Sensitization of the visual pigment in a photoreceptor

Detection of light in a photoreceptor begins with the absorption of quanta of light. Highly evolved photoreceptors should absorb light with a high probability. This can be important for an animal for several reasons, for instance in order to allow vision at low ambient intensities where only few quanta are available, or, when the animal has to detect small optical signals (small modulation of intensity) in a short time. The detection of such signals can be a problem even in bright light because the light quantum noise must be smaller than the signal, and the only way to achieve a low light quantum noise is to absorb many quanta per unit time