|MadSci Network: Physics|
Cheryl, You've asked a very good question. So good, in fact, that nobody really knows... I checked around a little bit, and according to a reliable source (The First Steps in Seeing, by Rodieck) no one has yet really gotten around to measuring it in the human eye. In chimpanzees, the ratio of Red to Green cones is about 1 to 1. In people, it seems like there is some variation. Some estimated results are around 1.2 green for every red, although some estimates are as high as 3.8 green for every red. What is known is that there are very few blue cones compared to the green and red. And what is also known is that there are 7 million cones together. OK, now for the background information you can skip if you want. First, cones aren't technically called red, green, and blue. Each cone is most sensitive to a particular wavelength of light based on the chemical structure of the photopigment inside of it. Some cones are most sensitive to Long wavelength light, and so are called L cones. This wavelenght happens to look red to most people. So red cones are really L cones. 'Green' cones are most sensitive to medium wavelength light that happens to look green, so those are M cones. And blue is a short wavelength, so a 'blue' cone is really an S cone. So how do you estimate the number of these cones? Well, each of us is most sensitive to one wavelength of light. In most people, this wavelength is in between the peak sensitivity of the L and M cones. Sooo, we can assume that our peak sensitivity is made up of a certain proportion of L and M cones. How close the peak sensitivity is to either the L or M cone peak sensitivity allows us to estimate which type of cone there are more of. Since it doesn't look like the S cones affect this sensitivity much at all, we assume that there are very few of them. Anyway, that's a long answer that doesn't really get at your question. The problem is that most people won't let you remove their retinas and put them under a microscope, since it's hard to put a retina back in someone's eye... Tom p.s. all the info here comes from that book by Rodieck, "The First Steps in Seeing." It's a handy book for all sorts of visual questions.
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