Additive color is a property of a model that predicts the appearance of colors made by coincident component lights with distinct colors, i.e. the perceived color can be predicted by summing the numeric representations of the component colors. Additive color models are applied in the design and testing of electronic displays that are used to render realistic images containing diverse sets of color using phosphors that emit light of a limited set of primary colors. Examination with a sufficiently powerful magnifying lens will reveal that each pixel in CRT, LCD and most other types of color video displays is composed of red, green and blue light emitting phosphors which appear as a variety of single colors when viewed from a normal distance.
Additive color does not, alone, predict the appearance of mixtures of printed color inks, dye layers in typical color photographs on film or paint mixtures. Subtractive color, is used to model the appearance of color (absorbing) from pigments or dyes, such as those in paints, inks, and the three dye layers in typical color photographs on film.
The combination of two of the common three additive primary colors in equal proportions produces an additive secondary color—cyan, magenta or yellow. Additive color is also used to predict colors from overlapping projected colored lights often used in theatrical lighting for plays, concerts, circus shows and night clubs.
The full gamut of color available in any additive color system is defined by all the possible combinations of all the possible luminosities of each primary color in that system. In chromaticity space, the gamut is a plane convex polygon with corners at the primaries. For three primaries, it is a triangle.
Systems of additive color are motivated by the Young–Helmholtz theory of trichromatic color vision, which was articulated around 1850 by Hermann von Helmholtz, based on earlier work by Thomas Young. For his experimental work on the subject, James Clerk Maxwell is sometimes credited as being the father of additive color. 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.
The following chart demonstrates an example of the mixing and perception of additive primaries, step by step.
|Light source||Green (medium wavelength) light and red (long wavelength) light radiate from two different projectors.|
|Projection screen||Both the green and the red light reflect off of a spot on the screen.|
|Retina||The mixed light activates the M and L cones on a spot on the retina about equally.|
|Brain||The brain interprets the equal amounts of M and L 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 (short to medium wavelengths) and yellow (medium to long wavelengths) light radiate from two different projectors.|
|Projection screen||Both the cyan and yellow reflect off of a spot on the screen.|
|Retina||M cones on a spot on the retina are strongly activated by both the cyan and yellow light, while S cones are activated by cyan and L cones by yellow.|
|Brain||The brain receives signals from the cones about some short, lots of medium, and some long wavelengths. It interprets the signal as pale (unsaturated) green.|
- David Briggs (2007). "The Dimensions of Color". Archived from the original on 2015-09-28. Retrieved 2011-11-23.
- "James Clerk Maxwell". Inventor's Hall of Fame, Rochester Institute of Technology Center for Imaging Science. Archived from the original on 2010-09-18.
- Robert Hirsch (2004). Exploring Colour Photography: A Complete Guide. Laurence King Publishing. ISBN 1-85669-420-8. Archived from the original on 2017-02-25.
- RGB and CMYK Colour systems.
- http://www.edinphoto.org.uk/1_P/1_photographers_maxwell.htm - Photos and stories from the James Clerk Maxwell Foundation.
- Stanford University CS 178 interactive Flash demo comparing additive and subtractive color mixing.