The human visual system has two types of photoreceptors - rods and cones (and a third photoresponsive system in the retinal ganglion cells). These two types of photoreceptors are distinct in their response to light, their position on the retina and their role in producing visual perception.
The photoreceptors are located in the retina and transform radiation into neural information. The photoreceptors absorb photons and produce neural impulses that are processed by other nerve processes in the retina which connect through ganglion cells and the optic nerve to the brain. The ratio of photoreceptors to each ganglion cell is significant in that cell's sensitivity to light, and that ratio varies by photoreceptor type and retinal location.
All rods are the same as each other. They respond to electro-magnetic radiation as described by the scotopic visual sensitivity function. Rods respond to light at all levels of adaptation, from photopic through mesopic down through scotopic. Rods are not found in the fovea, the two degree central visual field, but are present in the ten degree para-foveal field. Rods are most dense on the retina at about 20 degrees from the fovea, with decreasing density over the outer areas of the retina. Research by S. Berman and others indicates that rods may be significant in the opening and closing of the pupil, which changes the level of illumination onto the retina and the degree of visual acuity. Research by A. Lewis, He, M. Rea and others indicates that rods dominate the process of reaction to motion in the peripheral visual field. This may be related to the higher ratio of receptors to ganglion cells for rods (compared to cones) that corresponds to increased visual sensitivity.
Cones have three different varieties, depending on the spectral sensitivity of the photoreceptor. The three types of cones differ in their relative absorptance of photons over the visual spectrum, as shown in the following diagram.
The cones and retinal ganglia respond to radiation to produce color vision with an overall sensitivity described by the photopic sensitivity functions for luminances in the photopic range, over 3.4 cd/m2. Long (red) and middle (green) cones are predominant in the fovea, with some short (blue) cones also present. The relative density of short cones increases in the para-foveal region. Throughout the peripheral region there is a low density of all three types of cones.
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