Page:Journal of the Optical Society of America, volume 33, number 7.pdf/68

 Fig. 1b, has the same horizontal dimensions as the shorter one; but the vertical dimension has been increased by a factor of 4 to take account of the relatively greater lightness valence at the limen. Thus this model is designed to suggest the equation of the scales for the smallest, or just perceptible, color differences (9). Just why different equations should be required for perceived color steps of different orders of magnitude is still an unsettled question.

It may be noted that the conditions of viewing or method of observation were taken into account in the design of these models of a psychological, or “equal-sense-step,” color solid. Much as a single standard observer with standard observing conditions is assumed for all I.C.I. diagrams, so a normal observer with a set of standard observing conditions is required for the psychological color solid.

There is an important difference between the psychological solid presented in Fig. 1 and the analogous psychophysical solid in I.C.I. color space described and illustrated by MacAdam (7). The conditions for the psychological solid must be realizable in practice because this solid by definition represents real conscious responses. The I.C.I. system, on the other hand, fulfills its valuable functions of specification and transformation without the necessity of realizing its standard observer.

Figure 2 shows horizontal sections through the solid at Munsell value levels 1/ to 9/. The