Before this century, the answer was 2: rods and cones (see this textbook). However, there was good evidence that rods and cones could not account for all responses to light in mammals (1). In 2000, it was suggested that melanopsin-expressing retinal cells might be a third type of human photoreceptor cell and that melanopsin could function as a circadian photopigment (2).
In experiments with mice, evidence has been obtained for a population of retinal ganglion cells that make use of melanopsin as a photopigment and that send action potentials to the thalamus (3). In mice that lack rods and cones, light can still generate action potentials in melanopsin-expressing retinal ganglion cells. It is reported that light detected by melanopsin and action potentials carried by melanopsin-expressing neurons can influence the activity of a large fraction of neurons in the lateral geniculate nucleus of the visual pathway of mice. It is also possible that melanopsin contributes to human vision.
The figure to the right shows the melanopsin-expressing cells in mouse retina and their axons. Also shown (bottom of the figure) are axons that have reached the thalamus from the retina. There might be functionally distinct populations of retinal melanopsin-expressing cells in humans (relevant mouse studies).
The article by Brown et al. also shows the prominent axon projections of melanopsin-expressing retinal ganglion cells that project to the suprachiasmatic nucleus that are involved in the control of circadian rhythms. Other axons from melanopsin-expressing retinal ganglion cells project to the superior colliculus and are involved in reflex control of pupil diameter. These kinds of non-vision related functions for melanopsin have been the focus of attention in past human studies (for example). Melanopsin might play a role in human health problems that are related to circadian rhythms.
Patients with Leber hereditary opic neuropathy have loss of retinal ganglion cells. It was recently reported that melanopsin-expressing retinal ganglion cells seem to be relatively resistant to the neurodegeneration of Leber optic atrophy (Melanopsin retinal ganglion cells are resistant to neurodegeneration in mitochondrial optic neuropathies).
Another recent article suggests that mice can display responses to light even when they lack rods, cones and melanopsin (4).
Image credits. Copyright: © 2010 Figure 1 of Brown et al. from an open-access article distributed under the terms of the Creative Commons Attribution License