Metamerism

The phenomenon where spectrally different lights or surfaces produce identical color appearance because the eye reduces spectra to three cone responses.

Trichromacy: The principle that human color vision derives from three cone types with distinct spectral sensitivities. Three receptor classes mean only three numbers represent any spectrum, making metamers inevitable.
RGB displays: Screens that synthesize colors using only three narrow-band primaries: red, green, and blue. Display colors are almost all metamers of the real-world spectra they depict.
Illuminant metamerism: The failure of two surfaces to match under one light source while matching under another. The practical breakdown of metamerism when the viewing illuminant changes.
Chromatic adaptation: The visual system's adjustment that keeps object colors stable under changing illumination. Adaptation shifts which lights count as metamers over time and context.
James Clerk Maxwell: Physicist who demonstrated trichromatic mixing with spinning color tops and the first color photographs. His 1850s matching experiments put metamerism on quantitative footing.
Cone fundamentals: The spectral sensitivity functions of the L, M, and S cones, measured or derived psychophysically. Metamers are spectra with identical integrals against these three functions.
Grassmann's laws: Hermann Grassmann's 1853 rules stating that additive color mixing behaves linearly. They guarantee that metamers remain metamers when added to any third light.
Color rendering index: A measure of how faithfully a light source renders surface colors compared to a reference illuminant. Metameric white lights can score poorly because they match whites, not surfaces.
Tetrachromacy: Possession of four cone classes, allowing discrimination of some spectra that trichromats see as matches. A fourth cone splits metamers that three cones merge, refuting universality.
Hermann Grassmann: Mathematician who in 1853 gave color mixing its linear algebraic laws. His laws made metamer arithmetic predictable rather than merely observed.

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