"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 four-color 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 Ra, the color-rendering capability of a light source can be well clarified.