Ralph M. Evans

Prediction of Color Appearance with Different Adaptation Illuminations

Specifications are reported of colors which match in appearance with relatively complete adaptation to daylight, tungsten, and a third greenish illuminant. A computational procedure is described for predicting the color appearance of everyday objects. This procedure is based on the known spectral character of the given object in relation to the appearance data presented here. Certain implications for color vision theory are included.

Variables of perceived color.

The variables of perceived color are reconsidered critically. Two ways of viewing color are distinguished as modes of perception: (1) perceiving the effect of light entering the eye, as such, and (2) perceiving the effect produced by an object on the perceived color of light. For the light mode of perception, there are at least four and possibly six or more independent perceptual variables for the general case. For the object mode this number is reduced by at least one. Both modes can be reduced to three perceptual variables by simple restrictions but, in general, the three to which they reduce are not the same.Four perceptual variables are necessary (and perhaps sufficient) to describe the color perceptions produced by light sources, illumination, and both reflecting and transmitting objects. They each produce part or all of the perceptions of the four-dimensional color-perception continuum produced by a single homogeneous aperture color with a homogeneous surround.

Fluorescence and Gray Content of Surface Colors

An investigation of the colors in the Munsell 5R plane and an extension of this study to colors produced in a small aperture in a large white illuminated surround have led to the discovery of some interesting and novel relationships. It is found that under these conditions the domain of surface color perception includes the whole of the range from V = 0 to 10 and pc = 0 to 1.0 and under some conditions more. For a color of a given dominant wavelength there is a locus lying wholly within this space along which lie colors that do not appear to contain gray. If luminance or purity is increased above a point on this line, the sample takes on the appearance characteristic of a fluorescent material. If either is decreased below a point on this line, the color is perceived as having a gray component added to the purely chromatic component in increasing amounts until at pc = 0, there is no chromatic component perception of the color or at low values of V the sample appears black. Above a point somewhat higher than surround luminance, the appearance of fluorescence ceases and the surface mode changes to the illuminant mode, the saturation of the perceived color decreasing with increasing luminance above this point. An hypothesis is suggested to explain the facts and it is pointed out that more than one kind of “brightness” is necessarily involved.

Influence on Color Perception of Adaptation to Illumination

Six experienced observers made consistent determinations of various colors which appeared the same with adaptation to tungsten light and to artificial daylight. These observations were made with each eye viewing a different color patch and with the patches appearing juxtaposed at the middle of a fused binocular field. The method was to make the two juxtaposed patches match by adjusting one of them, sometimes when both eyes were adapted to the same illumination and sometimes to the different illuminations. Plots of the data in the CIE chromaticity diagram indicate a systematic shift in color appearance toward the blues when adaptation was changed from daylight to tungsten; or toward the yellows when adaptation was changed from tungsten to daylight. The magnitude of this color shift was substantial, at least in the considerable color region investigated, for here the average length of the representative vectors was 0.10 in CIE terms or of the order of 20 just perceptible color differences. Qualitatively, the results confirm those of Hunt and of Winch and Young. The theoretical implications will be discussed in a later paper.

Chromatic Strength of Colors: Dominant Wavelength and Purity

A new visual threshold lying between the colors that appear to contain gray and those that appear fluorescent has been determined as a function of dominant wavelength at high purity on a white (7000 K) background. The function is found to be an approximately constant multiple of the previously known purity threshold; it is independent of purity and hence is named chromatic threshold. The value of the function for any given chromaticity is shown to be a measure of its chromatic strength in (a) neutralizing its complementary color, (b) producing Munsell Chroma. It is considered also in terms of the Hering–Johansson “Natural Color System,” where it appears to specify the “clear” series, and in terms of the Ostwald System.

Chromatic Strengths of Colors, Part II. The Munsell System

An approximate model of the Munsell color space in terms of dominant wavelength and colorimetric purity is described. This approximation space describes the Value and Chroma relations as functions of chromaticity and the Chroma relations as a function of colorimetric purity, but does not describe the Hue spacing. A single curve describes (approximately) the chromaticities in all Chroma circles and this curve is the same as that describing the threshold between those colors which appear to contain gray and those that appear to be fluorescent (G0) as described in Part I. The loci of the G0 colors and of colors with constant gray content are also described for the approximation space.

Chromatic Strength of Colors, III. Chromatic Surrounds and Discussion

The relative luminance of a color at the threshold between gray content and apparent fluorescence (fluorence) is not much affected by the luminance of the surround but is greatly affected by its chromaticity. The results indicate that the continuum of color perceptions of a color stimulus seen against a variety of backgrounds is four dimensional. A new concept, “specific hue brilliance,” is shown to describe both the relationship of gray content to the purity of a color of a given dominant wavelength and the appearance of high-purity colors at constant luminance.

Color Constancy in Shadows

Because color samples perceived as surfaces or objects tend to look much the same under various viewing conditions, they are said to possess color constancy. The viewing condition chosen for study was an obvious shadow of daylight quality falling on a color sample and part of a surrounding white field; the less the effect of the shadow on the color appearance of the sample, the greater would be the color constancy. The purpose of the study was to make evaluations of the color constancy, both over all and by attributes, of ten color samples viewed one at a time under the standard shadow. The method was to present the color samples in the surface mode of appearance and to match them with a calorimeter, the field of which was also perceived in the surface mode. In some trials the shadow was present, in some absent, and in others there was no perceived shadow but rather the sample luminance alone was reduced proportionally. These match data were converted to the Munsell system of renotation. Brunswik-type constancy ratios were formed in terms of Munsell hue, chroma, and value taken separately. The results indicate the constancy of the hue, saturation, and lightness of the surface color perceptions. There was evidence of considerable constancy in all three attributes. After weighting the data for each attribute in accordance with an appropriate color difference formula, an estimate of the combined or over-all color constancy was obtained.

On some aspects of white, gray, and black.

In any system which defines color on the basis of the non-spatial, non-temporal properties of light it is shown that white, gray, and black are not colors. The perception of white is shown to be caused by a physical property of the object not associated with its selectivity. This property is the diffusion of light. White is shown to be one extreme of a series starting at clear and passing through various degrees of pale white to a maximum. Gray is shown to be the perception of the relative visual effectiveness of the light from two areas and cannot be seen unless at least two areas are present. Black is considered the extreme in lack of relative effectiveness.

Influence of shadow quality on color appearance.

Shadows of skylight quality and of daylight quality were cast upon various test samples. The color appearance of a test sample, both as shadowed and as unshadowed, was matched by adjusting the field of a visual colorimeter. Only the skylight shadow evoked any trend in hue, and this was toward the blue; but both shadows produced systematic losses in saturation and lightness. Still none of these perceptual changes was as great as the corresponding colorimetric change in the test stimulus would suggest; in other words, considerable color constancy was associated with both shadows. The difference in effect between the two kinds of shadows was appreciable as evaluated in Munsell hue but seemed almost negligible as regards value and chroma. These results refer to the attention-directed experimental observations; more casual viewing presumably would result in more constancy effect and even less differential effect.