In this topic, we're going to talk about color. And color vision has been of tremendous interest to everybody. An interest that far outweighs its biological importance. A lot of animals don't have color vision. As we'll see in people who are color deficient or color blind don't have that much of a problem getting along in the world. But color, as I said, is fascinating and all the natural philosophers from Isaac Newton to Thomas Young to Helmont and James Clerk Maxwell, we'll talk about these people as we go along, have been interested in color vision and as I say their devotion to it has outweighed it's biological importance. But it does say something very important about vision, of course, and anyone who wants to explain vision needs to explain color vision. As well as the black and white vision that we talked about in the previous topic. So let me begin with some definitions. First of all, what is color? Color is a set of perceptions that are elicited by the spectral distribution of light. What does that mean? Well before, when we talked about luminance, we talked about the totality of the energy that was falling on a detector, the photometer, or the retina. And now we're going to talk about the distribution of that energy because that's the basis for color vision. So, as you know and as we'll talk about in a minute Light has a variety of wavelengths and the distribution of wavelengths in any stimulus that is coming to your retina can be described in terms of a spectrum, the distribution across the range that we are able to see, 400 to 700 nanometers. The energy distribution across that range is the basis of color vision. There is of course a way of making physical measurements of the relative intensities of wave lengths measured by a spectrophotometer. A spectrophotometer is just a photometer. It is not too complicated an instrument, but an instrument that is made for the purpose of determining the distribution of energy across that bandwidth. This corresponds the physical measurement of color in terms of wavelength energy or energy of different wavelengths as we said before has a corresponding psychophysical measurement. And that measurement has to be reported. And it's the color that's reported by a normal subject, typically made, and we'll go into this in a few minutes, typically made by comparison.
