YoungHelmholtz

Young-Helmholtz theory, also known as the trichromatic theory of color vision, is a foundational concept named after Thomas Young and Hermann von Helmholtz. It proposes that the human eye contains three types of cone photoreceptors, each sensitive to different portions of the spectrum, roughly corresponding to short, middle, and long wavelengths. Color perception results from the relative stimulation of the three cone types, rather than from a single color signal. In color-matching experiments, observers can reproduce any perceived color by mixing three primary wavelengths, supporting a three-cone mechanism.

Historically, Young suggested in the early 19th century that color vision derives from several colors, and Helmholtz provided physiological grounding through experiments and measurements of human color matching. The theory explains additive color mixing and many aspects of normal vision. However, it cannot alone account for all phenomena; Ewald Hering's opponent-process theory, proposed later, emphasized opponent channels in later retinal and cortical processing and better explains afterimages and certain color-vision anomalies.

Modern understanding integrates both lines of evidence: the retina contains three cone classes (S, M, L) with peak sensitivities around 420–450 nm, 534–545 nm, and 564–580 nm, respectively; trichromacy at the receptor level is followed by luminance and chromatic opponent processing in subsequent neural stages. The Young-Helmholtz concept remains foundational in color science and informs display technologies using RGB color spaces and color vision testing.