The ability to detect colour, detail and movement and to see in varying light conditions relies on millions of rods and cones in the eyes responding to light by sending electrical impulses along nerves to the brain. Rods and cones are responsible for different areas of vision, with rods mainly being responsible for providing vision at night and cones providing vision in daylight.
Vision and the Retina
Images are created through the lens of the eye focusing light on the retina, located at the back of the eye. The human retina contains photoreceptors -- rods and cones -- that respond to this light. Rods and cones react to certain wavelengths of light, sending electrical signals along nerves to the areas of the brain that are responsible for vision. Rods and cones have very different roles in sending information to the brain to create different elements of vision and they are configured in a manner that enables them to work together in the most effective way. The average human eye contains around 120 million rods and between six and seven million cones.
In bright light and daylight, the human eye uses cones to send the electrical impulses along nerves that provide vision. Cones are responsible for the colour vision and detailed images that most people can see in daylight. There are three different types of cones in the human eye. Each responds to different wavelengths, and therefore colours, of light. These are red cones, green cones and blue cones. Almost two-thirds of all cones in the human eye are red cones, just over one-third are green cones and a minute proportion are blue cones, which are also the most sensitive. In order to see colours other than red, green and blue, a combination of these coloured cones send electrical impulses along nerves to the brain at the same time.
When light is limited (at night, for example), rods provide vision. Rods can provide vision when very little light is available as they are far more sensitive to light than cones and are capable of responding to even the smallest unit of light. They also detect movement. However, unlike cones, they are only capable of providing black and white images, with far less fine detail than those that are created by cones. Rods are less effective adaptors to changes in light conditions and work best when they have had around half an hour to adjust to a particular level of light.
The condition in which a person is completely colour blind is called monochromacy and can appear as one of two forms of the condition: either cone monochromacy or rod monochromacy. In cone monochromacy, the eye only has one of the three types of cones that a normal eye has, either red, green or blue. Vision is not severely affected by this condition although detecting colours can be a challenge. In rod monochromacy, there are either no cones present in the retina or cones are present but do not work. This condition causes a person to see only in black and white, with a lack of fine detail. Rod and cone monochromacy are both very rare conditions. In more common cases of colour blindness, known as anomalous trichromacy, a person has difficulty distinguishing between red and green due to an impairment of one of the three types of cones in the retina.