RGB color model
The RGB color model is an additive color model in which the red, green, and blue primary colors of light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three additive primary colors, red, green, and blue.
The main purpose of the RGB color model is for the sensing, representation, and display of images in electronic systems, such as televisions and computers, though it has also been used in conventional photography and colored lighting. Before the electronic age, the RGB color model already had a solid theory behind it, based in human perception of colors.
RGB is a device-dependent color model: different devices detect or reproduce a given RGB value differently, since the color elements and their response to the individual red, green, and blue levels vary from manufacturer to manufacturer, or even in the same device over time. Thus an RGB value does not define the same color across devices without some kind of color management.
Typical RGB input devices are color TV and video cameras, image scanners, and digital cameras. Typical RGB output devices are TV sets of various technologies, computer and mobile phone displays, video projectors, multicolor LED displays and large screens such as the Jumbotron. Color printers, on the other hand, are not RGB devices, but subtractive color devices typically using the CMYK color model.
Additive colors
To form a color with RGB, three light beams must be superimposed. Each of the three beams is called a component of that color, and each of them can have an arbitrary intensity, from fully off to fully on, in the mixture.The RGB color model is additive in the sense that if light beams of differing color are superposed in space their light spectra adds up, wavelength for wavelength, to make up a resulting, total spectrum. This is in contrast to the subtractive color model, particularly the CMY Color Model, which applies to paints, inks, dyes and other substances whose color depends on reflecting certain components of the light under which they are seen.
In the additive model, if the resulting spectrum, e.g. of superposing three colors, is flat, white color is perceived by the human eye upon direct incidence on the retina. This is in stark contrast to the subtractive model, where the perceived resulting spectrum is what reflecting surfaces, such as dyed surfaces, emit. A dye filters out all colors but its own; two blended dyes filter out all colors but the common color component between them, e.g. green as the common component between yellow and cyan, red as the common component between magenta and yellow, and blue-violet as the common component between magenta and cyan. There is no common color component among magenta, cyan and yellow, thus rendering a spectrum of zero intensity: black.
Zero intensity for each component gives the darkest color, and full intensity of each gives a white; the quality of this white depends on the nature of the primary light sources, but if they are properly balanced, the result is a neutral white matching the system's white point. When the intensities for all the components are the same, the result is a shade of gray, darker, or lighter depending on the intensity. When the intensities are different, the result is a colorized hue, more or less saturated depending on the difference of the strongest and weakest of the intensities of the primary colors employed.
When one of the components has the strongest intensity, the color is a hue near this primary color, and when two components have the same strongest intensity, then the color is a hue of a secondary color. A secondary color is formed by the sum of two primary colors of equal intensity: cyan is green+blue, magenta is blue+red, and yellow is red+green. Every secondary color is the complement of one primary color: cyan complements red, magenta complements green, and yellow complements blue. When all the primary colors are mixed in equal intensities, the result is white.
The RGB color model itself does not define what is meant by red, green, and blue colorimetrically, and so the results of mixing them are not specified as absolute, but relative to the primary colors. When the exact chromaticities of the red, green, and blue primaries are defined, the color model then becomes an absolute color space, such as sRGB or Adobe RGB.
Physical principles for the choice of red, green, and blue
The choice of primary colors is related to the physiology of the human eye; good primaries are stimuli that maximize the difference between the responses of the cone cells of the human retina to light of different wavelengths, and that thereby make a large color triangle.The normal three kinds of light-sensitive photoreceptor cells in the human eye respond most to yellow, green, and violet light. The difference in the signals received from the three kinds allows the brain to differentiate a wide gamut of different colors, while being most sensitive to yellowish-green light and to differences between hues in the green-to-orange region.
As an example, suppose that light in the orange range of wavelengths enters the eye and strikes the retina. Light of these wavelengths would activate both the medium and long wavelength cones of the retina, but not equally—the long-wavelength cells will respond more. The difference in the response can be detected by the brain, and this difference is the basis of our perception of orange. Thus, the orange appearance of an object results from light from the object entering our eye and stimulating the different cones simultaneously but to different degrees.
