Two broad categories of disorders are introduced in this topic: dyschromatopsia (color vision defects) and nyctalopia (night blindness).

Dyschromatopsia: involves the cone photoreceptors, of which there are three types:

• S-cones (short-wave)
• M-cones (medium-wavelength)
• L-cones (long-wavelength)

Depending on which cone type is affected, different portions of the color spectrum may become impaired.

Nyctalopia: results from dysfunction of the rod photoreceptors.