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

What the eye does

Three cone types project an infinite-dimensional spectrum onto three numbers. Everything colour science can and cannot do follows from that one collapse.
D65 spectrum400450500550600650700wavelength / nmx̄ → 95.04ȳ → 100.00z̄ → 108.90the area under each product is one coordinateCIE 1931 2° observer

Three numbers

A spectrum has as many degrees of freedom as anyone cares to give it. The eye reports three. Everything colour science can do, and every way it fails, follows from that one collapse.

 400450500550600650700wavelength / nmthe two coloursΔE00 = 6.2e-14spectra differ by 92%reflectances, under D65CIE 1931 2° observer

Two spectra, one colour

Metamerism is usually described and almost never demonstrated. It does not have to be — the metameric black space is enormous, so a matching pair can be constructed to order, verified, and then made to come apart by changing the light.

 400450500550600650700wavelength / nmx̄ȳz̄ȳ is the luminous efficiency functionCIE 1931 2° observer

Why colour is exactly three-dimensional

Matching every wavelength with three primaries requires, for some wavelengths, a negative amount of one of them. That physical awkwardness is why the colour-matching functions were transformed into XYZ, and why the horseshoe is curved.

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