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

Metamerism index — where it appears

A single number for how badly a metameric pair comes apart when the light changes. It is the colour difference between two samples under a test light, given that they matched under a reference one. A real pair rarely matches exactly to begin with, so the index is defined with a correction, and the two published corrections disagree.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

D65, and a source built to have its chromaticity and nothing else. The smooth curve is the CIE's daylight reconstruction at 6504 K. The other is five Gaussian emission bands whose weights were solved so that the two agree in chromaticity to 8.5e-10 — closer than any instrument could tell them apart when looking at the lamps. Three numbers were matched and seventy-eight were not, and everything either source falls on will report the difference.

There is no D65 lamp

D65 is standardised by three numbers, and three numbers do not pin a spectrum. A source built to hit them exactly — matched to a part in a billion, with a third of the visible band nearly empty — separates pairs that D65 says are identical by up to eight and a half units of colour difference.

light · Light How often the choice of correction changes a pair's grade. Eighteen metameric pairs walked to each of eleven reference mismatches, with the share whose index falls in a different band under the two corrections the standard allows. At an exact match the share is zero and must be: there is nothing for either correction to correct. It rises to 44 per cent at a reference mismatch of 2, which is the quality a dyehouse reaches rather than the quality a laboratory constructs. The banding is a five-step convention at 0.5, 1, 2 and 3, stated here rather than quoted, and how much the count depends on it is drawn separately.

The ambiguity is largest where the index is used

The metamerism index is defined for a pair that matches exactly under the reference light, no real pair does, and the two corrections the standard allows for the residual give different answers. Over eighteen pairs at eleven match qualities the gap is nearly a function of the mismatch alone — its middle half spans a factor of 1.4 at the mismatches a dyehouse reaches — so it could be tabulated. And it is largest exactly there: zero at a laboratory's match, and re-grading eight pairs in eighteen at a trade's.

matching · Gamut

Named alongside it

The objects these essays reach for when they reach for this one.

Illuminant metamerismMetamerismQuality controlSpecificationAcceptabilityColour differenceConventionThe D-series daylight illuminantsIlluminantMeasurement conditionSpectral power distributionStandard observer

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