Visual Adaptation Of The Eye

Rods and cones differ in their sensitivity to light and in the rate at which the bleaching and recombination of pigment takes place. This difference is important in visual adaptation, the adjustment in visual sensitivity in response to changes in the level of illumination. On a clear dark night we can see a single candle flame over 50 kilometers away.

We can also see quite well on a sunny, snow-covered ski slope where the levels of light energy affecting our eyes may be more than a trillion times greater than that of the candle flame, However, to function effectively in both situations, our visual system needs time to adjust its sensitivity to the level of illumination. This process of adjustment is called adaptation. Dark adaptation begins when you leave a brightly lighted environment and enter a darkened one; for example, when you leave a sunny street to enter a darkened movie theater. Light adaptation begins when this sequence is reversed; for example, when you leave the theater and step back into the sunlight. Your sensitivity to light is often so low on entering a darkened   theater that you find it difficult to locate a seat. Yet gradually, over a period of minutes, your sensitivity increases until even the faces of the audience are clearly visible. When you eventually emerge from the theater, your eyes are so sensitive to the previously comfortable sunlight that you may squint for a few moments until you adapt to the light.

How does this take place? As we have seen, the basic process of vision is a photochemical one-the breaking down or bleaching of chemical pigments   in rods and cones. It seems possible to account for much (but not all) of light and dark adaptation in terms of the balance between the bleaching and reconstitution of pigments in the rods and cones.

Pigment in the rods is much more sensitive to light than pigment in the cones. This means that it takes a less intense light stimulus to start the breaking down of the pigment in rods than it does in cones. In fact, when you are light-adapted you are relying primarily on the less-sensitive cones for vision, because most of the highly sensitive pigment in the rods is depleted. Only when you remain in a dimly illuminated environment long enough to dark-adapt completely does the highly sensitive pigment in the rods reach its highest concentration, This allows you to see very dim stimuli that are too weak to affect the less-sensitive cones.