtz, Handbuchder physiologischen Optik (Vos, Hamburg 1860); the relevant passages were substantially unchanged in the third edition that was translated by J. P. C.
Southall, published by the Optical Society of America, 1924, reprinted by Dover, New York, 1964. 18 Herman von Helmholtz, Pogg. Ann. 87, 45 (1852); Arch. Anat. Physiol. (MUller) 17, 461 (1852); Philos. Mag. Ser. 4, 519 (1852). 19 J. C. Maxwell, Trans. R. Soc. Edinburgh 21, 275 (1857). 20 Gordon Walls, J. History of Medicine 11, 66 (1956). 21 Ralph M. Evans, Scientific American 205, 118 (November 1961). 22 J. C. Maxwell, Proc. R. Soc. (Lond.) 10, 404 (1860). 23 G. Wyszecki and W. S. Stiles, Color Science (Wiley, New York, 1967), pp. 456 and 472. 24 W. R. J. Brown and D. L. MacAdam, J. Opt. Soc. Am. 39, 808 (1949); W. R. J. Brown, J. Opt. Soc. Am. 47, 137 (1957). 25
D. L. MacAdam, Official Digest, Fed. Soc. Paint Technol.
1487 (1965); K. D. Chickering, J. Opt. Soc. Am. 57, 537 (1967); D. L. MacAdam in Industrial Color Technology (Am. Chem. Soc., Washington, D.C., 1971); D. L. MacAdam, in
Color Metrics, edited by J. J. Vos, L. F. C. Friele, and P. L. Walraven (AIC/Holland, Institute for Perception TNO, Soesterberg, Holland, 1972). D. L. MacAdam, J. Opt. Soc. Am. 64, 1691 (1974).
26
27
F. E. Ives, Phot. J. 40, 99 (1900), abridgment in Sources of Color Science, p. 127.
28
D. L. MacAdam, J. Phot. Sci. 14, 229 (1966); Physics Today 20, 27 (January 1967).
4
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JOURNAL OF THE OPTICAL SOCIETY OF AMERICA
MAY 1975
Color essays* David L. MacAdam Research Laboratories, Eastman Kodak Company, Rochester, New York 14650 (Received 30 January 1975) Webster's New Collegiate Dictionary defines "essay" first as "an effort to do something; attempt; trial." Only second is it defined as "a literary composition, analytical or interpretative, dealing with its subject from a more or less limited or personal standpoint." This article is an essay (second meaning) about a few essays (first meaning) that have been made to understand color. Genesis, Aristotle, Plato, Newton, Palmer, Young, Helmholtz, Maxwell, and Frederic Ives are represented. Index Headings: Color; Vision; Colorimetry.
other. Seneca seems to agree, but Galen reverts to the aristotelian triad, which is also found in the Edda and in the Arabian literature, and in the West until the seventeenth century.
Colors have always attracted and fascinated mankind. Although Genesis Chapter 9 does not name the colors in the rainbow, they were certainly seen by the writers. There is nothing but colors to be seen in the rainbow. The uniqueness and glory of the rainbow, which made it appropriate as the token of God's first promise to mankind, results from the striking display of colors in it. Everyone knew what it looked like. Absence of a description of the colors is not surprising. There is very little description of anything in Genesis. The first color name occurs sixteen chapters later, where Esau is reported to have been born all red and hairy. Blue, purple, and scarlet are first mentioned in Exodus 26: 30.
COLOR SCIENCE
Plato4 recommended reticence concerning the study of color: "There will be no difficulty in seeing how and by what mixtures the colors are made... He however who should attempt to verify all this by experiment would forget the difference of the human and the divine nature. For God only has the knowledge and also the power that are able to combine many things into one and again to resolve the one into many. But no. man is or ever will be able to accomplish either the one or the other operation. The law of proportion, according to which the several colors are formed, even if a man
COLOR NAMES
Queen Victoria's prime minister, Gladstone, a leading authority on the Homeric poems, 1pointed out2 "that although Homer had used light in its various forms for his purposes with perhaps greater splendor and effect than any other poet, yet the color adjectives and color descriptions of the poems were not only imperfect but highly ambiguous and confused. " From the paucity of words for description of color in the language known to Homer, Gladstone concluded' that "the organ of color was but partially developed amongthe Greeks of Homer's age. " Subsequently, students of evolution and of color vision have rejected Gladstone's generalization that the ancient Greeks were color blind. On the other hand, philologers have found that, in their early stages of development, all languages have a dearth of color names. After black and white, red is usually the first color name to appear. Yellow comes later, then green and blue. Those four are the unique colors of perception; each can be perceived as uncontaminated with any of the others. Names for orange, scarlet, purple, pink, and violet, which are perceived as combinations of pairs of the perceptually unique colors, occur much later. The presence and absence of names for the various colors are taken as indications of the stage of development of each language. They are not, as Gladstone thought, indications of color blindness or incomplete development of color vision among the people who use that language, but of the sophistication of their language.
