CIE chromaticity (xy / u′v′)

The CIE chromaticity diagram is the map color science reasons on: a 2D plot where every visible color’s hue and purity land at a point, with brightness set aside. The 1931 version uses x, y coordinates; the 1976 revision uses u′, v′, respaced so equal distances look like roughly equal color differences (more perceptually uniform). On it, a display’s gamut is a triangle joining its three primaries, a white point like D65 is a point inside, and the horseshoe boundary is the spectral locus. When a colorist or a FilmLight talk shows you a “horseshoe,” this is it — and it’s the picture that makes gamut mapping make sense.

First used in: 3.1 · Color science foundations