Rods and cones function as photoreceptors in the retina of the eye, turning visible light into neuronal signals, which are sent to the brain. This process is called transduction. Rods and cones perform at different wavelengths and sensitivities allowing detailed perception of light and color.
Of the two types of photoreceptor, rods are the most numerous at 120 million. They are also more sensitive to light than cones, but not to color. Rods are more effective in low light conditions than cones, and are crucial for seeing at night. There are between 6 million and 6 million cones in the retina, providing color sensitivity which is of most use in the daytime. A large proportion of the cones are located in a central yellow spot called the macula. In the center of the macula there is a region, the fovea centralis, that has no rods at all, just thin and densely packed cones. Rods and cones are polarized cells, meaning that they have different architecture and functions at both ends of the cell. The outer layer of both types of photoreceptor is a large, modified cillium, and the center of the cell is made up of stacked layers of light-sensitive membrane.