knew, he would be foolish in telling,
for he could not
give any necessary reason nor, indeed, any tolerable or probable explanation of it. " Sir Isaac Newton was first to explain the cause of the rainbow. 5 He was able to do so as a result of his study of the colors exhibited by glass prisms, which were popular novelties and decorations. When he passed white light from the sun through a prism, he obtained a beautiful display of colors; because they were not corporeal, he called them spectra (i.e., specters, ghosts), and he called the array of colors a spectrum. Newton listed seven colors that he could see in the spectrum, and for which he knew names: violet, indigo, blue, green, yellow, orange, and red remained at its out that there is a continual and smooth progression and variation of colors between the colors so named. The quality that changes continuously along with spectrum is called hue. By combining the light from the two ends of the spectrum in various proportions, for example by increasing the violet content at first, while the red remained at its maximum strength, and then, with violet at its maximum, decreasing
the red, Newton found that he could
produce many hues that are not found anywhere in the spectrum.
Aristotle3 named three colors, red, green, and blue, in both the primary and the secondary rainbows. Ovid, in Metamorphoses vi 65-7, wrote that the rainbow has a thousand colors, each hardly distinguishable from its neighbor but with extremes very remote from each
In that way, he found that he could complete
the circuit, from the red of the spectrum through pinks, purples, and lavenders, back to the violet with which he began naming the hues. There are no abrupt changes of hue in the progression thus produced, from red through purple to violet. Therefore, Newton represented 483
DAVID L. MacADAM
484
Vol.
65
ORANGE
0
VIOLET
a) 0V
Q, INDIGO
FIG. 1. Newton's color circle.
the entire circuit of colors, through the spectrum from violet through green to red and then through purple back to violet again, with a circular diagram. 6 (Fig. 1.) Because he could reconstitute white light by recombining all of the colors of the spectrum,
Newton represented
white at the center of the circle and placed weak or strong tints of various hues on radii, at distances from the center that corresponded to their strengths. He pointed out that this representation is analogous to a center of gravity. In this analogy, each of the colors in the spectrum may be thought of as a weight. The weight of each color is proportional to the energy of that color. Newton con-
0.9 0.8 0.7 0.6 0.5 y
0.4 0.3 0.2 0.1
FIG. 2. Spectrum locus according to CIE 1931 standard observer for colorimetry.
500 600 Wavelength (nm)
FIG. 3. Weights per watt of spectrum colors, for center-ofgravity determinations with spectrum locus in Fig. 2.
ceived each weight to be located at the point on the circle that represents that particular color in the spectrum. This idea has been a powerful tool in color science, although the spectrum locus (shown in Fig. 2), is now known not to be a circle. 5 Nowadays, we label each color of the spectrum with a number, called wavelength, such as are shown in Fig. 2. The factor of proportionality between energy and weight is different for different wavelengths. It is shown in Fig. 3, which is the sum of the three curves, known as color-mixture functions, shown in Fig. 4, which are the basic data of modern colorimetry .7 Newton suggested that the center of gravity that corresponds to weights located around the spectral locus, proportional to the energies of the several spectrum colors in any particular light, represents the color of that light. The same idea applies to the colors of reflecting or transmitting objects, in which cases the weights are proportional to the energies of the various wavelengths that are reflected or
transmitted. The center of gravity is called the chromaticity of any color and the diagram, Fig. 5, that consists of the spectrum locus and the straight line that joins its short- and long-wavelength extremities (violet and red, respectively) is called the chromaticity diagram. The chromaticities of all physically possible colors are represented by points within or on the spectrum locus and the line between its extremities..
COLOR ESSAYS
May 1975
485
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I
E
M
0.8
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40 -
0.6-
40
0.4
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0.2
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0 400
500
600
Wavelength
FIG. 4. Color-mixture ver for colorimetry.
