S. M. Newhall

Comparison of Successive with Simultaneous Color Matching

The two matching methods were compared to see whether they gave substantially different results. Such a difference would be of interest because successive matching is common in everyday life whereas simultaneous matching is standard colorimetric practice. An experiment was performed in which 25 test colors were matched by both methods. The successive or memory method yielded (1) the higher variability of replicative matchings, (2) the shorter matching times, (3) systematically higher purities, and (4) somewhat higher luminances. Three supplementary experiments are cited which are confirmatory with respect to the principal finding of extra purity and luminance usually required for the memory matches. This increased apparent strength of the remembered colors seems to be a direct consequence of the selectivity of memory.

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.

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.

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.

Color perception in small test fields.

Several small test colors were varied in size and shape to discover possible effects on their color appearance. Color appearance was evaluated calorimetrically with a binocular septum viewer; the test color was viewed with one eye under the given conditions and matched with a variable mixture presented to the other eye under standard conditions. Increasing the area of the test colors from 25 to 500 sq min of visual angle evoked some significant increases in saturation and characteristic shifts in hue. These changes in perceived color were found to be independent of marked changes in rectangular shape. The observed changes are related to small area tritanopia.

I.C.I. specifications of difference limens for Munsell hue, value, and chroma.

In an earlier two-year study, color difference limens of three observers were determined separately for each of the Munsell analogs of hue, saturation, and lightness at each of a number of locations in the surface-color solid. The present paper makes these data available in terms of the standard I.C.I. system and so helps to make that system more visually meaningful. In addition to detailed tabulations of the x, y, and Y specifications, the means of the chromatic difference limens of the three observers are plotted in the I.C.I. chromaticity coordinates at three levels of reflectance. Absolute chroma limens for Munsell red, yellow, green, blue, and purple are tabulated at seven levels of reflectance. The general order of magnitude of all the liminal determinations may be indicated by the fact that the trichromatic coefficients and reflectance figures representing them are confined to the third and fourth decimal places. In terms of distances in the chromaticity diagram, the general mean hue and chroma limens were, respectively, 0.0023 and 0.0035; and the mean Fechner fraction from the value data turned out to be 0.0087.

Space error in color matching.

Recent reports indicate that stimulus position affects appearance in some visual situations. The present investigation was designed to yield information on the size of such a space error in color matching, the precision of color matching as a function of positional relations of the test and matching fields, and matching time as a function of position. Differences caused by position were not obtained. This is a reassuring result, since it has usually been assumed that no space error exists when symmetrical foveal areas are used for color matching.

Effects of Visual Angle on Visual Perception

If the visual angle subtended by an object is varied, at by varying the viewing distance, the appearance of the object may also change. Appropriate comparisons can reveal obvious changes as in perceived size, perceptible detail, eye-comfort, apparent color and realistic effect. There are, however, a number of factors in commercial motion-picture viewing situations which tend to minimize differential effects of visual angle on motion-picture perceptions.