Carotene is the source of retinal which is involved in the formation of rhodopsin of rod cells. Retinal, a derivative of vitamin A, is the light-absorbing part of all visual photopigments. Rods have rhodopsin pigment while cones have three different photopigments, iodopsin, porpyrosin, and cyanopsin. Rhodopsin is a purplish red protein, also known as visual purple, and is associated with vision under low light.