trichromatic

Trichromatic refers to a color vision system that uses three independent channels to encode color information. In humans and many other primates, this trichromacy arises from three types of cone photoreceptor cells in the retina, each with a different spectral sensitivity.

The cones are commonly labeled S, M, and L, corresponding to short (blue), medium (green), and long (red) wavelengths. S cones peak around 420–450 nm, M around 534–545 nm, and L around 564–580 nm. The brain interprets color by comparing the relative excitation of these three cone types, effectively encoding a three-dimensional color signal. This arrangement allows hue discrimination and, to a degree, brightness perception through combined input.

Historically, the trichromatic theory of color vision was proposed by Thomas Young and Hermann von Helmholtz in the 19th century, later supported by color-matching experiments. The principle is that most perceived colors can be matched by mixing three primary colors, which corresponds to the activity of the three cone types. This concept underpins modern color displays and digital imaging in the RGB color model.

Variations exist among species. While humans are typically trichromats, many mammals are dichromats, whereas birds and some fish are tetrachromats with an additional cone type. In humans, color vision deficiencies can arise from anomalies or absences in one cone type (for example, protanopia or deuteranopia for red–green deficiencies, tritanopia for blue deficiencies). Genetic differences in cone opsin genes influence color discrimination and can show sex-linked patterns due to X-chromosome inheritance.