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

The thread: Three numbers

An infinite-dimensional spectrum is projected onto three cone responses, and everything colour science can do — and every way it fails — follows from that single collapse.
D65 spectrum400450500550600650700wavelength / nmx̄ → 95.04ȳ → 100.00z̄ → 108.90the area under each product is one coordinateCIE 1931 2° observer What the eye does

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 / nmA0.448, 0.407D500.346, 0.358D650.313, 0.329E0.333, 0.333normalised to 100 at 560 nmCIE 1931 2° observer What light is

A spectrum is not a colour

What arrives at the eye is a function of wavelength. What the eye reports is three numbers. Keeping the two apart is the single most useful habit in the subject, and almost every confusion in applied colour comes from letting them merge.

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

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 What the eye does

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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