The receptors are the sensitive elements that absorb light and start the electrophysiological process that sends visual signals to the brain.

The rods and cones are not evenly distributed across the retina.  Most of the cones are in the foves.  In the very center of the fovea there are very few if any rods.  Click on distribution to see how the receptors are distributed across the retina.

When light enters the eye some of it will eventually find its way to the outer segment of the receptors. The outer segments contain photopigment molecules. When light (more properly speaking, a photon) is absorbed by one of these photopigment molecules it undergoes a morphological (shape) change called isomerization.

There are four classes of receptors: 1. Rods, which are used to see at night or under very low illumination. They are very sensitive but color blind. Rods are not shown in this diagram of the retina. 2. L- receptors are ones which are most sensitive to long wavelength light. Long wavelengths are the ones which appear red to us. 3. M- receptors are most sensitive to middle wavelengths which appear green to us. 4. S- receptors are most sensitive to short wavelengths which appear blue to us.

People with normal color vision have L- sensitive, M- sensitive and S- sensitive receptors. People with color variant (sometimes called color defective, or inappropriately called color blind) vision are missing one or more of these receptors.

About 8% of the population has color variant vision; most of these are men.

Select on receptor to see how rods and cones appear when viewed through a light microscope and through an electron microscope.

The ends of the receptors are called the outer segments. These outer segments contain photopigments with known spectral absorbencies which when they absorb light, that has entered the eye, starts the neurophysiological process that eventually sends signals to our brain.

Select RETURN to see the diagram of the retina again.

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