Deane B. Judd

Color in Business Science and Industry

For a number of years the Optical Society of America has run parallel sessions one of which is devoted to papers on spectroscopy and the other to papers on color. The ardent spectroscopist has thus been carefully isolated from color problems since instantaneous transitions between the quantum levels of the Keystone and Georgian rooms have a low probability if they are not altogether excluded by some corollary of the Pauli principle. The few spectroscopists who have by some mischance gotten into the wrong session have probably left quickly since the technical language used by the color people is as forboding to applied spectroscopists as the terminology of spectroscopists is to their uninitiated brethern. Color is nevertheless a branch of applied spectroscopy even if it does not deal with the analysis of materials but rather with the appearance of materials.

Spectral Distribution of Typical Daylight as a Function of Correlated Color Temperature

Spectral distributions of 622 samples of daylight (skylight, and sunlight plus skylight) have been subjected to characteristic vector analysis, as composite data and in three subgroups (99 distributions measured by Budde; 249, by Condit; and 274, by Henderson and Hodgkiss). The chromaticity coordinates (x,y) computed from these distributions have been compared with direct visual determinations of chromaticity coordinates of daylight by Nayatani and Wyszecki, and by Chamberlin, Lawrence, and Belbin. It was found that the chromaticities indicated by the spectral distributions and by direct visual colorimetry cluster about the curve: y = 2.870x−3.000x2−0.275. This curve of typical daylight chromaticities falls slightly on the green side of the Planckian locus. From the mean and the first two characteristic vectors of the composite data, spectral distribution curves have been reconstituted by choice of scalar multiples of the vectors such that the chromaticity points fall on the curve of typical daylight chromaticities at places corresponding to correlated color temperatures of 4800°, 5500°, 6500°, 7500°, and 10 000°K. The representative character of these reconstituted spectral-distribution curves has been established by comparison with the measured curves from each subgroup yielding the closest approximation to the same chromaticities. The agreement so found suggests that this family of curves is more representative of the various phases of daylight between correlated color temperatures 4800° and 10 000°K than any previously derived distributions.

FINAL REPORT OF THE O.S.A. SUBCOMMITTEE ON SPACING OF THE MUNSELL COLORS

This report presents the characteristics of a modified and enlarged Munsell solid which has been evolved from the 1940 visual estimates of the Munsell Book of Color samples. All three dimensions have been carefully reviewed and extensively revised. The newly defined loci of constant hue have been extended closer to the extremes of value while the loci of constant chroma have been extrapolated to the pigment maximum. The dimension of value has been redefined without substantial departure from the Munsell-Sloan-Godlove scale. By the above changes a solid is achieved which approaches more closely to A. H. Munsell’s dual ideal of psychological equispacing and precise applicability. The new solid is defined in terms of the I.C.I. standard coordinate system and Illuminant C.

Hue Saturation and Lightness of Surface Colors with Chromatic Illumination

The visual mechanism of a normal observer is so constructed that objects keep nearly their daylight colors even when the illuminant departs markedly from average daylight. The processes by means of which the observer adapts to the illuminant or discounts most of the effect of a nondaylight illuminant are complicated; they are known to be partly retinal and partly cortical. By taking into account the various fragments of both qualitative and quantitative information to be found in the literature, relations have been formulated by means of which it is possible to compute approximately the hue, saturation, and lightness (tint, value) of a surface color from the tristimulus specifications of the light reflected from the surface and of the light reflected from the background against which it is viewed. Preliminary observations of 15 surfaces under each of 5 different illuminants have demonstrated the adequacy of the formulation and have led to an approximate evaluation of the constants appearing in it. More detailed and extensive observations have been carried out in the psychological laboratories of Bryn Mawr College. and these observations have resulted in an improved formulation.

A Maxwell Triangle Yielding Uniform Chromaticity Scales

A colorimetric coordinate system has been found by trial and error whose Maxwell triangle has the useful property that the length of any line on it is a close measure of the chromaticity difference between the stimuli represented at the extremes of the line. Such accurate chromaticity scales may be derived from this triangle merely by stepping off equal intervals on it that it has been called the “uniform-scale triangle.” The definition of the system is given, and also a comparison of experimental sensibility data with corresponding data derived from the triangle. An important application of this coordinate system is its use in finding from any series of colors the one most resembling a neighboring color of the same brilliance, for example, the finding of the nearest color temperature for a neighboring non-Planckian stimulus. The method is to draw the shortest line from the point representing the non-Planckian stimulus to the Planckian locus.

