Three different types of cone cells in the eye process different wave lengths of the visible spectrum.There are only two forces in the world, the sword and the spirit. In the long run, the sword will always be conquered by the spirit. - Napoleon Bonaparte
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How Do We See Color?

Rods and Cones

The retina of the eye contains two types of photoreceptors which are sensitive to light: cones and rods. Rods process light at lower levels of illumination while cones process at higher levels. The light energy which is absorbed by the cones and rods is converted into chemical and electrical signals. These signals are processed by the neurons of the eye and the brain.

Three different types of cone cells in the eye process different wave lengths of the visible spectrum.

Cones are used to perceive color. There are three types, each being more sensitive to a different part of the visible spectrum. The three types are called

  • short-wavelength sensitive cones
  • middle-wavelength sensitive cones
  • long-wavelength sensitive cones

They are also called S-cones, M-cones and L-cones. Sometimes they are referred to as R-cones, G-cones and B-cones (for red, green and blue), however that is a simplification and not quite accurate. For example, the very short wavelengths will only stimulate the S-cones, but the color sensation will have both a reddish and a bluish component.

Perceived Color

The color we perceive is the result of comparisons of the amount of light absorbed by the different cone types. What the different types of cone cells output is processed by our visual system to determine the color we perceive. This process occurs at different stages, starting at the retina and ending at the cerebral cortex in the brain.

Two more things influence our perception of color: the ratio of cone cell excitement in the surrounding areas and also the overall level of cone cell excitement.

Photopic and Scotopic Vision

Photopic vision (vision at normal light levels) is more sensitive to lower frequency colors (reds and yellows) than scotopic vision (vision at low light levels) which is more sensitive to the higher frequency colors like blue and green. This is known as the Purkinje effect, as it was discovered by the Czech scientist Johannes Purkinje (1787 - 1869.) Specifically, the scotopic vision is most sensitive at 555 nm and the photopic at 507 nm.

This explains why red light does not cause bleaching of the rods and, therefore, can be used without resetting the visual pigment. That is why instrument panels are often reddish and why pilots and astronomers use a red filter over their flashlight to read at night.


Read more about color vision.

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