(Write your answers on the printed Worksheet).
Human color vision relies on special cells in the retina of the eye called cones.
There are three types of cones. "Red" cones are sensitive to red light, "green" cones
are sensitive to green light, and "blue" cones are sensitive to blue light.
When we look at a beam of light that stimulates only the red cones, but not the green or blue cones, we see pure red. Light that stimulates only the green cones, but not the red or blue cones, is pure green. Light that stimulates only the blue cones, but not the green or red cones, is pure blue.
But can see many more colors than just red, green, and blue. How can we see other colors? All the other colors that we see result from the stimulation of combinations of red, green, and blue cones. For example, if we look at a beam of light that stimulates both the red and green cones equally, but not the blue cones, we see yellow. Light that stimulates the blue and green cones equally, but not the red cones, results in a blue-green color called cyan. Light that stimulates the blue and red cones equally, but not the green cones, results in a bluish-red color called magenta. Light that stimulates all three types of cones equally is white or gray. All of the thousands of colors that we can see are the simply the result of weaker or stronger stimulation of the red, green, and blue cones.
These three colors red, green, and blue are called the primary colors for human color vision. (Primary colors are any set of colors from which all other colors may be derived).
Color television sets and computer monitors use these same three primary colors
to produce all the other colors. Use a small magnifying glass or the Flexcam
to look closely at the screen of a television set or computer monitor and you can see
tiny dots or bars of red, green, and blue. These colors were chosen to match
the three types of cones in the human retina. Notice that there are no yellow
dots, so how does the TV make yellow? And what about purple and brown and pink
and other colors? All of the thousands of colors that a color TV can display are the simply the
result of different brightness of the red, green, and blue dots. The
color dots are so tiny we can see them individually; they blend into the
colors we see.
You can experiment with making colors on your computer screen. Launch
the freeware application RGBtoHEX.exe.
The three sliders at the top of the windows allow you to control the
brightness of the red, green, and blue primary colors individually. The
resulting color mix is shown in the box to the right center.
Each slider has a range from zero (fully to the left) to 255 (fully to the right). With all three sliders at zero, the resulting color is black. Now, move each slider indivdually to the right, while the other two sliders are left at zero. Use the magnifying glass or microscope to inspect the computer monitor screen inside the color box so you can see the color dots or bars change in brightness as you adjust the sliders.
How many different colors could you make if you limit yourself to only two values for each primary color - off (zero) or fully on (255)? Two values each of three colors gives 2 x 2 x 2 combinations (23).
(The conventional name for the color made with red + blue (but no green) is magenta, which some people confuse with red but which is not quite the same as pure red. The conventional name for the color made with green + blue (but no red) is cyan, which some people confuse with blue but which is not quite the same as pure blue).
Suppose you had three colored
spotlights, one each red, green, and blue, shining on the wall of a darkened room.
If you partially overlap the three spotlight beams, you get the pattern shown
on the left. When all three spotlights overlap, you get white light! That's because
white light consists of all colors. When only the red and blue spotlights overlap, you get
magenta light. When only the green and blue spotlights overlap, you get
cyan light. When only the green and red spotlights overlap, you get
yellow light. With no spotlights on, you get black. (Click the figure and try
dragging the color squares).
Colored spotlights are often used for stage lighting at music concerts and plays. If you look up at the ceiling of the theater you may be able to see the individual lights. Colored filters are used to get the colors. Sometimes red, green, and blue colored spotlights are combined to get white light. Why don't they simply use white lights when they want white light, rather than combining red, green, and blue colored spotlights? One reason is that the lighting manager can adjust the tint of the stage light gradually by changing the brightness of the colored lights. For example, they could make a warm, pink-tinted light, to suggest a romantic or happy mood, by using slightly more red brighness that the other colors. Or they could make cool, blue-tinted light, to suggest a cold, spooky mood, by using slightly more blue brighness that the other colors. Another reason for using colored spotlights is that the combination of colored lights gives interesting colored shadows and more colorful sparkling highlights from stage jewelry (sequins and fake diamonds) and metal props.
Can you think of a common color that is not in this table? _______ Try to make it in RGBtoHEX.exe. What mix of primary colors did it take? _________________ How many colors could be made if you limit yourself to four values of each primary color? ____________________
Computers use
256 different values for each of the three primary colors, so the total number
of combinations is ____ x ____ x ____ = __________! This includes just about every
possible shade of every color imaginable, more than you could possibly name. Why
do computers make so many colors? The answer is that, as far as the computer is
concerned, every different combination of Red, Green, and Blue counts as a different color,
even if you couldn't tell the difference just by looking. The computer counts every
slightly different shade of each color as different color. For example, look at this
picture on the left of a green frog sitting on a green leaf. If
I asked you what color the frog is, or what color the leaf is, you'd say "green", right?
But actually there are 16,020 different colors used in this picture! Why so many?
