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Q & A: Color and visual acuity

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Q:
Does color have an effect on visual acuity. If so, why? Also what color effects it the greatest for the better and wich for the worse.
- Andrew Harless (age 12)
Franklin middle School, Springfield Illinios
A:
Yup, people's eyes respond differently to light of different colors, both in the focusing and in the retina. The retina has four kinds of light-sensitive cells -- "rods", and three kinds of "cones" (names given because of the cell's shapes). Some cones are most sensitive in the red end of the visual spectrum, others in the middle (green), and the third type is more sensitive to the blue end of the visual spectrum. The rods do not distinguish colors. Cones are concentrated near the fovea, a very dense and sensitive part of the retina located along the axis of the lens. Other parts of the retina give us our peripheral vision, while the fovea gets the image of what we are looking straight at. Most cones are concetrated near the fovea, while the rods are spread out away from the fovea.

The blue-sensitive cones are the least numerous, and are also spread out away from the fovea. This is one reason why it is harder to get a sharp visual impression of something blue than something of other colors.

The focusing of the eye is accomplished by the aqueous and vitreous humors, and the lens. These share a property with prisms, in that they split light into its various color components. So it is impossible to focus simultaneously on a red light and a nearby blue light. I personally have a harder time bringing a small, bright blue light into focus with my eyes, particlarly if there are lights of other colors around.

I never noticed this before blue LED's came out just a few years back, because I'd never had the occasion to look at small blue lights (the sky is blue but is very diffuse and broad, so there's no reason to focus on it). Sometimes, illuminated blue signs on restaurants or stores are hard to read from a distance when you're driving down a road.

Tom

(republished on 07/27/06)

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