Use of the three primary colors is not sufficient to reproduce all colors; only colors within the color triangle defined by the chromaticities of the primaries can be reproduced by additive mixing of non-negative amounts of those colors of light.
History of RGB color model theory and usage
The RGB color model is based on the Young–Helmholtz theory of trichromatic color vision, developed by Thomas Young and Hermann von Helmholtz in the early to mid-nineteenth century, and on James Clerk Maxwell's color triangle that elaborated that theory.Photography
The first experiments with RGB in early color photography were made in 1861 by Maxwell himself, and involved the process of combining three color-filtered separate takes. To reproduce the color photograph, three matching projections over a screen in a dark room were necessary.The additive RGB model and variants such as orange–green–violet were also used in the Autochrome Lumière color plates and other screen-plate technologies such as the Joly color screen and the Paget process in the early twentieth century. Color photography by taking three separate plates was used by other pioneers, such as the Russian Sergey Prokudin-Gorsky in the period 1909 through 1915. Such methods lasted until about 1960 using the expensive and extremely complex tri-color carbro Autotype process.
When employed, the reproduction of prints from three-plate photos was done by dyes or pigments using the complementary CMY model, by simply using the negative plates of the filtered takes: reverse red gives the cyan plate, and so on.
Television
Before the development of practical electronic TV, there were patents on mechanically scanned color systems as early as 1889 in Russia. The color TV pioneer John Logie Baird demonstrated the world's first RGB color transmission in 1928, and also the world's first color broadcast in 1938, in London. In his experiments, scanning and display were done mechanically by spinning colorized wheels.The Columbia Broadcasting System began an experimental RGB field-sequential color system in 1940. Images were scanned electrically, but the system still used a moving part: the transparent RGB color wheel rotating at above 1,200 rpm in synchronism with the vertical scan. The camera and the cathode-ray tube were both monochromatic. Color was provided by color wheels in the camera and the receiver. More recently, color wheels have been used in field-sequential projection TV receivers based on the Texas Instruments monochrome DLP imager.
The modern RGB shadow mask technology for color CRT displays was patented by Werner Flechsig in Germany in 1938.
Personal computers
s of the late 1970s and early 1980s, such as the Apple II and VIC-20, use composite video. The Commodore 64 and the Atari 8-bit computers use S-Video derivatives. IBM introduced a 16-color scheme with the Color Graphics Adapter for its IBM PC in 1981, later improved with the Enhanced Graphics Adapter in 1984. The first manufacturer of a truecolor graphics card for PCs was Truevision in 1987, but it was not until the arrival of the Video Graphics Array in 1987 that RGB became popular, mainly due to the analog signals in the connection between the adapter and the monitor which allowed a very wide range of RGB colors. Actually, it had to wait a few more years because the original VGA cards were palette-driven just like EGA, although with more freedom than VGA, but because the VGA connectors were analog, later variants of VGA eventually added true-color. In 1992, magazines heavily advertised true-color Super VGA hardware.RGB devices
RGB and displays
One common application of the RGB color model is the display of colors on a cathode-ray tube, liquid-crystal display, plasma display, or organic light emitting diode display such as a television, a computer's monitor, or a large scale screen. Each pixel on the screen is built by driving three small and very close but still separated RGB light sources. At common viewing distance, the separate sources are indistinguishable, which the eye interprets as a given solid color. All the pixels together arranged in the rectangular screen surface conforms the color image.During digital image processing each pixel can be represented in the computer memory or interface hardware as binary values for the red, green, and blue color components. When properly managed, these values are converted into intensities or voltages via gamma correction to correct the inherent nonlinearity of some devices, such that the intended intensities are reproduced on the display.
The Quattron released by Sharp uses RGB color and adds yellow as a sub-pixel, supposedly allowing an increase in the number of available colors.