A set of primary colors is a set of pigments, colored lights, or abstract elements of a mathematical color space model that can be combined in varying amounts to produce a range or "gamut" of colors. Deriving many colors from several primaries facilitates technological and artistic applications such as painting, electronic displays, and printing. Any small set of realizable primary colors are "imperfect" in that they cannot generate all perceptible colors, but some sets of primaries can yield a far wider gamut than others.
For an additive set of primary colors for human vision, as in a television or computer display screen, projector, or other emissive electronic visual displays, the usual choice is red, green, and blue (RGB), although the primaries' specific chromaticities can vary.
For a set of subtractive primary colors for humans, as in mixing of pigments for printing, cyan (a bright greenish-blue), magenta (a bright reddish purple), and yellow are often used (usually supplemented by black to make CMYK).
Red, yellow, and blue (RYB) are a well-known traditional set of subtractive primaries in the art field. Artists often use more than three chromatic pigments and so are not limited to a colorspace such as RYB even if they think of these three as their primary colors in an abstract or conceptual sense, for example when constructing a color wheel.
The precise set of primary colors to be used in a specific color application depends on gamut requirements as well as on application-specific constraints such as cost, power consumption, lightfastness, mixing behavior, etc.
Abstract colorspaces include the additive CIE XYZ, in which all visible colors can be represented mathematically by combinations of its X, Y, and Z primaries, but each primary itself does not directly correspond to a physically-possible light source.
Video Primary color
Color applications based on primary mixing
The response of the eye to a "mix" of primary colors is predicted by different models for different applications.
Additive mixing of light
In additive color mixing, the total light present is simply the sum of individual light sources. For example, coincident blue and red spotlights on a surface will make a purple, brighter than either of the spotlights alone. Additive mixing of coincident spot lights was applied in the experiments used to derive the CIE 1931 colorspace. The original monochromatic primaries of the (arbitrary) wavelengths of 435.8 nm (violet), 546.1 nm (green) and 700 nm (red) were used in this application due to the convenience they afforded to the experimental work.
Red, green, and blue light are the ideal primaries for additive color mixing since primary lights with those hues provide the largest triangular chromaticity gamuts. Small red, green, and blue elements in electronic displays mix additively in the eye at an appropriate viewing distance to synthesize compelling colored images.
The exact colors chosen for additive primaries are a technological compromise between the available phosphors (including considerations such as cost and power usage) and the need for large chromaticity gamut. The ITU-R BT.709-5/sRGB primaries are typical.
Subtractive mixing of ink layers
Subtractive color mixing describes overlaid partially absorbing materials on a reflecting or transparent surface. Each layer partially absorbs some wavelengths of light from the illumination spectrum while letting others pass through (combining multiplicatively), which results in a colored appearance. Overlapping layers of ink in printing mix subtractively over reflecting white paper in this way, to generate photorealistic color images. The typical number of inks in such a printing process ranges from 3 to 6 (e.g., CMYK process, Pantone hexachrome). In general, using fewer inks as primaries results in more economical printing but using more achieves potentially better color reproduction.
Cyan, magenta, and yellow are good subtractive primaries in that the reflectance curves corresponding to idealized inks can be combined for the largest chromaticity gamuts. An additional key ink (shorthand for the key printing plate that impressed the artistic detail of an image, usually black) is also usually used since it is difficult to mix a dark enough black ink using the other three inks. Before the color names cyan and magenta were in common use, these primaries were often known as blue and red, respectively, and their exact color has changed over time with access to new pigments and technologies.
Mixing paints in limited palettes
The perceived color of mixed paints, slurries of pigment particles typically suspended in water or linseed oil, isn't well approximated by either subtractive or additive mixing model. Color predictions that incorporate light scattering effects of pigment particles and paint layer thickness require approaches based on the Kubleka-Munk equations. Even such approaches cannot predict the color of paint mixtures precisely since small variances in particle size distribution, impurity concentrations etc. can be difficult to measure but impart significant effects on the way light is reflected from the paint. Artists typically rely on mixing experience and "recipes" to mix desired colors from a small initial set of primaries and do not use mathematical modelling.
