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''Visual clarity'' or ''feeling of contrast'' of object colors under illumination is affected significantly by changing color-rendering property of light source used. Though the feeling of contrast is considered one of the most important characteristics on color-rendering properties of light sources, it cannot be estimated adequately by using the present R a method. The new index FCI is proposed for estimating the effect of feeling of contrast quantitatively under any light sources. The FCI is derived using a simple transformation of the gamut area, which is constituted by a specially selected fourcolor combination in CIE LAB color space. The FCI correlates well with the illuminance ratio for equal feeling of contrast (or equal visual clarity) on various light sources reported so far. Using the FCI together with the present CIE R a , the color-rendering capability of a light source can be well clarified.

A newly modified opponent-colors theory is proposed, with the introduction of a concept of relative chroma. Its maximum value is different for different hues and is determined by the chromatic strength of each hue. The newly modified theory is able to clarify the following four points: (1) a contradiction found between color attributes (whiteness, w; blackness, s; and grayness value, gv) for different chromaticness values in the NCS color-order system; (2) the difference between a constant hue interval and its corresponding perceived hue change (or hue color difference) in the neighborhood of each of the four dual colors raised by Hård et al. (Color Res Appl 1996;21:180 -205, 206 -220)

The following questions were raised to chroma and hue scales of the nonlinear color‐appearance model: 1) significant nonuniformities of the chroma scales for different hues, and 2) deviations of hue scale between the model and the Munsell and the NCS schemes. It was suggested that the problems were caused by the use of the coefficient es(0) proposed by Hunt. Instead of es(0), a new coefficient Es(0) was proposed, which corresponds to the chromatic strengths of spectral colors (including colors on the redpurple locus). By using Es(0), the nonlinear color‐appearance model could predict the hue and chroma scales of the Munsell and the NCS schemes quite nicely. the method in the present study is generally applicable for determining hue and chroma perceptions of object colors irrespective of the color‐appearance model used.