As a consequence
700
10
(nm)
functions for CIE 1931 standard obser-
400
of the center-of-gravity
principle,
1
I
520 0
0.8
1
1
510 550 06560 0.6
700
transmit any yellow spectrum light. the spectral transmittance of a piece of yellow glass; the curve shows that no wavelength that appears yellow in the spectrum gets through the glass. The wavelengths of the portion of the spectrum that appears yellow are simply not transmitted by the glass. Yet, in white light, the glass appears yellow! Color is therefore not wavelength. Rather, it is the resultant of the combined effects of all of the wavelengths that are present, which is represented by the center of gravity of all of those wavelengths, weighted in proportion to their energies.
I
530
07
500 600 Wavelength (nm)
FIG. 6. Spectral transmittance of a yellow glass that does not
it is wrong to say, as some teachers and textbooks do, that color is wavelength. For example, Fig. 6 shows
0.9
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0.5
580
y
590 600 610 620
0.4 0.3
THREE-COLOR VISION
490
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Ithree-quarters
.S470 \400 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
FIG. 5. Chromaticity diagram for CIE 1931 standard observer for colorimetry.
Newton's contributions to color science were many and great, but he never stated the great simplification that makes color photography, color television, and color printing feasible. He never mentioned that, to produce any color whatever, a mixture of three suitable colors, in suitable amounts, is sufficient. That fact was stated, more or less clearly, by several people during the last quarter of the seventeenth and the first of the eighteenth centuries. 3 As a matter of fact, Aristotle recognized the primacy of red, green, and blue in color vision, and stated that all other colors (he mentioned yellow and orange specifically)
are inter-
mediate between those three, either mixtures of them or result from contrast between them. 3 Twenty centuries after Aristotle (in 1681) a French
DAVID L. MacADAM
486
physicist, Mariotte, 9 said again that three colors are sufficient for the production of all other colors by mixture. Mariotte, however, specified yellow, rather than green, as one of his primaries. He did not propose any theory of color vision. COLOR-VISIONTHEORIES
Lomonosov, a Russian manufacturer of tinted glass, who cited Mariotte and accepted Mariotte's conclusion that three primaries are sufficient and also Mariotte's choice of red, yellow, and blue as primaries, in 1756 associated red with salt, yellow with sulfur, and blue with mercury, loand asserted "their concordance with the salt, sulfur, and mercury, respectively, found in our optic nerves enables us to see these colors... other colors arise from a mixture of these primaries. "' Lomonosov did not suggest that there are different classes of nerves or receptors, each specifically sensitive to one of the primary
colors.
That concept was introduced by Palmer, an English chemist, who omitted alchemy. Like Mariotte and Lomonosov, but without mentioning them, Palmer named red, yellow, and blue as primaries. Palmer ascribed color vision to the presence in the retina of three different kinds of "particles, " each specifically "moved by" one of those colors. Palmer was the first to attribute color blindness to deficiency of sensitivity of any of the
classes of color-specific receptors. Until 1956 completely unheard of, Palmer published a fictitious dialogue"' between himself and a character named Johnson, whose manner of expression seems to mimic that of Dr. Samuel Johnson. Palmer won all of the exchanges. Despite this misrepresentation, forwhich the choleric
Doctor may have made London too hot for
Palmer, the statements concerning color and vision are exceedingly interesting, and a quarter of a century ahead of their time. For example, he wrote, "There is no color in the light... Each ray of light is composed of three rays only: one appears yellow, one red, and the other blue... These rays are present in different proportions, which they keep exactly, notwithstanding decrease or increase of the intensity of the lights... The surface of the retina is composed of particles of three different kinds, corresponding to the three rays of light; each of these particles is moved by its own ray... The complete and uniform motion of these particles produces the sensation of white... The absolute want of motion of these particles, whether by interception of the light, or by reflection from a black substance, produces the sensation of black... Differing excitations of these particles by rays from colored objects or from refracting prisms, produce the sensation of color... Motions of these particles by rays that are all decreased by the same amounts produce only sensations of more or less white; but not of colors. " Nine years later, in Paris, in a French brochure' 2 that he wrote to promote the sale of an artificial daylight lamp for use by artists, Palmer wrote (my translation), "It is quite evident that the retina must be composed of three kinds of fibers, or membranes, each susceptible of being stimulated
by only one of the three primary
Vol. 65
rays.* Equal sensibility of these three classes of fibers constitutes normal vision; any deficiency of sensibility, as well as excessive sensibility of any class, constitutes anomalous color vision.. . Even the best-organized eyes are disturbed temporarily when they look attentively at colors. If we look for a long time at a vivid red and then close our eyes, we see green... When sunlight shines directly in the eye, it causes violent motion of the fibers. For several minutes thereafter, we have sensations of various colors in succession, because the different fibers come to rest at different rates."
