Page:Journal of the Optical Society of America, volume 30, number 12.pdf/47

DECEMBER, 1940

Johns Hopkins University, Baltimore, Maryland

The spacing problem consists in the detection and correction of errors of allocation in surface color space of the regular 1929 Munsell samples. This amounts to the adjustment of imperfections in a real chromatic skeleton of 400 units so that it will more properly fit into an ideal chromatic body. For the last five years various observers have been making visual estimates of the color spacings and the accumulated data have been summarized and are presented herewith in tabular form. These data will provide a psychological basis for defining smooth contours of Munsell hue, value, and chroma in terms of the 1931 I.C.I. colorimetric coordinate system. The plan is to present those contours in the final report. The present report includes charts of a preliminary smoothing of chroma based on the earlier visual estimates.

HE familiar principle of specifying color by spatial location underlies the design of various color solids. The ideal of this investigation is a psychological color-solid in which cylindrical coordinates of Euclidean space represent the principal at tributes of colors perceived as belonging to surfaces and equal linear extents represent equal sense-distances. Along the color scales there is variation in but one attribute at a time, and the scalar graduations are perceptually uniform. Furthermore, any horizontal section through the solid would define a plane of constant lightness (Munsell value) while any vertical plane originating at the achromatic axis would be a plane of constant hue. Finally, a cylindrical section concentric with the axis would constitute a surface of constant saturation (Munsell chroma). Just how closely this psychological ideal can be realized in either theory or practice still remains somewhat conjectural.

One question arises at once. A marked influence of surrounding-field reflectance on surface-color perception has long been evident (,, , , , , , p. 546; ). Since the color perceived as belonging to a surface may vary with the background, there is the question of what ground to employ. Present views suggest grounds similar to samples for ideal viewing but continual changes in observational conditions seemed neither feasible nor desirable in the present study. Therefore, the subcommittee decided to have all observations made against three approximately neutral grounds of high, low, and intermediate reflectance (white, black and gray), and to decide later, by reference to the comparative data thus secured, what to do regarding this important problem of specifying an ideal or standard viewing ground.

The principal practical advantage of defining