Translations:The Eye and Retina/15/en

Photoreceptors consist of two broad classes of cells: rods and cones. Rods are concentrated at the outer edges of the retina and are used in peripheral vision. They are more sensitive to light than cones, and are almost entirely responsible for night vision (also called scotopic vision). Cones are more concentrated towards the center of the retina, and are the only photoreceptor type found in the center of the retina (the fovea). Cones are responsible for color vision (also called photopic vision). Mammals usually have either two or three different types of cone cells, because in order to specify the wavelength of a stimulus (i.e., its color), the outputs of at least two cone types must be compared.

These photoreceptor cells contain proteins called opsins in their outer segments (really just modified cillia) which bind a chromophore derived from Vitamin A, allowing these cells to convert absorbed light into an electrical impulse. Different wavelengths of light can be detected by varying the structure of the opsin protein expressed in the cell.