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

Difference and uniformity

How far apart two colours are, whether the space you measured in was uniform, and how MacAdam's ellipses settle the question by measurement.
ΔE76ΔE94ΔE2000same pairs, three answersCIELAB, D65

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.

 0.00.20.40.60.80.00.20.40.60.8xyellipses at 10× scaleCIE 1931 2° observer

MacAdam measured it

In a perceptually uniform space the just-noticeable-difference contours would be circles of equal size. MacAdam's ellipses are neither, by a factor of eighty — and transforming them into each candidate space settles which spaces improved matters and by how much.

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

The midpoint is not half

Code 128 sits halfway along the sRGB scale and carries about a fifth of white's luminance. Half the luminance is code 188. Almost every gradient, blur and resize on the web gets this wrong, and the errors are visible once known.

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