The fact that the effect on a photopigment of absorbing a photon is independent of its wavelength. Thus the response of a photoreceptor is determined solely by the number of photons absorbed and is not sensitive to the wavelengths of those photons. This has the important consequence that there is no information in the response of a single photoreceptor about the wavelength of the light which affects it. A single photoreceptor with a single photopigment is incapable of distinguishing between an intense light at a wavelength to which it is relatively insensitive and a weak light at a wavelength to which it is relatively sensitive. Chromatic as opposed to black-and-white vision thus requires multiple photoreceptor types.