Photoreceptor implicated in non-image-forming responses to light. May be able to isomerize covalently bound all-trans retinal back to 11-cis retinal (By similarity). Highest level in the iris, high level in the inner nuclear layer, possibly in horizontal cells, and lowest level in retinal pigment epithelium. Expressed in melanophore cells of the skin. Belongs to the G-protein coupled receptor 1 family. Opsin subfamily.