In previous papers a correlate for lightness (<i>L</i>) was found in terms of CIE-tristimulus values for colors of approximately constant luminous reflectance (<i>Y</i>). <i>L</i> could be expressed by a second degree equation with <i>X</i>, <i>Y</i>, <i>Z</i> as variables. In the present study about 100 samples were selected to cover the gamut of existing surface colors. The lightness-equivalent neutrals were estimated by three observers for these colors under the same conditions as described in a previous paper. The original type of equation could not be used to fit the observational data because of the large differences in luminous reflectance among the samples. However, the <i>L</i>/<i>Y</i> ratios could be expressed by a second degree equation with chromaticity coordinates <i>x</i>, <i>y</i> as variables. This type of equation appears to be valid for all reflecting surface colors.
C. L. SANDERS and G. WYSZECKI, "L/Y Ratios in Terms of CIE-Chromaticity Coordinates," J. Opt. Soc. Am. 48, 389-391 (1958)