Colour Science & Perception
what the eye settles, and what it cannot
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The thread: Matching is not appearance

CIE XYZ predicts when two lights will match 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.
on a dark fieldon a light fieldboth patches are #818181asserted equal What the brain does

These two patches are identical

It is the most repeated and least checkable sentence in visual perception, because the whole point is that it does not look true. Every instance on this site is an assertion in code that stops the build if it ever stops being the case.

 ΔE76ΔE94ΔE2000same pairs, three answersCIELAB, D65 Difference and uniformity

How far apart are two colours

ΔE is meant to be a distance with the property that the same number means the same perceived difference everywhere. Three successive formulae have tried, they disagree with each other by more than a just-noticeable difference, and the disagreement decides real matching questions.

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

Seventeen observers in 1931

The standard observer that governs every colour specification in industrial use is an average over seventeen young British men, measured with equipment from the 1920s. It is known to be wrong in the blue, the correction has existed since 1951, and it has never been adopted.

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

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.

 normal trichromatprotanopiadeuteranopiatritanopiaBrettel–Viénot–Mollon 1997, severity 1.0a simulation is not an experiencesRGB, D65 Where the model breaks

Simulating what cannot be simulated

A colour-blindness simulation cannot show what anybody sees. What it can show is which discriminations survive, and that is a narrower claim, a checkable one, and the only one worth making.

 400450500550600650700wavelength / nmunder D650.428, 0.474under A0.500, 0.467reflectance is a fraction, 0 to 1CIE 1931 2° observer What the brain does

Constancy is the default

A sheet of paper looks white in daylight and white under a tungsten lamp, although the light reaching the eye differs enormously. The visual system is solving one equation with two unknowns, and it solves it by assumption.

 400450500550600650700wavelength / nmunder D650.490, 0.417under A0.561, 0.413reflectance is a fraction, 0 to 1CIE 1931 2° observer What light is

The illuminant is half the answer

An object has a reflectance, not a colour. The colour appears when a specified light falls on it, which is why two surfaces can match in a shop and clash outside, and why every serious matching standard names the light.

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

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.

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