Günter Wyszecki

Color Science: Concepts and Methods, Quantitative Data and Formulas

Physical Data. The Eye. Colorimetry. Photometry. Visual Equivalence and Visual Matching. Uniform Color Scales. Visual Thresholds. Theories and Models of Color Vision. Appendix. References. Author and Subject Indexes.

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.

New Color-Matching Ellipses

Three new sets of color-matching experiments have been made by three observers binocularly viewing a bipartite field (each half subtending 3°) of a three-primary colorimeter. Each set of color-matching data refers to 28 test colors scattered over the chromaticity gamut provided by the colorimeter. The luminance of each test color was 12 cd · m−2; the white surround (subtending 40°) was maintained at 6 cd · m−2. The elliptical cross sections (for Y = const) of the observed color-matching ellipsoids are compared with results previously published by MacAdam, and Brown and MacAdam. In view of the inherent experimental uncertainties of data of this kind, the new color-matching ellipses correlate well with those obtained by Brown, and Brown and MacAdam, but show significant deviations from those obtained by MacAdam’s observer P. G. Nutting, Jr. The discrepancies are puzzling. A note of caution is added with regard to the usefulness of color-matching ellipses in testing line elements. A set of color-matching ellipses can reveal only little of the visual mechanism that governs the precision of color matching and its assumed direct correlation with judging small color differences. It appears that fundamentally different line elements can reproduce almost equally well a given set of color-matching ellipses.

Wavelength discrimination for point sources.

Wavelength discrimination curves have been measured for two normal trichromats for three field sizes and various intensities. In order to obtain high-intensity levels in the blue region, a high-pressure xenon arc was used as a source. Results for the 1° field agree essentially with those of previous workers. The higher blue intensities have led to the establishment of a definite minimum at 420 mμ. Results for the smaller field sizes differ considerably from previous data. The chief reason for this seems to he in the viewing technique used. While previous authors employed strict fixation, this paper reports data obtained with a scanning technique. The effect of different methods of plotting is briefly discussed.

Counting metameric object-color stimuli using frequency-limited spectral reflectance functions

This paper is a supplementary article on counting metamers to one presented earlier in this journal. Whereas in the previous study the basic collection of object-color stimuli contained “jagged” spectral reflectance curves, including a great many that exhibit extreme reflectance variations within small spectral ranges, the present paper makes use of basic collections which contain “smooth” spectral reflectance curves generated by frequency-limited functions. The results obtained in the present study are similar to those obtained previously, particularly when the limiting frequency of the spectral reflectance functions is set at ω = 1/50. Such a limiting frequency leads to spectral reflectance curves of typically four oscillations within the visible spectrum, making them resemble practical spectral reflectance curves.

Correlate for Lightness in Terms of CIE Chromaticity Coordinates and Luminous Reflectance

This is a report on a side-line experiment of the OSA Committee on Uniform Color Scales. It gives new data on L/Y ratios in terms of CIE chromaticity coordinates, based on observations made by seventy-six observers on forty-three colored ceramic tiles. The mean results are given in tabular and graphical form and compared with similar results obtained previously by other investigators.

High-level trichromatic color matching and the pigment-bleaching hypothesis

Abstract Maxwell-type color matches with reference to a given “white” stimulus at a retinal illuminance of 100,000 td can be predicted from Maxwell-type color matches made with reference to the same “white” stimulus but at a retinal illuminance of 1000 td by means of a simple hypothesis of bleaching of the visual pigments. The spectral distributions of the fundamentals of color vision, the densities of the visual pigments of the 1000 and 100,000 td levels, as well as the spectral transmit tances of the lens and macula pigments which must be assumed as essential parameters of the visual mechanism emerge with values which appear consistent with those obtained from other investigations. While both the “red” and the “green” fundamentals exhibit strong and predictable bleaching characteristics, the “blue” fundamental appears to bleach only very little under the given experimental conditions. The somewhat unexpected behavior of the “blue” fundamental is not explained, but it is suggested that the pigment associated with the “blue” fundamental may be in a near-diluted state already at moderate levels of retinal illuminance, making it difficult to detect bleaching at higher levels.

Evaluation of Metameric Colors

A method is described which allows a quick evaluation of a large number of spectral reflectance (transmittance) curves which with respect to a given light source and the 1931 C.I.E. standard observer produce metameric colors. The method is based on a precalculated set of linearly independent metameric blacks and demonstrated by a numerical example.

Color-Difference Matches

For a given pair of colors (i) and (j) of the same brightness, an observer generates in a visual colorimeter a third color (i,j) such that he perceives all three colors to be equally bright and the three differences between them to be of equal size. In color-perception space, the three colors form an equilateral triangle on a surface of constant brightness. The precision of setting the third color (i,j) can be characterized by ellipsoids in color space. Three observers take part in the experiment and over 30 pairs of colors (i) and (j) are presented to them. In approximately 50% of the cases a given observer will agree, within the uncertainty of his own data, with the mean color (i,j) of another observer. The corresponding color-difference-matching ellipses (cross sections of the ellipsoids) of the different observers show a satisfactory correlation. The experimental data presented may be used to test color-difference formulas and the chromaticity spacing in UCS diagrams.

Luminosity functions for various field sizes and levels of retinal illuminance.

Luminosity functions have been measured for four normal trichromats for three field sizes (1°, 12′, 1.5′) at various levels of retinal illuminance. As the illuminance decreases the reverse Purkinje effect is observed, i.e., there is a decrease in spectral sensitivity of the eye to blue relative to red. The dependence of the spectral sensitivity on field size is not pronounced for the experimental conditions used here. Considerable differences are found between the luminosity functions of the observers. These differences are several times larger than those obtained on repeated curves by any observer. It is shown that the use of an average luminosity function may give very misleading results in some cases.A comparison is made with the data obtained by previous workers.