It is now clear that, besides rods and cones, there is another photoreceptor type in the retina, consisting of a sub-population of retinal ganglion cells that are intrinsically-photosensitive by virtue of expressing the visual pigment, melanopsin.  These ganglion-cell photoreceptors are involved in primarily non-image vision (such as circadian photoentrainment and pupillary light reflex), but also in supposedly some subtle aspects of image vision.  Like rods, these cells have a high light-signaling efficiency by being capable of signaling a single absorbed photon to the brain.  The underlying phototransduction mechanism uses a phospholipase C-mediated signaling pathway as found in fly eye, instead of a cyclic nucleotide-mediated pathway as found in rods and cones.  In nocturnal and crepuscular sub-primate mammals, melanopsin also mediates a local pupillary light reflex in the iris, in addition to the conventional pupillary light reflex involving the retina and neuronal circuitry through the brain.