Color Vision
The human eye is a marvelous design that employs many of the features that we have described in the design of a camera. Light enters the eye through the cornea, the transparent front opening of the eye. Most of the bending or focusing of the light occurs at this surface. Behind the cornea is the iris which adjusts to let more or less light pass as required, just as the iris diaphragm in a camera; it is the iris that gives color to our eyes. Behind the iris is the lens of the eye, also called the crystalline lens. The index of refraction of the lens is not much different from the fluid inside the eye on either side of it so it does not bend the light a great deal. The lens acts as a fine adjustment to the focus. It is pliable and can change its shape as the ciliary muscles attached to it exert forces on it. When they are fully relaxed, the lens is flattest, offering the least additional bending of light. As the ciliary muscles exert forces on the lens it becomes thicker in the middle, bending the light more. An image is focused on the light-sensitive back of the eye, called the retina. The retina in the eye corresponds to the film in a camera.
The retina has a complex arrangement of light-sensitive receptors known as rods and cones because of their shape. The cones are color-sensitive and require higher levels of illumination. The rods are far less color-sensitive and come into play when the light is dim. That is why a dimly-lit scene will also have very little color information in it. Near the center is the fovea, about 0.3 mm in diameter where the density of cones is extremely high. In detailed work requiring a great deal of information, like reading, the image is placed on the fovea for maximum information and color discrimination. Here is a microphotograph of a piece of the retina, with its light sensitive rods and cones, as viewed under a microscope. Human vision is a complex process involving the retina of the eye and the cortex of the brain. Edwin Land (of Polaroid fame) coined the word "retinex" to indicate the integral part played by both the retina and the cortex.
As you already know, the human eye is sensitive to light with wavelengths from about 700 nm, which we see as red, to about 400 nm, which we see as violet. There are three kinds of color-sensitive cones with maximum sensitivities each to light with wavelengths of 575 nm, 535 nm, and 445 nm. We can refer to these three kinds of cones as being sensitive to red, green, and blue light, respectively.
We see the color of an object by the light that it reflects. When white light, such as sunlight, shines on an object that means it is illuminated with light of all wavelengths. If red light is reflected while light of other colors is absorbed, we will see the object as red as shown ihere.
Here we have used incoming rays of red, green, and blue to indicate the full range of colors that is in white light. Objects that we see having different colors reflect different colors from white light and absorb the rest; this is here for a green object and a blue object.
We have used red, green, and blue to represent the many colors present in white light such as sunlight and because these colors correspond to the three kinds of color-sensitive cones in our retinas. However, these color-sensitive cones are sensitive over a fairly wide range of wavelengths or colors. And very few objects absorb or reflect light over a very short range of wavelengths. Below is a sketch which illustrates what we see when white light shines on something that absorbs light from the red end of the spectrum while the rest of the light is reflected. White light without its reds and oranges will be bluish-green or aqua-colored or turquoise; we call this color cyan.
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(C) 2003, Doug Davis; all rights reserved
