Adams Chromatic Valence Color Space - Chromance

Chromance

In chromaticity scales, lightness is factored out, leaving two dimensions. Two lights with the same spectral power distribution, but different luminance, will have identical chromaticity coordinates. The familiar CIE (x, y) chromaticity diagram is very perceptually non-uniform; small perceptual changes in chromaticity in greens, for example, translate into large distances, while larger perceptual differences in chromaticity in other colors are usually much smaller.

Adams suggested a relatively simple uniform chromaticity scale in his 1942 paper: and

where are the chromaticities of the reference white object (the n suggests normalized). (Adams had used smoked magnesium oxide under CIE Illuminant C but these would be considered obsolete today. This exposition is generalized from his papers.)

Objects which have the same chromaticity coordinates as the white object usually appear neutral, or fairly so, and normalizing in this fashion ensures that their coordinates lie at the origin. Adams plotted the first one the horizontal axis and the latter, multiplied by 0.4, on the vertical axis. The scaling factor is to ensure that the contours of constant chroma (saturation) lie on a circle. Distances along any radius from the origin are proportional to colorimetric purity.

The chromance diagram is not invariant to brightness, so Adams normalized each term by the Y tristimulus value: and

These expressions, he noted, depended only on the chromaticity of the sample. Accordingly, he called their plot a "constant-brightness chromaticity diagram". This diagram does not have the white point at the origin, but at (1,1) instead.