There are hundreds of commercially available pigments for visual artists to use and mix (in various media such as oil, watercolor, acrylic, and pastel). A common approach is to use just a limited palette of primary pigments (often between four and eight) that can be physically mixed to any color that the artist desires in the final work. There is no specific set of pigments that are primary colors, the choice of pigments depends entirely on the artist's subjective preference of subject and style of art as well as material considerations like lightfastness and mixing heuristics. Contemporary classical realists have often advocated that a limited palette of white, red, yellow, and black pigment (often described as the "Zorn palette") is sufficient for compelling work.
A chromaticity diagram can illustrate the gamut of different choices of primaries, for example showing which colors are lost (and gained) if you use RGB for subtractive color mixing (instead of CMY).
Maps Primary color
Biological basis
A contemporary description of the color vision system provides an understanding of primary colors that is consistent with modern color science. The human eye normally contains only three types of color photoreceptors, known as long-wavelength (L), medium-wavelength (M), and short-wavelength (S) cone cells. These photoreceptor types respond to different degrees across visible electromagnetic spectrum. The S cone response is generally assumed to be negligible at long wavelengths greater than about 560 nm while the L and M cones respond across the entire visible spectrum. Every visible wavelength stimulates at least two cone types. Since there is no visible wavelength that stimulates only one type of cone, L, M and S are considered imaginary (or abstract) primaries. Humans cannot normally see a color that corresponds to a pure L, M or S response but essentially every visible color can be mapped to a triplet specifying the coordinates in LMS color space. Humans and other species with three such types of color photoreceptor are known as trichromats.
The L, M and S response curves (cone fundamentals) were deduced from color matching functions obtained from controlled color matching experiments (e.g., CIE 1931) where observers matched the color of a surface illuminated by monochromatic light with mixtures of three monochromatic primary lights illuminating a juxtaposed surface. Practical applications generally use a canonical transformation of LMS space known as CIEXYZ. The X, Y, and Z primaries are typically more useful since luminance (Y) is specified separately from a color's chromaticity. There are many useful derivatives of CIEXYZ including CIELUV and CIELAB, each colorspace possessing its own set of primaries that are used to specify essentially any color. All of the respective primaries of these colorspaces which are conceptually rooted in LMS space are also necessarily abstract and complete in the sense that no color space contains more colors than another . All of these primaries are also arbitrary in the sense that they can be subjected to various types of mathematical transformations that are one-to-one and still specify each color in the original space exactly and completely. The color matching context is always three dimensional (since LMS space is three dimensional) but more general color appearance models like CIECAM02 describe color in six dimensions and can be used to predict how colors appear under different viewing conditions.
Thus for trichromats like humans, we use three (or more) primaries for most general purposes. Two primaries would be unable to produce even some of the most common among the named colors. Adding a reasonable choice of third primary can drastically increase the available gamut, while adding a fourth or fifth may increase the gamut but typically not by as much.
Most placental mammals other than primates have only two types of color photoreceptor and are therefore dichromats, so it is possible that certain combinations of just two primaries might cover some significant gamut relative to the range of their color perception. Meanwhile, birds and marsupials have four color photoreceptors in their eyes, and hence are tetrachromats. There is one scholarly report of a functional human tetrachromat.
The presence of photoreceptor cell types in an organism's eyes do not directly imply that they are being used to functionally perceive color. Measuring functional spectral discrimination in non-human animals is challenging due to the difficulty in performing psychophysical experiments on creatures with limited behavioral repertoires who cannot respond using language. Limitations in the discriminative ability of shrimp having twelve distinct color photoreceptors have demonstrated that having more cell types in itself need not always correlate with better functional color vision.
History
There are numerous competing primary colour systems throughout history. Scholars and scientists engaged in debate over which hues best describe the primary color sensations of the eye. Thomas Young proposed red, green, and violet as the three primary colors, while James Clerk Maxwell favoured changing violet to blue. Hermann von Helmholtz proposed "a slightly purplish red, a vegetation-green, slightly yellowish, and an ultramarine-blue" as a trio. 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.
Psychological primaries
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 theory states that every color can be described as a mix along the three axes of red vs. green, blue vs. yellow and white vs. black. The six colors from the pairs might be called "psychological primary colors", because any other color could be described in terms of some combination of these pairs. Although there is a great deal of evidence for opponency in the form of neural mechanisms, there is currently no clear mapping of the psychological primaries to neural substrates.
The three axes of the psychological primaries were applied by Richard S. Hunter as the primaries for the colorspace ultimately known as CIELAB. The Natural Color System is also directly inspired by the psychological primaries.
See also
- Color vision
References
Source of the article : Wikipedia
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