This principle states that once a photon has been absorbed by a photoreceptor, all information about the wavelength of the photon is lost. This is because the photopigment's activation is like a switch. Either the photopigment is acting as a catalyst or it is not. Isomerization of the chromophore caused by the absorption of one photon is identical to the isomerization caused by the absorption of any other photon irrespective of the photon's wavelength. Color is in the Brain of the Beholder Animals extract information about the spectrum of light striking different regions of their retinas by contrasting the responses of photoreceptors containing different photopigments.
Animals generate different photopigments by expressing different opsins.
Opsins with different amino acid sequences may have drastically different absorption spectra. Figure 7A depicts the absorption spectra of the three types of photopigment that mediate human color vision and the four types of photopigment that mediate color vision in the European starling. Figure 7B depicts the transformation from an incident spectrum of light to the relative responses that each of these photopigments would generate. The numbers inset in the plots are the integrals of the correspondingly-colored curves. Note that since starlings have four cone photopigments, there are four different outputs, and note that none are identical to any of the three outputs of the human cones.