Color Vision
Theories of Color Vision

Current Understanding of Color Vision

A detailed examination of the retina reveals that there are two basic kinds of receptors on the retina, rods and cones. The rods all contain a single form of biochemical visual substance that is very sensitive to light. Because this substance is present in only one form in the rods, vision that depended on the rods alone would be achromatic�that is, the rods have no way of distinguishing one part of the spectrum from another. The cones, on the other hand, are found to contain three forms of a less sensitive visual substance, although any given cone contains only one form of this material. Thus the cones appear to correspond to the three kinds of receptors of the Young-Helmholtz theory. The distribution of rods and cones on the retina is not entirely uniform. The cones are most closely spaced on the central part of the retina which corresponds to the center of our field of view. This small part of the retina is called the fovea and covers only about the central 2� of our field of view. When we examine something closely we are using the fovea. Because of the close spacing of the cones, in this region, foveal vision is very sharp. The density of cones becomes progressively less at increasing distances from the fovea.

The distribution of rods is somewhat different. On the central 1� of the fovea there are no rods at all. Rods begin to appear toward the outer part of the fovea, and their concentration is greatest about 30� from the fovea. Like the cones, the density of rods drops off toward the periphery of the retina.

It is normally assumed that the rods and cones form two generally distinct visual systems. At high light levels (characteristic of normal viewing conditions) the more sensitive rods are saturated and do not contribute to vision. Under these conditions the cones operate effectively and produce normal color vision (photopic vision). At low light levels the cones are not sensitive enough to be stimulated. Thus the rods take over and produce night vision (scotopic vision); Scotopic vision is colorless or achromatic because the rods contain only a single form of visual material. Also, since the central 1� of the fovea contains no rods, scoptopic vision suffers from a �blind spot� at the very center of the visual field. On a dark night after you have allowed your eyes to adapt to the low light level (that is, allowed the rods to unsaturate), try looking at a very faint star. If the star is sufficiently faint, you will not be able to see it when you look straight at it. By looking slightly to one side of the star, however, it will become visible because the image of the star will now be focused just outside the fovea where the concentration of rods is greater. This technique of using �averted vision� is one that people who normally work under low light conditions try to develop.