a special pigment found in the rods and cones of the retina

Pigments in photoreceptors that change their conformation when they absorb a photon. The change in conformation of the photopigments in response to light is the first step in the process that leads to the photoreceptor producing a neural output. Different photopigments have different probabilities of absorbing a photon of a given wavelength and it is these differences that give rise to the different spectral sensitivity characteristics of different types of photoreceptors.