Wednesday, November 7, 2012

What is a Photoreceptor?

How do we see? How do our eyes actually translate light into something that our brains can "see"? It all starts in the retina, the thin film of tissue that lines the inside of our eyes. Everything in the front of the eye, the cornea and the lens, focus light on the retina. It's then the job of the PHOTORECEPTORS to turn that focused light into electrical energy.

Maybe you remember something about this from high school biology class! There are two types of photoreceptors, the RODS and the CONES. People who have normal color vision actually have 3 separate types of cones; we'll chat about this sometime when we talk about color blindness.

There are cones scattered throughout the retina but the vast majority of them are in the very center of the retina, the area called the macula. In the middle of the macula is the fovea and here there are nothing but cones. As you move outward from the fovea and leave the macula you enter a part of the retina that is covered almost totally with rod photoreceptors.

Cone photoreceptors are sensitive to subtle differences in light intensity and wavelength. It is this sensitivity to wavelength that allows us to see color. Different cones are activated by different wavelengths giving us our sense of color. Subtle differences in light intensity drive our sharp central vision. Cone photoreceptors are most active in bright light and do not work well in the dark. That's why you have to look just to the side of a star to see it at night!

Rods are the photoreceptors for side vision and vision at night. They are very sensitive to low levels of light, and they also "fire" when there is some evidence of movement. Some night vision problems are caused by diseases of the rods. Complex chemical reactions occur when either a rod or a cone is "excited" by the energy in light, and these reactions create the electrical impulses that eventually become sight!

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