Two different spectra that are the same colour
Drawn at its defaults, in metamerism. It takes no options at all, so every essay calling it gets this exact drawing.
Called by 7 essays
the blast radius of changing it
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.
What the eye doesTwo 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.
What light isThere 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.
Matching and measuringThe laws that make colour add up
Colorimetry is an integral, and an integral assumes matching is linear. Grassmann's four laws are exact for a linear observer, to floating point — and they fail at both ends of the light range, by two different mechanisms, leaving colorimetry an operating band of three and a bit decades that no standard states.
Difference and uniformityA tolerance is a probability
A colorimeter reports one number and a specification compares it with another, and both are computed for an observer who does not exist. Handed to two hundred people, the same pair at ΔE00 1.0 is read from 0.8 to 3.7 — and which of those two ranges applies depends on something no specification records.
Matching and measuringThe metamerism index has two corrections
The index for a metameric pair is defined for a pair that matches exactly under the reference light, and no real pair does. The standard's remedy is to correct the sample first, and it names two corrections — scale the tristimulus values, or add the difference. On six pairs matched to one colour difference, the two answers differ by a tenth to four tenths of an index unit; at two, by a whole one.
Matching and measuringThe 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.