Trichromacy
Color vision built on three cone cell types, so any visible color can be matched by mixing three primary lights in suitable amounts.
Cone cell: A photoreceptor in the retina operating in daylight; humans have L, M, and S cone types. The three cone types are the biological implementation of trichromacy.
Thomas Young: Polymath who proposed in 1802 that the retina contains three kinds of color receptors. His three-receptor hypothesis is the founding statement of trichromacy.
Opponent process theory: Hering's theory that colors are coded by red–green, blue–yellow, and light–dark opposing channels. Hering's rival account explains afterimages that trichromacy alone cannot.
CIE 1931 color space: The standard numeric system specifying colors by three coordinates derived from human matching data. The institutional codification of trichromacy as an engineering standard.
Visible spectrum: The range of electromagnetic wavelengths, roughly 380 to 700 nanometers, that human eyes detect. Trichromacy is a sampling scheme over this wavelength range.
Opsin: The protein in a photopigment that determines which wavelengths a receptor absorbs. Small opsin differences separate the three cone classes' peak sensitivities.
Hermann von Helmholtz: Physiologist who developed Young's hypothesis into a quantitative three-receptor theory in the 1850s. The Young–Helmholtz theory made trichromatic mixing predictions testable.
Dichromacy: Color vision with only two functioning cone types, leaving colors reducible to two primaries. The commonest deviation from trichromacy, revealing what the third cone adds.
RGB color model: Color representation using red, green, and blue intensities as the three primaries. Displays exploit trichromacy by exciting only three receptor channels.
Spectral power distribution: A function describing how much light energy a source emits at each wavelength. Three cone types reduce any such infinite-dimensional function to three numbers.