A set of primary colors is a small, arbitrary set of pigmented physical media, lights or a purely abstract elements of a colorspace. Distinct colors from a larger gamut can be specified in terms of a mixture of primary colors which facilitates technological applications such as painting, electronic displays and printing. Any small set of pigments or lights are "imperfect" physical primary colors in that they cannot be mixed to yield all possible colors that can be perceived by the psychophysical human vision system. The abstract (or "imaginary") primaries X, Y and Z of the CIEXYZ colorspace can be mathematically summed to specify essentially all colors that can be perceived but these primaries cannot be physically realized due to the underlying structure and overlapping spectral sensitivities of each of the human cone photoreceptors. The precise set of primary colors that are used in a specific color application depend on gamut requirements as well as application-specific constraints such as cost, power consumption, lightfastness, mixing behavior etc.
For additive combination of colors, as in coincident projected lights or in electronic visual displays, the primary colors normally used are red, green and blue (but the precise visible light spectra for each color can vary significantly). For a subtractive combination of colors, as in mixing of pigments or dyes for printing, the colors magenta, yellow and cyan are normally used. See RGB color model, and CMYK color model for more on these popular sets of primary colors.
Primary colors are not a fundamental property of light but are related to the psychophysical response of the eye to light. The human eye normally contains only three types of color receptors that are associated with specialized cone cells. Each color receptor responds to different ranges of the color spectrum although there is no single wavelength that stimulates only one color receptor type. Humans and other species with three such types of color receptors are known as trichromats. These three receptors are why colorspaces based on color matching experiments (generally derived CIE1931) are fundamentally three-dimensional. Color appearance models like CIECAM02 describe color in six dimensions that can be used to predict how colors appear in different viewing conditions.
Before the nature of colorimetry and visual physiology were well understood, scientists such as Thomas Young, James Clerk Maxwell and Hermann von Helmholtz expressed various opinions about what should be the three primary colors to describe the three primary color sensations of the eye. Young originally proposed red, green and violet, and Maxwell changed violet to blue; Helmholtz proposed "a slightly purplish red, a vegetation-green, slightly yellowish (wavelength about 5600 tenth-metres), and an ultramarine-blue (about 4820)". In modern understanding, human cone cells do not correspond precisely to a specific set of primary colors, as each cone type responds to a relatively broad range of wavelengths.
The peak response of the three human color receptors varies, even among individuals with "normal" color vision; in non-human species this polymorphic variation is even greater, and it may well be adaptive. Most placental mammals other than primates have only two types of color receptors and are therefore dichromats. Many birds and marsupials are tetrachromats with four color receptor types. There is no currently peer reviewed scholarly work that has confirmed the existence of a functional human tetrachromat. In general, the presence of extra color receptor types does not directly imply that they are being used functionally for improved spectral discrimination in an animal as vision systems are made up of complex sets of neurons that affect perception in ways that are difficult to interrogate.
Examples of Primary Colors
RGB for Electronic Displays
Television and other computer and video displays are a common example of the use of additive primaries and the RGB color model. The exact colors chosen for the primaries are a technological compromise between the available phosphors (including considerations such as cost and power usage) and the need for large color triangle to allow a large gamut of colors. The ITU-R BT.709-5/sRGB primaries are typical.
Additive mixing of red and green light produces shades of yellow, orange, or brown. Mixing green and blue produces shades of cyan, and mixing red and blue produces shades of purple, including magenta. Mixing nominally equal proportions of the additive primaries results in shades of grey or white; the color space that is generated is called an RGB color space. The experiments used to derive the CIE 1931 color space used monochromatic primary colored lights with the (arbitrary) wavelengths of 435.8 nm (violet), 546.1 nm (green) and 700 nm (red) due to the convenience they afforded to the experimental work.
Some recent TV and computer displays are starting to include yellow as a fourth "primary" color, often in a four-point square pixel area, so as to achieve brighter pure yellows and a larger color gamut. Even the four-primary technology does not yet reach the range of colors that the human eye can see from light reflected by illuminated surfaces (as defined by the sample-based estimate called the Pointer Gamut), with 4-primary LED prototypes providing typically about 87% and 5-primary prototypes about 95%. Several firms, including Samsung and Mitsubishi, have demonstrated LED displays with five or six "primaries", or color LED point light sources per pixel. A recent academic literature review claims a gamut of 99% can be achieved with 5-primary LED technology. While technology for achieving a wider gamut appears to be within reach, other issues remain; for example, affordability, dynamic range, and brilliance. In addition, there exists hardly any source material recorded in this wider gamut, nor is it currently possible to recover this information from existing visual media. Regardless, industry is still exploring a wide variety of "primary" active light sources (per pixel) with the goal of matching the capability of human color perception within a broadly affordable price. One example of a potentially affordable but yet unproven active light hybrid places an LED screen over a plasma light screen, each with different "primaries". Because both LED and plasma technologies are many decades old (plasma pixels going back to the 1960s), both have become so affordable that they could be combined.
CMYK color model or four-color printing
In the printing industry, to produce the varying colors the subtractive primaries cyan, magenta and yellow are applied together in varying amounts. Before the color names cyan and magenta were in common use, these primaries were often known as blue-green and purple or in some circles as blue and red, respectively, and their exact color has changed over time with access to new pigments and technologies.
Mixing yellow and cyan produces green colors; mixing yellow with magenta produces reds, and mixing magenta with cyan produces blues. In theory, mixing equal amounts of all three pigments should produce grey, resulting in black when all three are applied in sufficient density, but in practice they tend to produce muddy brown colors. For this reason, and to save ink and decrease drying times, a fourth pigment, black, is often used in addition to cyan, magenta and yellow.
These results are described by the CMYK color model. The abbreviation stands for cyan, magenta, yellow and key—black is referred to as the key color, a shorthand for the key printing plate that impressed the artistic detail of an image, usually in black ink.
The opponent process is a color theory that states that the human visual system interprets information about color by processing signals from cones and rods in an antagonistic manner. The three types of cones have some overlap in the wavelengths of light to which they respond, so it is more efficient for the visual system to record differences between the responses of cones, rather than each type of cone's individual response. The opponent color theory suggests that there are three opponent channels: red versus green, blue versus yellow and black versus white. Responses to one color of an opponent channel are antagonistic to those of the other color. The theory states that the particular colors considered by an observer to be uniquely representative of the concepts red, yellow, green, blue, white and black might be called "psychological primary colors", because any other color could be described in terms of some combination of these.
No specific hues, set of pigments or visible light spectra are the primary colors; all choices for primary colors in a specific application are arbitrary.
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