Sixteenyears later, or 25 years after Palmer's first publication of the theory, Thomas Young, an English medical doctor, commenting on Newton's studies of color, proposed the same theory of color vision. '3 In the first of a famous series of lectures that dealt with, and presented for the first time much of physical optics, Young included just this one paragraph: "As it is almost impossible to conceive each sensitive point of the retina to contain an infinite number of particles, each capable of vibrating in perfect unison with every possible undulation, it becomes necessary to suppose the number limited, for instance to the three principal colors, red, yellow and blue, and that each of the particles is capable of being put in motion more or less forcibly by undulations differing less or more from perfect unison. Each sensitive filament of the nerve may consist of three portions, one for each principal color." Note that Young, like Palmer in 1777, referred to the
retinal receptors as "particles. " Alsolike Palmer, Young named yellow and blue as primaries. We do not know whether Young's thinking was influenced in any way by Palmer's pamphlets. Within a year, however, he amended his theory with a brief note in the Philosophical Transactions, 14 "It becomes necessary to modify the supposition that I advanced in the last Bakerian lecture, respecting the fibers of the retina, substituting red, green, and violet for red, yellow, and blue." These two passages contain nearly all that Young ever published about color-sensitive particles, or nerve filaments, or fibers, in the retina. The idea is missing from the 1845 edition of Young's collected lectures that Maxwell quoted. 15 That edition merely states that "the perfect sensations of yellow and of blue are produced respectively by mixtures of red and green and of green and violet light, respectively. From these three simple sensations, with their combinations, we obtain seven primitive distinctions of color; the different proportions in which they may be combined afford a variety of tints beyond all calculation. The three simple sensations being red, green, and violet, the three binary combinations are yellow, consisting of red and green; crimson, of red and violet; and blue, of green and violet; the seventh is white light, composed of all three united. The blue produced by combining the whole of the green and
violet is not the blue of the spectrum. " In 1872, Maxwell wrote,
16
concerning
"the use of sen-
sation as a means of investigating the anatomical structure, " that "a remarkable instance of this is the deduction of Helmholtz's theory of the structure of the retina from that of Young with respect to the sensations of color.
I; :
. 11
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. i
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COLOR ESSAYS
May 197 5
Young asserts that there are three elementary sensations of color; Helmholtz asserts that there are three systems of nerves in the retina, each of which has for its function, when acted upon by light or other disturbing agent, to excite in us one of these three sensations. " However, Helmholtz' 7 (in his Physiological Optics (1860))wrote that "Young's theory of the color sensations, like so much else that this marvelous investigator achieved in advance of his time, remained unnoticed until Maxwell directed attention to it. " Actually, Helmholtz18 himself had in 1852 cited and accurately abstracted Young's three-receptor theory of the retina. Maxwell's only mention of Young's proposal of that theory was in a lecture 19 he gave, 3 years later,
in 1855.
After that, Maxwell seems to have forgotten Young's presentation of it. The theory is not mentioned again in Maxwell's collected works until 1872, when, as quoted, he attributed it to Helmholtz! Because Helmholtz called attention to it, 18whereas Young's editor suppressed it, and probably as a result of Maxwell's inaccurate
1872 summary16 of its history,
the theory of the three kinds of color receptors in the retina has been called the Young-Helmholtz theory. Neither Helmholtz nor Maxwell seems to have known of Palmer's
statement
of it 25 and 75 years prior to Young
and Helmholtz, respectively. Therefore, it is curious that Palmer's publications were discovered (in 1956) by Gordon Walls20 as a result of a search on which he was started by an obscure reference in Helfnholtz' s Physiological Optics. 17 Apparently, Helmholtz never saw or even heard of Palmer's pamphlets. His reference was to a second-hand reference to a case of color blindness that Palmer mentioned in his 1786 pamphlet. COLOR PHOTOGRAPHY
To illustrate the three-receptor theory of color vision, in the lecture he delivered in 1855, Maxwell'9 said "This theory of color may be illustrated by a supposed case taken from the art of photography. Let it be required to ascertain the colors of a landscape by means of impressions taken on a preparation equally sensitive to rays of every color. Let a plate of red glass be placed before the camera and an impression taken. The positive of this will be transparent wherever the red light has been abundant in the landscape, and opaque where it has beenwanting. Let it now be put in an optical projector along with the red glass. A red picture will be thrown on the screen. Let this operation be repeated with a green and a violet glass, and by means of three projectors, let the three images be superimposed on the screen. The color of any point on the screen will then depend on that of the corresponding point of the landscape. By properly adjusting the intensities of the projectors, a complete copy of the landscape, as far as color is concerned, will be thrown on the screen. The only apparent difference will be that the copy will be more subdued, or less pure in tint, than the original, because the process is performed twice-first on the screen and then on the retina. This illustration shows how the function that Young attributes to the three systems of nerves may be imitated by optical apparatus.