Because in this picture there are so many intermediate shades of green, light green,
dark green, blue-green, yellow-green, almost-black, and almost-white, etc. Every
one counts a different color, even though we don't have different names for every one.
If you tried to represent this picture with only four colors (green, dark green,
really dark green, and almost-black), it would look like the picture on the right.
Producing and mixing colors in paper with ink differs
from mixing colors of light on a TV screen. With color printing,
you start out with a white peice of paper and them apply colored inks. White
contains all the colors:
The function of the inks is to subtract some of the colors from the white light reflected from the paper so the light is colored when it hits your eye. The colors that you get when you subtract colors from white can be demonstrated with the RGBtoHEX.exe prohram. Start with all the sliders to the right (at 255), giving white. Now remove one of the colors (set its slider to zero) while leaving the other two colors at 255. Do this for each of the three primary colors.
We can represent what
happens by means of "color arithmetic". For example, if you take away green from white, you
are left with blue and red, which combine to give magenta:
white - red = blue + green = cyan
white - blue = green + red = yellow
This is illustrated graphically by this diagram. The three colors that you get by this subtraction process - cyan, magenta, and yellow - are called the "process primary colors" because they are the colors of ink that are used in the modern color printing process. You can prove this by looking at the colors of the ink cartridge of an ink-jet computer printer. An easy way to do this is to activate the ink cartridge cleaning mode of your printer. (For a Hewlet-Packard ink jet printer, double-click on the printer icon on the desktop, select the "Printer services" tab, and click on "Clean the Print Cartridge"). This causes the printer to print a series of four color bands, one for each of its colors - cyan, magenta, and yellow - and one for the black ink cartridge (used for printing black text).
In order to produce colors other than cyan, magenta, and yellow, the inks are combined on the paper, either by depositing one layer of ink on top of another or by printing tiny dot of colored ink side-by-side. By mixing the inks in this way, the primary colors red, green, and blue can be produced. The more ink is deposited, the darker the resulting color. The more different colors are combined, the darker the resulting color. If all three colors are combined, the result is black (or dark grey); this is the opposite of mixing light, where combining all three primary colors gives white. You can try this yourself by means of the color mixing applet; click on the "color printing" tab; that allows you to vary the amount of ink deposited on the paper for each of the three process primary colors. What combination of ink colors and numbers do you need to create orange?
In the diagram on the left, the inks are overlaid. In the diagram on the right, each color patch is made by alternating tiny dots of the indicated process primary colors. For example, the blue colored square is actually made of alternating dots of magenta and cyan, but no yellow dots. (Click on the color patches to magnify by a factor of 4, so you can see the individual color dots, then click the left arrow to return to this page). Note that the primary colors so produced are lighter than pure blue, green, and red. The bottom square consists of alternating dots of all three process primary colors. It is gray, rather than black. In practical color printing, black ink is used to get darker colors and pure black.
Why does color printing use different primary colors than color television
or the human eye? Why not use red, green, and blue inks for color printing? It
would work, sort of, but the mixed colors would be dull and muddy because
red, green, and blue inks subtract more than one color at a time.
In the diagram on the left, each color patch is
made by alternating tiny dots of the indicated primary colors.
(Click on the color patches to magnify by a
factor of 4, so you can see the individual color dots, then click the left arrow to return to this page).
For example, an attempt to produce magenta by using alternating dots of
red and blue results instead in a dark magenta or purple color.
An attempt to produce yellow by using alternating dots of
green and red results instead in a sickly dark yellow color. Much
brighter color mixes can be produced by using cyan, magenta, and yellow inks,
as shown in the previous diagram. That is why color printing does not use
red, green, and blue inks.
You may have been taught that the "primary colors" were red, blue, and yellow.
This is a common simplification, but it is really a combination of two different
systems: two primary colors (red and blue) and one process primary color (yellow).
This was done because for many years true magenta and cyan paints were not
available, so red and blue were traditionally substituted. However, using red and blue
instead of true magenta and cyan limits the range of colors that can be mixed and
results in duller colors, as you can see on the left. Cyan, magenta, and yellow are
a better set of primary colors for mixing paint (or ink) than red, blue, and yellow,
but this is
not so important now, because plenty of bright paint colors are now available
and it is not necessary to mix your own paints from a set of primary color paints.
The famous painter George Seurat was dissatisfied with the traditional methods of mixing paint. Inspired by research in optical and color theory, he juxtaposed tiny dots of colors that, through optical blending, form a single and more brilliantly luminous hue in the viewer's eye. Using this techique, he created huge compositions with tiny, detached strokes of pure colour too small to be distinguished when looking at the entire work but making his paintings shimmer with brilliance. One of his most famous paintings that uses this technique is A Sunday on La Grande Jatte. It is a very large painting (2 x 3 meters), yet the dots of paint are so small they are hard to distinguish.