Colour Science & Perception
what the eye settles, and what it cannot
Field

Matching and measuring

CIE XYZ, chromaticity, and gamut — a system for predicting when two lights match, frequently mistaken for a system that predicts how they look.
0.00.20.40.60.80.00.20.40.60.8xyD65460480500520540560580600620hatched: outside sRGB15% of the visible area is reachableat luminance Y = 0.55CIE 1931 2° observer

Most of this diagram cannot be shown

The chromaticity horseshoe is the canonical illustration of colour science, and nearly every printed copy is filled edge to edge with colours the page cannot produce. The honest version marks them, and the marking covers most of the picture.

 on a dark fieldon a light fieldboth patches are #868686asserted equal

Matching is not appearance

CIE XYZ predicts when two lights will look the same under identical viewing conditions. It was never a model of how anything looks, and most of the confusion in applied colour comes from using it as one.

 0.00.20.40.60.80.00.20.40.60.8xysRGBP3Rec. 2020areas are diagram areas, not perceptualCIE 1931 2° observer

What a gamut costs

Three primaries reach a triangle and the visible region is not a triangle, so something has to give. Widening the primaries helps, has a price in precision and compatibility, and runs into a limit that is geometric rather than technological.

 code 128 → 21% luminance50% luminance → code 188code valuerelative luminancegamma 2.2sRGBthe diagonal would be a linear displayIEC 61966-2-1

A hex code is not a colour

Six hexadecimal digits identify three numbers. Turning three numbers into a colour needs a colour space, a transfer function, a white point and a display, and leaving any of them unstated is the everyday version of every confusion on this site.

All essays