489
It is therefore unnecessary to search for any direct connection between the wavelengths of the various kinds of light and the sensations produced by them. The threefold partition of the properties of light maybe performed by physical means. The remarkable correspondence between the results of the experiments on different individuals indicates some anatomical arrangementidentical
in all. " The quality of the picture that Maxwell ultimately (in 1861) showed to illustrate his example suffered because he did not have what he prescribed, " a preparation equally sensitive to rays of every color. " The photographic plates available in 1861 were not at all sensitive to red, yellow, or green. How he could have gotten results as good as he did is an interesting puzzle, which is the subject of a very interesting article by Ralph
Evans. 2'
COLORIMETRY
Having thus casually invented the principle of modern color photography, Maxwell went on to perform experiments that elucidated the properties of color vision. He
determined three spectral sensitivities that are typical of normal human color vision, which he had suggested should be mimicked by the three photographs. From his work, 22 modern colorimetry has developed. He learned that the colors of the spectrum should not be represented by a circle, but by a horseshoe-shaped curve, such as is the basis of the modern chromaticity diagram. The chromaticity diagram shown in Fig. 5 was recommended7 in 1931, for general technical use, by the International
Commission
on Illumination
(CIE).
In 1960,
the CIE adopted a modified form of the chromaticity diagram, 23for use "whenever a diagram yielding color spacing more nearly uniform than the (x, y) diagram is desired. " The color plate (opposite) illustrates the arrangement of colors on that diagram. The printed colors are merely schematic. For instance, colors of sufficient saturation to be represented by points near the spectrum locus cannot be printed. For qualitative, suggestive illustration, however, less-saturated colors have been printed all the way out to the spectrum locus; they indicate the appropriate hues and suggest the outward increase of saturation. The distribution of colors is appropriate only when the plate is illuminated by daylight. The abrupt changes of color that are evident in the color plate are intentional deviations from the everywhere smooth and continuous changes of color that are represented by locations in the diagram. A painting that had smooth, continual, gradual changes of color would be too dull and featureless to be worth reproducing. The abrupt, although unrealistic, changes that are evident in the color plate (opposite) are much more effective than would be the correct, gradual changes of colors, in conveying the correct impressions of the changes of hue and saturation from one area to the next. However, for these reasons, the colors printed in the plate are not accurate and should not be used for any quantitative work. The diagram is, however, illustrative of the general idea and trends of distributions of colors represented in the 1960 chromaticity diagram.
DAVID L. MacADAM
490
Small samples of colored materials are often fastened at the locations of points on chromaticity diagrams that represent those materials in some specified quality of illumination. Those samples are, of course, correctly located on the diagram only when they are illuminated by that light, but visual adaptation to moderate changes of illumination is so rapid and complete that the appearance of an array of samples so mounted on the chromaticity diagram changes very little. Therefore, a statement that such an array of samples on the chromaticity diagram represents their appearance in the prescribed illumination, regardless of the quality of light actually used to examine the chart, is substantially correct and didactically useful. For the same reason and those mentioned in the preceding paragraph, the illustration facing page 489 is instructive. Despite the severe restrictions and limitations of validity of the distribution of colors on it, I hope readers will find the illustration helpful. Because printed or painted colors absorb every wavelength to some extent, they necessarily reflect less than 100%of the light with which they are illuminated. Because their colors result from absorption distributed over more or less of the spectrum, there is an upper limit of the percentage of light that is reflected by samples that correspond to any particular point in the chromaticity diagram. That limit depends on the quality of the illumination. The limits for the entire chromaticity diagram, for illumination with artificial daylight (CIE Illuminant C) are shown by the contours in Fig. 7. The limits of reflectance of light from an incandescent tungsten lamp (CIE Illuminant A, 2854 K) are shown in Fig.
