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Name: Ryan M.
Status: student
Age: 18
Location: N/A
Country: N/A
Date: 9/22/2004


Question:
As both an aspiring artist and scientist, I have always been perplexed by the contradiction between the two as far as primary colours are concerned.In science the primary colours are red, green and blue. This makes sense as it can be proved with a ray box. However, in art the primary colours are said to be blue, red and yellow and red which is believable as with these colours all other colours can be obtained.


Replies:
The theory of color is NOT straightforward. It involves chemistry, physics, physiology, and psychology. In fact it is possible to create color from "black and white" transparencies exposed using different filters!! And the now common place digital cameras produce colors from electronic pixels that have no intrinsic color at all. What is a poor artist to do?? I have just finished reading the best presentation of this complicated subject from an artist's perspective, which you will find very readable and fascinating. The book is titled: "Bright Earth: Art and the Invention of Color" by Philip Ball [ISBN 0-226-03628-6] The University of Chicago Press. Fascinating reading for scientist and artist alike.

Vince Calder


The first thing to realize is that the number of "primary" colors depends on the type of creature that is doing the looking. Normal cones in the human eye sense light in broad color bands in the red, green and blue-violet regions of the spectrum. These bands overlap a great deal.

Thus, for humans, mixing various amounts of three colors of light (red, green and blue) will produce all colors that can be perceived. That is for an additive source like a computer monitor or TV screen that produces light.

The human eye is sensitive to broad bands of light wavelengths, and thus the eye is not a spectrometer: it interpolates colors depending which of the three cones are receiving light. For example, there are two ways to make "yellow" light. 1) use equal amounts of red and green light. These stimulate the red and green cones, and the brain sees "yellow". 2) use a wavelength of light that is between red and green (about 565nm). Because the response of the red and green cones overlaps, this single color also stimulates both the red and green cones. The brain sees "yellow in both cases.

There are three types of color blindness, depending on which color sensor is defective.

For pictures using paint or ink, it gets a little more complicated because what happens to the light before it strikes the eye is a two-step process. Inks are subtractive. They block various colors of light and only the colors that are NOT blocked are reflected from the paper to your eyes.

For example, yellow ink absorbs blue light, but allows red and green light to reflect from the paper. The human eye sees yellow which is red and green. Cyan, which is a sort of green/blue, absorbs red, but lets blue and green light reflect from the paper. So if you put both yellow and cyan ink on the paper, only green light gets through to the eye. Similarly, magenta is a purplish-red color that absorbs green.

Dogs seem to have only two kinds of color receptors in the eye. They see fewer separate colors. Many birds have four or more types of cones (the extra one is ultraviolet). They see potentially many more colors than humans, depending on how the brain perceives this color. Birds would need a TV with four primary colors, or paper with four different kinds of inks for their full color vision.

More information can be found in encyclopedias.

Bob Erck


The easiest thing to do is point you to the following web site which I believe does a good job describing the differences between Red-Green-Blue, Red-Blue-Yellow, and Cyan-Magenta-Yellow and what is meant by Primary Colors.

http://www.cecs.csulb.edu/~jewett/colors/index.html

Bob Hartwell


Ryan,

The colors of paint and crayons do not add color to the light we see; they absorb light from it. A "perfect" set of paint colors is in fact magenta, yellow, and cyan. For most human eyes, magenta is very close to red and cyan is very close to blue. Yellow is yellow. Red, yellow, and blue will work almost as well as magenta, yellow, and cyan. Also, red, yellow, and blue are all well known colors, easier to talk about.

Magenta paint will remove green from white light, reflecting back red and blue. Yellow removes blue light, reflecting back red and green. Cyan absorbs red, reflecting green and blue. If you combine cyan and yellow paint, red and blue are absorbed. In white light, this reflects only green. This is why blending blue and yellow paint gives you green.

Ken Mellendorf
Math, Science, Engineering
Illinois Central College


Ryan,

The spectral primaries are pure colors and deal with light sources. The light impinging on an object is, for example, red. No matter what "color" the object might appear when illuminated with white light it will appear red when the light source is pure red (assuming at least some of the light is reflected instead of absorbed).

In are you very rarely have light of a pure color. Instead, an object appears red because it absorbs all colors except red. The red light is reflected to our eyes.

This difference is why when you mix red, green, and blue light sources you perceive white light but if you mix red green and blue colors you get black. The red color absorbs green and blue, green absorbs red and blue and blue absorbs red and green. The net result is all colors of light get absorbed and you perceive black.

Good luck.

Greg Bradburn



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