Light is registered in the retina when photons hit molecules of the pigment rhodopsin and change the molecules' shape  Even when they have not been struck by photons of light, rhodopsin molecules sometimes change shape because of normal molecular motion, thereby introducing error into the visual system
The amount of this molecular motion is directly proportional to the temperature of the retina
The temperature of an animal's retina depends on the amount of light the retina is absorbing.