'x-
FIG. 7. Maximum possible luminous reflectance (or transmittance) of nonfluorescent objects in Illuminant C (artificial daylight).
t
Vol. 65
0.5
y 0.4 0.3 0.2-
0.1 0
0
QI1
0.2
0.3
0.4
0.5
0.6
0.7
FIG. 8. Maximum possible luminous reflectance (or transmittance) of nonfluorescent objects in Illuminant A (incandescenttungsten lamp, 2854 K).
VISUAL SENSITIVITIES TO COLOR DIFFERENCES
The ability of human observers to see small color differences varies throughout the chromaticity diagram; it is different in different directions in any region of the diagram. Those variations, for an observer who had normal color vision, are shown in Fig.' 9. Under the most favorable viewing conditions, he could barely distinguish two equally light colors that are represented by points separated in the chromaticity diagram by distances equal to one-third of any radius of any of the ellipses in Fig. 9. For intermediate positions in the chromaticity diagram, he was barely able to notice color differences intermediate between one third of the lengths of the radii in the same direction in the most nearly neighboring ellipses. The ellipses in Fig. 9 transform to those shown in Fig. 10 in the 1960 CIE chromaticity diagram. Because the goal of the CIE, when it recommended the 1960 diagram, was to provide a diagram on which all of such ellipses would be transformed to equal-size circles, as nearly as possible, Fig. 10 indicates that the goal was only crudely attained. Ellipsoids whose radii in all directions represent color differences involving lightness differences (represented by distances perpendicular to the chromaticity diagram) have also been obtained. 24 Such ellipsoids are so difficult to use that many attempts have been made to develop algebraic formulas with which the noticeability of color difference can be computed, 25 rather than determined graphically. As a result of such a study, the Optical Society Committee on Uniform Color Scales is preparing a set of some four or five hundred colors, each of which shall be, as nearly as possible, equally
COLOR ESSAYS
May 1975
y0.4
491
590
610 0.3 -
490
U
620 FIG. 10.
of Fig. 9 transformed
to CIE 1960 chromatic-
ity diagram.
&
0.2
Ellipses
O,011 47 68
0
0.1
0.2
0.3
0.4
0.5
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0.7
success by the pioneers of color television. Possibly, they were steered towards Maxwell's principle and spectral sensitivities by Ives's son, Herbert, of the Bell Telephone Laboratories. Herbert Ives was President of the Optical Society of America in 1924 and 1925. He
FIG. 9. Equally noticeable color differences around 25 chromaticities,
on CIE 1931 chromaticity
diagram.
Each point on any
ellipse represents a color about three times a just-noticeable difference from an equally light color having the chromaticity represented by the center of the ellipse,
noticeably different in color and lightness (combined) from each of twelve other nearest neighbors in that set of colors.
26
MAXWELL-IVESCRITERION FOR SPECTRAL SENSITIVITIES FOR COLOR REPRODUCTION
Thirty years after Maxwell invented three-color photography,
dye sensitization
had been found to make pho-
tographic plates sensitive in the green, yellow, and red portions of the spectrum. Frederic Ives, whom the Ives Medal commemorates, applied that discovery to color photography.27 Ives had previously invented the halftone screen, the forerunner of those with which pictures are still printed in newspapers, magazines, books, and much of the graphic arts. Ives wanted to extend the use of halftone screens to printing color pictures. He adopted Maxwell's idea, including the principle that had been neglected by color photographers for three decades, that the spectral sensitivities for color photography should be quantitatively related to the spectral sensitivities that Maxwell had determined for color vision. In his attempts to follow that principle, Ives was probably the first American to dye sensitize photographic plates for color photography. Although the foremost contemporary English investigator of color, Willam Abney, agreed with Ives concerning the appropriateness of simulating the spectral sensitivities of the eye for color photography, and admired color photographs that Ives made with such sensitivities and exhibited in London, color photographers rejected the idea. The idea languished for fifty more years, until it was taken up with enthusiasm and
was donor of the Ives medal and the fifth recipient of it. He was a leading investigator of color vision, colorimetry, and heterochromatic photometry. He invented and demonstrated one of the first television systems. Even today, the spectral sensitivities of most films for color photography do not conform to the MaxwellIves prescription. 28 One of the most frequently noticed consequences is that blue flowers, e.g., heavenly blue morning glories, ageratums, and orchids, appear reddish in color photographs. The bottom and middle pairs of pictures on the color plate that faces p. 492 are reproductions of pairs of photographs taken with commercially available (left) color film and with specially prepared (right) film that approximated the MaxwellIves type of color sensitizations. The upper-left section of the color plate facing p. 492 is a reproduction of the color photograph of a tartan bow that Maxwell originally projected by additive combination in his 1861 demonstration, which was the first demonstration of color photography. The upper-right section of the color plate is a reproduction of one of the sets of additive color (Chromoscope) photographs made by Frederic Ives, who used spectral sensitivities that were as nearly as he could produce in accordance with the Maxwell principle.
