Rods and cones are known as photoreceptors. They are specialized cells that are located on the retina, in the back of the eye. Their roles are very specific: to receive and process signals of light and color, which gives us our vision. Because humans rely primarily on vision over other senses, rods and cones are very important to us. The effect of malfunctioning or deficient photoreceptors can be serious.
The most basic and crucial function of photoreceptors is to perceive light. This is the job of rods. Rods are located throughout the retina except for the very center, or fovea. They are specialized to pick up light signals; they determine light and shadow. Most people have about 120 million rods, which are each more than a thousand times as sensitive as individual cones. They pick up signals from all directions, improving our peripheral vision, motion sensing and depth perception. However, rods do not "see" color: they are responsible for only light and dark.
Color perception is performed by cones. There are 6 million to 7 million cones in the average human retina. They are mostly concentrated in the center of the retina, around the fovea. There are three types of cones: red cones (about 64 percent of the total), green (32 percent) and blue (2 percent). Cones are best for detecting fine details, but only work in bright light. They must reset after sending a nerve signal to the brain, which is why the eye is constantly moving: this causes light to fall in different spots on the retina, to keep color perception steady. Color blindness occurs when a person is missing one type of cone. This is more common in males, about 8 percent of whom are considered color-blind. Only 0.5 percent of females experience color blindness. Interestingly, color vision is best in birds and primates, and worse in other species. This may be a compensation for a weaker sense of smell, or it may have evolved to help us identify plants.
Stereoscopic and Night Vision
Human vision benefits from the configuration of rods and cones in two ways: stereoscopic vision and night vision. Stereoscopic vision is vision "in stereo"--with two eyes. Each of our eyes has strong peripheral vision thanks to the wide distribution of rods. This enables us to see in more than one direction at a time. It improves our depth perception and awareness of our surroundings. Night vision is also known as scotopic vision. It takes several minutes to become effective when we enter a dark room, and is best after about half an hour. Scotopic vision is possible because of the sheer sensitivity of rods. However, rods do not perceive the color red: a room with red light does not impede our night vision, which is why many dashboard lights use red lenses so that we can see the darkness outside our vehicle as well as the lights inside.