The light absorbing part of a photopigment. The photopigments contained in photoreceptors consist of two components: an opsin bound to a chromophore. The chromophore in human (and all mammalian) photopigments is retinal (a form of vitamin A). Upon absorbing a photon, retinal changes its conformation which results in its separation from the opsin group. This chemical change initiates the visual response. The differences in spectral sensitivity among the photopigments found in the different types of human photoreceptors are due to differences in their opsins.