*Ives Medal Address at Houston Meeting of Optical Society of America,
J. Opt. Soc. Am. 64, 1371A (1974).
'William Ewart Gladstone, Studies on Homer and the Homeric Age (Oxford University 2
Press,
1858), Vol. 3, Sec. 4, p. 457,
"Homer's Perceptions and Use of Colour. " William Ewart Gladstone,
The Nineteenth Century 2, 366 (1877);
in this article, Gladstone also mentions that "the fragrance of flowers is nowhere noticed in the Old Testament until we reach the Song of Solomon. "
DAVID L. MacADAM 3
Aristotle, Meteorologica 111, 2 translation by E. W. Webster (Clarendon Press, Oxford, 1923); also in D. L. MacAdam,
'OM. Lomonosov, Oratio de origine lucis ...
(This book will be referred
to by title only, sub-
Plato,
G. Palmer, Theory of Colors and Vision, (Leacroft, London, 1777); relevant passages in Sources of Color Science, p. 40.
1971;
by Benjamin Jowett,
Tinzaeus 68, Translation
A microfiche copy of Refs. 11 and 12, complete, combined on one film, can be obtained from Microfiche Publications, 440 Park Ave., South, New York, N. Y. 10016. Order ASIS/
also in Sources of Color Science, p. 1. 5Isaac Newton, Philos.
Trans.
R. Soc. (Lond.) 80, 3075 (1671);
also in Sources of Color Science, p. 14. 6
Isaac Newton, Opticks (SmithandWalford,
London, 1704); rele-
vant passage in Sources of Color Science, p. 33. Figure 1
NAPS 00701. Price $1.50, with order.
12G. Palmer,
greenness)
and that by "blue" he referred
to the greenish
blues for which many workers in color photography and color printing have adopted the name " cyan. "s See Ralph M.
Evans, The Perception of Color (Wiley-Interscience, New York, 1974), p. 65.
If Newton's usage of "blue", had been
retained throughout the subsequent three centuries,
we
would not have had the long-continued dispute as to whether green paint or dye mixtures can or cannot be made by mixing yellow and blue pigments or dyes. As Newton used and most of his the name "blue," and as his predecessors during the 18th century seem to have used it, to successors
refer to greenish short-wave hues, that statement is correct. But, to use "blue" in that sense we should have retained and used "indigo" to refer to the hues of the short wavelengths
that appear neither greenish nor reddish, as Newton did. Instead of this, Newton',s successors came to use "blue", for the hues that Newton called "indigo. " We still do so. This has had two troublesome consequences. First, students of color for the past two centuries have conjectured that Newton had some kind of anomalous color vision that enabled him to see a distinctly different hue in the spectrum between violet (the
slightly reddish, shortest, visible wavelengths) and the greenfree blues.
Second, the shift of the meaning of "blue" from
Newton's greenish blues to the green-free blues left unnamed a broad spectral band that is of great importance for color photography and all of the graphic arts, including painting. That lacuna had to be filled-and
has been, by the color-photog-
raphers' term "cyan." The extremely difficult, and so far futile, task of re-education of artists, teachers and studentsto get them to say "yellow and cyan paints make green"-would
not have been necessary if we had preserved Newton's meaning for "blue" and had kept his name "indigo" for green-free blues. 7 Commission Internationale de l'Eclairage (CIE) Proceedings of the 8th Session, Cambridge 1931; also Deane B. Judd, J. Opt. Soc. Am. 28, 359 (1933); also A. C. Hardy et al., Handbook 8 9
of Colorimetry (MIT Press, Cambridge, Mass., 1936), pp. 61-85. R. A. Weale, Nature 179, 648 (March 30, 1957). E. Mariotte, Oeuvres (Van de Aa, Leyden, 1717).
