n. a substance in a retinal rod or retinal cone that interacts with light to initiate a chemical cascade resulting in the conversion of light energy into an electrical signal. All rods contain the photopigment rhodopsin, whereas cones have one of three different photopigments (iodopsins), each with a different wavelength sensitivity (see long-wavelength pigment; medium-wavelength pigment; short-wavelength pigment). Photopigment is located in disks of membrane in the outer segment of a rod or cone. Also called visual pigment.