Rod cells are one of two types of photoreceptor cells that can be found in the retina of the eyes of humans and other mammals. Rod cells are very sensitive to light. They allow night vision. They are concentrated at the edge of the retina, where they also allow peripheral vision. Scientists in 1992 counted an average of 60 million rod cells per human retina. They counted only 3 million cone cells, the other kind of receptor, which are bigger and perceive color.