Primary Colors
The human eye does not function like a
machine for spectral analysis, and the same color sensation can be produced
by different physical stimuli. Thus a mixture of red and green light
of the proper intensities appears exactly the same as spectral yellow,
although it does not contain light of the wavelengths corresponding
to yellow. Any color sensation can be duplicated by mixing varying quantities
of red, blue, and green. These colors, therefore, are known as the additive
primary colors. If light of these primary colors is added together
in equal intensities, the sensation of white light is produced. A number
of pairs of pure spectral colors called complementary colors also exist;
if mixed additively, these will produce the same sensation as white
light. Among these pairs are certain yellows and blues, greens and blues,
reds and greens, and greens and violets.
Most colors seen in ordinary experience are caused by the partial absorption
of white light. The pigments that give color to most objects absorb
certain wavelengths of white light and reflect or transmit others, producing
the color sensation of the unabsorbed light.
The colors that absorb light of the additive primary colors are called
subtractive primary colors. They are red,
which absorbs green; yellow, which absorbs blue; and blue, which absorbs
red. Thus, if a green light is thrown on a red pigment, the eye will
perceive black. These subtractive primary colors are also called the
pigment primaries. They can be mixed together in varying amounts to
match almost any hue. If all three are mixed in about equal amounts,
they will produce black. An example of the mixing of subtractive primaries
is in color photography and in the printing of colored pictures in magazines,
where red, yellow, black, and blue inks are used successively to create
natural color. Edwin Herbert Land, an American physicist and inventor
of the Polaroid Land camera, demonstrated that color vision depends
on a balance between the longer and shorter wavelengths of light. He
photographed the same scene on two pieces of black-and-white film, one
under red illumination, for long wavelengths, and one under green illumination,
for short wavelengths. When both transparencies were projected on the
same screen, with a red light in one projector and a green light in
the other, a full-color reproduction appeared. The same phenomenon occurred
when white light was used in one of the projectors. Reversing the colored
lights in the projectors made the scene appear in complementary colors.