Metamerism

Two fabrics match perfectly under the shop lights and stop matching in the car park.³

The effect is called metamerism, and it follows directly from the fact that the eye measures light with only three sensors.¹

The retina contains three classes of cone, each with a different spectral sensitivity.¹ Each one obeys the principle of univariance: a cone responds the same way to every photon it absorbs, whatever that photon's wavelength, so intensity and wavelength are confounded in its output and no single cone can report colour at all.¹ The consequence is that any light arriving at the eye is reduced to three numbers, and two lights with quite different spectra that produce the same three numbers will look identical.¹ Two surfaces that behave this way are a metameric pair: their spectral reflectance curves differ, but they match under at least one combination of illuminant and observer.³ In practice those curves cross one another at least three times, and a crossing count of three or more is the standard instrumental test for a metameric pair.³ Change the light and the curves no longer integrate to the same three numbers. The match fails.³

The reference observer the whole industry calculates against was built from two experiments that ran separately.^{4, 5} John Guild tested seven observers and W. David Wright tested ten; their data were combined mathematically and adopted as the standard observer in 1931.⁵ The colour-matching functions in use today descend from that combined dataset.⁴ The match was made across a 2-degree field, which confines it to the fovea, and the mismatch with wider real-world viewing was enough that a supplementary 10-degree observer was added in 1964.⁵ Individual observers differ too, which is why observer metamerism is defined as its own category: a pair can match for one person and fail for another under identical light.³

Colour laboratories therefore refuse to trust a single source.³ The sample is assessed in a booth under a reference source, then under a significantly different one, and a third is brought in whenever the first two agree, because the second source may simply fail to expose the mismatch.³ The quantity reported is a metamerism index: the colour difference that emerges when the illuminant or the observer is changed, quoted alongside the name of the difference formula used to compute it.²