any of a number of related substances that function in light reception (photoreception) by animals by transforming light energy into electrical (nerve) potentials.

It is believed that all animals employ the same basic pigment structure, consisting of a coloured molecule, or chromophore (the carotenoid retinal, sometimes called retinene), and a protein, or opsin, of moderate size. Retinal₁ is derived from vitamin A₁; retinal₂ is derived from vitamin A₂.

Many vertebrate animals have two or more visual pigments. Scotopsin pigments are associated with vision in dim light and, in vertebrates, are found in the rod cells of the retina; the retinal₁ forms are called rhodopsins, and the retinal₂ forms porphyropsins. Photopsin pigments operate in brighter light than scotopsins and occur in the vertebrate cone cells; they differ from the scotopsins only in the characteristics of the opsin fraction. The retinal₁ forms are called iodopsins; the retinal₂ forms cyanopsins.