Dichromacy

Dichromacy is color vision based on two independent signals, usually because only two classes of cone photoreceptors function normally. It reduces the range of colors that can be distinguished compared with trichromatic vision.

Cone cell: A light-sensitive retinal cell that supports color vision and detailed vision in bright conditions. Dichromacy usually results when one of the three cone classes is absent or nonfunctional.
Protanopia: A congenital form of red–green color vision deficiency caused by absent or nonfunctional long-wavelength-sensitive cones. It is dichromacy associated with loss of the long-wavelength cone signal.
Trichromacy: Color vision based on three independent signals, usually generated by three classes of cone photoreceptors. Trichromacy retains a third cone signal that dichromacy lacks.
Ishihara test: A color-vision screening test that uses plates of colored dots containing numbers or paths. Its plates commonly detect red–green deficiencies that include protanopia and deuteranopia.
Opsin: A light-sensitive protein that combines with a retinal molecule to form a photopigment. Cone opsins determine spectral sensitivities, and variants can disrupt one signal class.
Spectral sensitivity: The variation in a photoreceptor’s response across wavelengths of light. The sensitivities of the two functioning cone classes determine a dichromat’s color signals.
Deuteranopia: A congenital form of red–green color vision deficiency caused by absent or nonfunctional medium-wavelength-sensitive cones. It is dichromacy associated with loss of the medium-wavelength cone signal.
Anomalous trichromacy: A form of color vision in which three cone classes function, but two have unusually similar spectral sensitivities. It preserves three signals, unlike dichromacy, though its color matches can resemble dichromatic ones.
Color assessment and diagnosis: The evaluation of color discrimination using behavioral tests or measurements of visual responses. Tests distinguish dichromatic patterns from other forms of color-vision deficiency.
Cone opsin gene: A gene encoding an opsin used in a cone photopigment. Changes or deletions in cone opsin genes can cause inherited dichromacy.

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Color matching (trichromacy)Compared with: Reduced matching dimensionality makes dichromats the sharpest test of the theory.