Normal color vision is based on three light-sensitive pigments, called photopigments, that are found in the cones of the human retina: one sensitive to short wavelengths, one sensitive to middle wavelengths, and one sensitive to long wavelengths. The cones that contain these pigments are abbreviated as S, M, and L cones, respectively, and together, they are responsible for normal trichromatic vision.
Color vision deficiencies (CVD) are caused by the absence of one of the normal cone pigments, and the type of CVD depends on which photopigment is affected.
Defects caused by the absence of L cones use the prefix Protan-, those caused by the absence of M cones use the prefix Deutan-, and those caused by the absence of S cones use the prefix Tritan-. Congenital Tritan defects are extremely rare, affecting less than 0.01% of the population, but Protan and Deutan defects, collectively known as red-green color vision deficiencies, affect approximately 8% of all males, making red-green CVD the most common single-locus genetic disorder in humans.