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Additive color mixing: adding red to green yields yellow; adding all three primary colors together yields white.

File:RGB illumination.jpg

A rendered model, showing red, green and blue lights combining.

An additive color model involves light emitted directly from a source or illuminant of some sort. The additive reproduction process usually uses red, green and blue light to produce the other colors. Combining one of these additive primary colors with another in equal amounts produces the additive secondary colors cyan, magenta, and yellow. Combining all three primary lights (colors) in equal intensities produces white. Varying the luminosity of each light (color) eventually reveals the full gamut of those three lights (colors).

Computer monitors and televisions are the most common form of additive light. The colored pixels do not overlap on the screen, but when viewed from a sufficient distance, the light from the pixels diffuses to overlap on the retina. Another common use of additive light is the projected light used in theatrical lighting, such as plays, concerts, circus shows, and night clubs.[1]

Results obtained when mixing additive colors are often counterintuitive for people accustomed to the more everyday subtractive color system of pigments, dyes, inks and other substances which present color to the eye by reflection rather than emission. For example, in subtractive color systems green is a combination of yellow and blue; in additive color, red + green = yellow and no simple combination will yield green. Additive color is a result of the way the eye detects color, and is not a property of light. There is a vast difference between yellow light, with a wavelength of approximately 580 nm, and a mixture of red and green light. However, both stimulate our eyes in a similar manner, so we do not detect that difference. (see eye (cytology), color vision.)

File:Tartan Ribbon.jpg

The first permanent color photograph, taken by James Clerk Maxwell in 1861.

File:J C Maxwell with top.jpg

James Clerk Maxwell with his color top that he used for investigation of color vision and additive color

James Clerk Maxwell is credited as being the father of additive color.[2] He had the photographer Thomas Sutton photograph a tartan ribbon on black-and-white film three times, first with a red, then green, then blue color filter over the lens. The three black-and-white images were developed and then projected onto a screen with three different projectors, each equipped with the corresponding red, green, or blue color filter used to take its image. When brought into alignment, the three images (a black-and-red image, a black-and-green image and a black-and-blue image) formed a full color image, thus demonstrating the principles of additive color.[3]

The following flowchart demonstrates an example of the process, step by step.

Light source Medium wavelengths, or green, and long wavelengths, or red, radiate from two different projectors.
Projection screen Both the medium and long wavelengths reflect off of a spot on the screen.
Retina The medium and long wavelengths activate M and L cones on a spot on the retina.
Brain The brain interprets the equal amounts of medium and long signal as yellow.

To fully understand the process, it should be demonstrated how dull colors are obtained using cyan, magenta, and yellow instead of red, green, and blue.

Light source Cyan, or SM, and yellow, or ML, radiate from two different projectors.
Projection screen Both the SM and ML reflect off of a spot on the screen.
Retina Some short, lots of medium, and some long wavelengths activate cones on a spot on the retina.
Brain The brain receives signals from the cones about some short, lots of medium, and some long wavelengths. It interprets the signal as light green.
If the background is not black, it interprets the signal as dull green.

See also[edit | edit source]

References[edit | edit source]

  1. David Briggs. The Dimensions of Color. URL accessed on 2011-11-23.
  2. James Clerk Maxwell. Inventor's Hall of Fame, Rochester Institute of Technology Center for Imaging Science.
  3. Robert Hirsch (2004). Exploring Colour Photography: A Complete Guide, Laurence King Publishing.

External links[edit | edit source]

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