Color-all in the eye of the beholder
![[Corel] A colorful fractal-design](fractal-colors.jpg) Q: Your column (Apr. 25, 2002) says that the primary colors are green, blue and red. I must have
gone to the wrong school, because I learned them as yellow, blue and red. Where have I gone
wrong? -Gerry R., Williamsville, New York
A: Several other readers also wrote in wondering about green-blue-red versus yellow-cyan-magenta
primary colors. You haven't gone wrong. The two sets of primary colors complement each other and
result from how the eye functions.
[Corel] A colorful fractal-design
We start with the human eye. Our eyes absorb blue, green, and red light best. That's why these colors
are primary. It's an empirical result.
Suppose we look at a red rose growing in the bright of day. Light hits the red of the rose. The red
pigments in the rose absorb light in the green and blue frequencies and reflect the red light to our eyes.
The reflected red light stimulates the red-sensitive cone cells. Our brains interpret the result as red.
If the red of the rose isn't a true red, then the reflected light stimulates the red-sensitive, the blue-sensitive, and the green-sensitive cells to various degrees. Our brain assigns a color based on adding the responses: maybe a deep rose red
(red with some blue). By adding different primary-color responses, our brains can distinguish some 10,000,000 different colors-the
gamut is great.
This takes us to the crux of the problem. Those who define magenta, yellow, and cyan to be the primary colors are perfectly right. They,
however, are talking about mixing pigments to make colors. Yellow pigment absorbs blue light, cyan pigment absorbs red light, and
magenta pigment absorbs green light.
"When exposed to white light, a mixture of these three pigments controls the mixture of reflected lights (red, green, and blue) and thus you
can see any possible color," says Louis A. Bloomfield, physics professor at the University of Virginia.
The eyes define both the true "primary" colors and, consequently, the pigment primary colors.
Let's wrap up this discussion by answering some questions.
(Answered by April Holladay, science correspondent, May 8, 2002)
Q: I thought you combined yellow and blue to make green. Not true? -Gerry R., Williamsville, New York
A: True, except yellow and "blue" are speaking loosely. It's really yellow and cyan. Again, we're talking about pigment primaries. It
works like this. Mix equal amounts of yellow and cyan. Shine a light on the mixture. The cyan pigments absorb the red light and reflect
blue and green. But the yellow pigments absorb the blue light and reflect red and green. The combination, then, absorbs both red and blue
light, leaving only green reflected light.
(Answered by April Holladay, science correspondent, May 8, 2002)
Q: How can magenta, cyan, and yellow produce all the colors on a newspaper? -Jack W., Washington D.C.
Printing a newspaper image involves creating colors by mixing pigments. Magenta, cyan, and yellow are the pigment primary colors.
They reflect the true primary colors and by mixing them, we can reflect any color we please to the reader's eyes.
(Answered by April Holladay, science correspondent, May 8, 2002)
Q: You cannont make yellow out of any combination of red, green, and blue. -Thomas
Actually you can: by mixing lights, not pigments. You add red and green colored lights to get yellow. Your eyes and brain do this
automatically when you look at, say, a lemon. You can also demonstrate the same phenomenon with lights.
Shine three light projectors on a white screen. Project red light with one projector, blue light with another, and green with the third. What
happens? Where all three colored light beams overlap: the screen shows white. The three primary colors combine to make white.
The red and blue lights overlap to make magenta. The blue and green lights overlap to make cyan. The red and green lights overlap to
make yellow.
 [Corel] The inside surface of a television tube
By overlapping colored lights in varying amounts and intensities, we can make any color we please. That, in
fact, is the way TV screens work.
"...television sets and computer monitors produce the range of colors on the monitor by the use of of red, green
and blue phosphors," says Tom Henderson in his cogent Physics-Classroom website.
The inside surface of a color TV tube is composed of tiny dots or stripes of red, blue, and green. See figure.
We adjust the brightness balance of these glowing spots to produce all the colors displayed on the TV screen.
(Answered by April Holladay, science correspondent, May 8, 2002)
Q: Can your brain tell the difference between seeing a yellow light and seeing an equal mixture of red and green lights?
A: No, it can't. Your brain gets essentially the same message both ways. The yellow light stimulates both the red cones and the green
cone cells about the same amount. Your brain adds the two fairly equal responses and thinks, "Yellow." When you look at an equal
mixture of red and green lights, on the other hand, again, the red and the green cones respond equally to give yellow. The brain can't tell
the difference between the two sources.
(Answered by April Holladay, science correspondent, May 8, 2002)
Further Surfing:
USATODAY.com, WonderQuest: Primary colors
Tom Henderson, The Physics Classroom: color and vision
John Meyer, N. Carolina State U: color vision
How stuff works: television
Conceptual Physics by Paul G. Hewitt: Properties of Light, Color
|
