Color Addition

We know white light is the presence of all colors. We see sunlight as white and we have discussed the experiments of Sir Isaac Newton in which he recombined the spectrum of all colors to produce white light. Our eyes and brains can be fooled somewhat. Above are shown three spotlights -- of red, blue, and green light -- shining onto a screen in an overlapping pattern. These three colors are called primary colors&emdash;or additive primary colors. They correspond to the three kinds of color-sensitive cones in our retinas. In the region where both red and blue light strike the screen our eyes and brains see a rosy color we call magenta. In the region containing only blue and green light we see a blue-green turquoise color known as cyan. Those two do not seem unusual. But in the region where only red and green light are found, our eyes and brains see yellow! Perhaps even more unusual, in the central region where all three -- and only those three -- colors are found, our eyes and brains see white light! We see or distinguish or experience the same thing as if real white light, with all wavelengths, were there, even though only light of three wavelengths is present.

This characteristic of human vision is very useful in photography, printing, art, and television. As sketched below, a color television screen or a color computer monitor is made up of thousands of tiny dots -- called phosphors -- which glow red, green, or blue when excited by an electron beam. All the colors that we see on a television screen are made of differing intensities and combinations of red, green, and blue .

Here are particular examples:

Interactive Color Vision Module

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Color Vision

Color Subtraction

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(C) 2003, Doug Davis; all rights reserved