Estimation of Chromaticity Differences and Nearest Color Temperature on the Standard 1931 ICI Colorimetric Coordinate System

Estimation of chromaticity differences has been facilitated by the preparation of a standard mixture diagram showing by a group of ellipses the scales of perceptibility at the various parts of the diagram. The distances from the boundaries of the ellipses to their respective “centers” all correspond approximately to the same number (100) of “least perceptible differences.” The estimation of nearest color temperature has been facilitated by the preparation of a mixture diagram on which is shown a family of straight lines intersecting the Planckian locus; each straight line corresponds approximately to the locus of points representing stimuli of chromaticity more closely resembling that of the Planckian radiator at the intersection than that of any other Planckian radiator.

Appraisal of Land’s Work on Two-Primary Color Projections

An analysis of the results of Land’s experiments with two-primary color projections has been carried out in terms of the known phenomena of object-color perception. It is shown that no new theory is required for the prediction of Land’s result that two-primary color projections can produce object-color perceptions of all hues; nor for his result that many choices of pairs of primaries yield substantially the same object-color perceptions. Land’s hypothesis that when the colors of the patches of light making up a scene are restricted to a one-dimensional variation of any sort, the observer usually perceives the objects in that scene as essentially without hue, is new; several special cases of it are supported by previous work as well as Land’s. This hypothesis deserves the serious attention of research workers in object-color perception.

CHROMATICITY SENSIBILITY TO STIMULUS DIFFERENCES

There is described in the present paper a method, discovered empirically, for computing the approximate number of “least perceptible differences” between any two colors of the same brightness whose specifications are available. This method has been stated in simple form as an empirical relation; it is shown to be in substantial agreement with extant sensibility data of the following types: (1) the least wave-length difference perceptible in the pure spectrum as a function of wave length (Steindler, Jones); (2) the least dominant-wave-length difference perceptible at constant purity as a function of purity (Watson, Tyndall); (3) the least purity difference perceptible at constant dominant wave length as a function of purity (Donath); (4) the least purity difference perceptible near zero purity as a function of dominant wave length (Priest, Brickwedde), and (5) the least color-temperature difference perceptible as a function of color temperature (Priest). A mixture diagram is included showing colors specified by their trilinear coordinates and by the dominant wave length and purity of their stimuli. From this diagram the number of “least perceptible differences” separating any two colors of the same brightness may be read with a degree of certainty indicated by the comparisons here presented.The empirical relation was originally expressed in terms of distribution (“sensation,” “excitation”) curves which suggest a three-components theory of vision (Young-Helmholtz); but it has been re-expressed in terms of curves which suggest an opponent-colors theory (Hering). Since this re-expression is nearly as convenient as the original expression, it is concluded that such success as has been demonstrated for the empirical relation can not be used as an argument for either form of theory; rather is a theory suggested which is more complex than either, such as that of G. E. Muller.

Terms, Definitions, and Symbols in Reflectometry

Angular conditions of incidence are described as hemispherical, conical, or directional; the same adjectives are used to describe the angular conditions of collection. This classification of angular conditions leads to nine kinds of reflectance; symbols for them are proposed in which 2π, g, and θ0, ϕ0 refer to hemispherical, conical, and directional incidence, 2π, g′, and θr,ϕr refer to the corresponding kinds of collection. Use of the perfectly reflecting mirror and of the perfectly reflecting diffuser as reference standards in reflectometry is discussed. Three of the nine reflectance ratios, specimen to perfect diffuser, in which the collection is directional have already been named radiance [luminance] factor. It is proposed to differentiate them by angular condition of incidence. It is also proposed to name the other six ratios: reflectance factor qualified by the same adjectives identifying the type of incidence and collection as are used for reflectance. The interrelationships of these 18 concepts are shown both by formulas for computing one from another and by diagrams indicating the process (integration, summation, averaging, equality, reflectance of perfect difluser, and reciprocity) by which values of one concept may be computed from those of another.

Tritanopia with Abnormally Heavy Ocular Pigmentation

The chromaticity confusions characteristic of tritanopia may be represented on the (x, y)-chromaticity diagram by a family of straight lines intersecting at a copunctal point near the shortwave extreme of the spectrum locus. A case of congenital tritanopia is reported that departs from typical tritanopia, first by having a luminosity function abnormally curtailed on the shortwave end, second by having chromaticity confusions among object colors describable by straight lines on the (x, y)-plot intersecting in an area surrounding the spectrum locus at 460 mμ instead of near the shortwave extreme, and third by confusing incandescent lamp light at a color temperature of 2900°K with the spectrum at 586 mμ instead of the typical tritanopic value of 579 mμ. It has been found that all three of these disagreements with typical tritanopia are to be expected from a tritanope possessing normal macular pigmentation combined with an ocular pigment five times normal. We believe, therefore, that this case of atypical tritanopia departs from typical tritanopia because of abnormally heavy ocular pigmentation.