additive color : Spreety TV Online Glossary
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additive color n. A means of producing color by
mixing lights of red, green, and blue (technically, blue-violet)—the three
primary additive colors. Varying proportions of these primary colors combine to
create light of all other colors, including white (which is a mixture of all of
the visible wavelengths) and black (which is the absence of all light).
The first color photography experiments, conducted by James
Clerk Maxwell in 1861, were based on additive color. He used a still camera and
colored filters to take three pictures of the same subject. One photograph was
shot through a red filter, another was shot through green, and the last was
shot through blue. He printed positives of these images on glass magic lantern
slides and projected them onto the same screen using three magic lanterns. The
original red-filtered photograph was projected through a red filter, etc. The
result was a full-color projected image.
One of the first color motion picture systems used the same
method. The camera shot frames through alternating filters (red, green, and
blue) and the resulting print was projected through similarly alternating
filters. This resulted in a full-color image, but the process broke down if the
subject moved; it also caused eye strain in the viewer (color-bombardment). The
original two-strip Technicolor process developed in 1914 also used additive
color. In this case, recording red and green filtered images on a double strip
of black-and-white film that was later projected through a double projector
fitted with corresponding red and green filters. The additive process for color
motion picture photography had two serious failings: it required special
cameras and projectors, and the filters absorbed much of the light. This meant
that it took more light to expose the images and more light to project them.
Solutions to these issues were eventually found in subtractive color.
Video systems, on the other hand, continue to use the additive
color process. The traditional CRT (cathode ray tube) television or computer
monitor uses red, green, and blue phosphors to create an additive color image.
In a CRT, an electron gun fires a focused electron beam into the back of the
colored phosphors that coat the inside of the screen, causing them to glow in
varying amounts, creating all the different colors in an image. If one looks
closely at a color CRT screen, one can see the individual image pixels, each
consisting of a cluster of three colored phosphors. Plasma, LCD (liquid crystal
display), and DLP (digital light projection) video displays each operate in a
different fashion, but still produce full color images by mixing the primary
additive colors of light (red, green, and blue) in varying amounts.
Compare subtractive color.
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