Harry Helson

The Effect of Chromatic Adaptation on Achromaticity

The tristimulus values of a small (foveal) spot which appears achromatic to an eye which is adapted to a uniform chromatic background have been found experimentally. Various colors have been used for the background, and a considerable range of luminances has been employed for both the background and the spot. It is found that the chromaticity coordinates of the achromatic spot can be predicted by interpolation between the coordinates of a “white point,” which is independent of the state of adaptation, and those of the background. An empirical interpolation formula, which expresses the effect of adaptation as a function of the tristimulus values of the background and the achromatic spot, is presented.

Changes in Hue, Lightness, and Saturation of Surface Colors in Passing from Daylight to Incandescent-Lamp Light

198 Munsell samples, representing 13 hues at various value levels and chromas, including 9 neutrals, were viewed separately on black, gray, and white backgrounds under Illuminant A and matched by samples viewed in Illuminant C by 7 observers, making a total of over 4000 matchings. With these samples, hues containing red and blue become redder in Illuminant A as compared with Illuminant C, while hues from Munsell purple-blue to yellow-red shift toward reddish yellow. Two invariant hues emerge, one between 10R and 5YR with all backgrounds, the other between 5PB and 10B with white background, and between 5PB and 10B with gray and black backgrounds. Greatest shifts occur with black background, least with white. General trends are qualitatively in accord with the Helson-Judd principles of color conversion and in the direction to be expected from the I.C.I. specifications of the samples in the two illuminants. The changes in hue and chroma and the invariant hues as they appear in the I.C.I. plots are not far different from those observed, but the computed changes are much greater than the observed because the I.C.I. computations are based on a completely non-reflecting background and observers screened from the illuminants. Further theoretical treatment of the data seems warranted.

Man as a meter

The use of units such as the foot and the mile, which are obviously of physiological origin, indicates that physical measurements are based on direct human experiences. The definitions of many fundamental physical quantities have resulted from attempts to quantify direct sensory experiences. Thus, our scale of masses has grown from the distinction “heavier‐lighter”; our temperature scale from “hotter‐colder”; our light intensity scale from “brighter‐dimmer”. It is true that the physicist has, to a large extent, replaced these personal judgments by the pointer readings or the counts given by quantitative instruments, but surprisingly accurate quantitative measurements may be made by the human organism without the aid of such instruments. In comparing the human meter with devices which the physicist is more inclined to trust, we shall find it advisable to set up a parallelism between the terms used by the physicist and those used by the psychologist. The signal supplied to a meter is exactly analogous to th...