Mariotte,
seems to have been the first to use "blue" to refer to the color that Newton called "indigo. " All subsequent color-vision theorists until (and including) Thomas Youngfollowed Mariotte's lead in this. Newton's "blue", (the color-photographers' "cyan") would not have the properties assigned to "blue" by Mariotte, Lomonosov (Ref. 10), Palmer (Refs. 11 and 12), 1 of his statement of or Young (Ref. 13). Young's amendment
his theory seems to have resulted from a partial recognition of the problem. But, by shifting "blue" back to the greenish hues in accord with Newton, without recognizing the corresponding need for Newton's term "indigo, " Young was forced to overcorrect and specified "violet" as his primary.
Theory of Light (in French) (Hardouin and Gattey,
Paris, 1786); see note under Ref. 11 for availability of this otherwise nearly inaccessible item; translations of relevant passages in Sources of Color Science, p. 48.
and study of Newton's many uses of the names "indigo" and
"blue" suggest that Newton meant by "indigo" the hues that we now call "blue" (i. e., hues with no appreciable redness or
translation from
11
sequent to this.)
4
65
Russian to Latin by G. Kositzki (Acad. Sci., Petersburgh, 1757); translations of relevant sections to English in Ref. 8.
Sources of Color Science (MIT Press, Cambridge, Mass., 1970), p. 8.
Vol.
13
Thomas Young, Philos. Trans. R. Soc. Lond. 92, 20 (1802). Thomas Young, Philos. Trans. R. Soc. Lond. 92, 395 (1802). i 5Thomas Young, in A Course of Lectures on Natural Philosophy and the Mechanical Arts, edited by P. Kelland (Taylor and Watson, London, 1845), p. 344. Young's return to Newton's concept of "blue" is brought out clearly in the quoted passages of this edition of Young's Lectures. Note, particularly, Young's characterization of blue as one of the three binary combinations (along with "yellow" and 14
"crimson"), "blue" being the combination "of green and violet." Young puts the matter entirely beyond doubt by his statement "The blue produced by combining the whole of the green and violet is not the blue of the spectrum." The crucial point is not
the predicate of Young's sentence, which is certainly truecyan is two thirds of the spectrum, not a pure spectrum colorbut the defining clause. Young's blue, produced by combining the whole of the green and violet, is the color-photographer's cyan. Young was left with no color name by which to refer to
the red-free and green-free short wavelengths. He reassigned "blue" and did not recognize
the significance
of Newton's "in-
digo. " 16
J. C. Maxwell, Proc.
17
R. Inst. G. B. 6, 260 (1972).
Herman von Helmholtz, Handbuchder physiologischen Optik (Vos, Hamburg 1860); the relevant passages were substantially unchanged in the third edition that was translated by J. P. C.
Southall, published by the Optical Society of America, 1924, reprinted by Dover, New York, 1964. 18 Herman von Helmholtz, Pogg. Ann. 87, 45 (1852); Arch. Anat. Physiol. (MUller) 17, 461 (1852); Philos. Mag. Ser. 4, 519 (1852). 19 J. C. Maxwell, Trans. R. Soc. Edinburgh 21, 275 (1857). 20 Gordon Walls, J. History of Medicine 11, 66 (1956). 21 Ralph M. Evans, Scientific American 205, 118 (November 1961). 22 J. C. Maxwell, Proc. R. Soc. (Lond.) 10, 404 (1860). 23 G. Wyszecki and W. S. Stiles, Color Science (Wiley, New York, 1967), pp. 456 and 472. 24 W. R. J. Brown and D. L. MacAdam, J. Opt. Soc. Am. 39, 808 (1949); W. R. J. Brown, J. Opt. Soc. Am. 47, 137 (1957). 25
D. L. MacAdam, Official Digest, Fed. Soc. Paint Technol.
1487 (1965); K. D. Chickering, J. Opt. Soc. Am. 57, 537 (1967); D. L. MacAdam in Industrial Color Technology (Am. Chem. Soc., Washington, D.C., 1971); D. L. MacAdam, in
Color Metrics, edited by J. J. Vos, L. F. C. Friele, and P. L. Walraven (AIC/Holland, Institute for Perception TNO, Soesterberg, Holland, 1972). D. L. MacAdam, J. Opt. Soc. Am. 64, 1691 (1974).
26
27
F. E. Ives, Phot. J. 40, 99 (1900), abridgment in Sources of Color Science, p. 127.
28
D. L. MacAdam, J. Phot. Sci. 14, 229 (1966); Physics Today 20, 27 (January 1967).
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