Univariance

Univariance is a property of a photoreceptor in the visual system. A single photoreceptor’s response is determined by the combined effects of light wavelength (color) and light intensity (luminance). Because different spectral compositions can produce the same receptor excitation at a given intensity, that receptor by itself cannot convey unambiguous information about either wavelength or brightness.

Consequently, color perception relies on comparing signals from multiple photoreceptor types that have different spectral sensitivities. In humans, three cone classes—short-, middle-, and long-wavelength sensitive—provide separable signals that the brain combines to infer spectral composition, with luminance largely derived from overall light input across receptors or via a separate pathway. The luminance channel is not encoded by a single receptor alone, but by aggregated activity across the system.

Mathematically, the response R of a receptor to a light with spectral power distribution E(λ) is R = ∫ E(λ) S(λ) dλ, where S(λ) is the receptor’s spectral sensitivity. Different E(λ) can yield the same R, a phenomenon known as metamers. Univariance therefore explains why a single receptor cannot distinguish between certain color and brightness combinations and why color vision requires multiple, differently tuned receptors.

Historically, univariance is a central concept in color vision theory and underlies the classical trichromatic model. It helps explain why color appearance arises from patterns of activity across receptors rather than from the output of any one receptor alone.