Normalised absoption spectra of human rod (R) and cone (S,M,L) cells. Note that the wavelength scale is not represented linearly.
Rod cells are photoreceptor cells in the retina that function in less intense light. Rods are named for their cylindrical shape. They are concentrated at the outer edges of the retina (see peripheral vision). There are about 120 million rod cells in the human retina.
Rods require less light to function than cone cells, and therefore are the primary source of visual information at night. A rod cell can respond to a single photon of light. Cone cells, on the other hand, require tens to hundreds of photons to become activated. Additionally, multiple rod cells converge on a single interneuron, amplifying the signal. This convergence is at a cost to visual acuity, however, since the pooled information from multiple cells is less distinct than if the visual system received information from each rod cell individually.
The convergence of rod cells also tends to make peripheral vision very sensitive to movement.
Experiments by George Wald and others showed that rods are more sensitive to the blue area of the spectrum, and are completely insensitive to wavelengths above about 640 nm (red). Thus, rods have little, if any, role in color vision.
See also: Sensory receptor