Four kinds of simple estimation equations are
The phenomena are well-known and are identified with the Helmholtz-Kohlrausch effect (abbreviated as the H-proposed for the Helmholtz-Kohlrausch effect. Two of them can be used for luminous colors, and the other two K effect hereafter). A large number of studies have been reported so far. 1 The effect is also called the B/L (bright-for object colors. In each of luminous and object colors, the two estimation equations are given to each of the ness/luminance) or the L/Y (lightness/luminance-factor) ratio effect. The definition of the H-K effect used in the Variable-Achromatic-Color (VAC) and the Variable-Chromatic-Color (VCC) methods. All the equations are present article is described in Appendix I in detail.
It is believed that the H-K effect is caused by a contri-similar in type to the Ware-Cowan equation. They give the ratio between luminance (or metric lightness) of test bution of chromatic component in a chromatic color stimulus to its brightness (or perceived lightness). However, color stimulus and its equivalent luminance (or equivalent lightness) directly. Though their computations are the problem is more complex than expected. The contribution of chromatic component to brightness (or per-simple, they can apply to various H-K effects including their adapting luminance dependency. The applicable ceived lightness) is different for different hues of chromatic color stimuli.
fields of the proposed equations are wider than those of the Ware-Cowan equation. The proposed equations can
For this reason, very few theoretical studies have been reported on the H-K effect, though many experimental be applied to predict the H-K effect within the whole chromaticity gamut including spectral colors, spectral lu-studies have been done by various researchers. Only an empirical formula called the Ware-Cowan equation 2 is minosity functions based on direct color matching from 0.01 Td to 100 000 Td using the photopic and the scotopic well-known, and it has been used within CIE. 3 On the H-K effect, the present author has already re-spectral luminosity functions specified by CIE, equivalent lightness values of NCS colors, and others. ᭧ 1997 John