Color model
The human color space is a horse-shoe-shaped cone such as shown here (see also CIE chromaticity diagram below), extending from the origin to, in principle, infinity.
This makes it easy to, for example, describe the possible colors (gamut) that can be constructed from the red, green, and blue primaries in a computer display.
However, the overall luminosity function (which in fact is a weighted sum of these three curves) is subjective, since it involves asking a test person whether two light sources have the same brightness, even if they are in completely different colors.
[6][7] This model was used for printing by Jacob Christoph Le Blon in 1725 and called it Coloritto or harmony of colouring,[8] stating that the primitive (primary) colors are yellow, red and blue, while the secondary are orange, green and purple or violet.
RGB is a device-dependent color model: different devices detect or reproduce a given RGB value differently, since the color elements (such as phosphors or dyes) and their response to the individual red, green, and blue levels vary from manufacturer to manufacturer, or even in the same device over time.
[13] It is possible to achieve a large range of colors seen by humans by combining cyan, magenta, and yellow transparent dyes/inks on a white substrate.
The dyes used in traditional color photographic prints and slides are much more perfectly transparent, so a K component is normally not needed or used in those media.
Many are in the shape of a sphere, whereas others are warped three-dimensional ellipsoid figures—these variations being designed to express some aspect of the relationship of the colors more clearly.
The vertical axis of the color sphere, then, is gray all along its length, varying from black at the bottom to white at the top.
All impure (unsaturated hues, created by mixing contrasting colors) comprise the sphere's interior, likewise varying in brightness from top to bottom.
In an attempt to accommodate more traditional and intuitive color mixing models, computer graphics pioneers at PARC and NYIT developed[further explanation needed] the HSV model in the mid-1970s, formally described by Alvy Ray Smith[16] in the August 1978 issue of Computer Graphics.
In the same issue, Joblove and Greenberg[17] described the HSL model—whose dimensions they labeled hue, relative chroma, and intensity—and compared it to HSV.
Albert Munsell began with a spherical arrangement in his 1905 book A Color Notation, but he wished to properly separate color-making attributes into separate dimensions, which he called hue, value, and chroma, and after taking careful measurements of perceptual responses, he realized that no symmetrical shape would do, so he reorganized his system into a lumpy blob.
In the 1940s, the Optical Society of America made extensive measurements, and adjusted the arrangement of Munsell colors, issuing a set of "renotations".
The trouble with the Munsell system for computer graphics applications is that its colors are not specified via any set of simple equations, but only via its foundational measurements: effectively a lookup table.
[20][21][22][23] The Swedish Natural Color System (NCS), widely used in Europe, takes a similar approach to the Ostwald bicone at right.
Because it attempts to fit color into a familiarly shaped solid based on "phenomenological" instead of photometric or psychological characteristics, it suffers from some of the same disadvantages as HSL and HSV: in particular, its lightness dimension differs from perceived lightness, because it forces colorful yellow, red, green, and blue into a plane.
The grayness of an ink is m/M, where m and M are the minimum and maximum among the amounts of idealized cyan, magenta, and yellow in a density measurement.
[25] The International Commission on Illumination (CIE) developed the XYZ model for describing the colors of light spectra in 1931, but its goal was to match human visual metamerism, rather than to be perceptually uniform, geometrically.
In the 1960s and 1970s, attempts were made to transform XYZ colors into a more relevant geometry, influenced by the Munsell system.
Though these two theories were initially thought to be at odds, it later came to be understood that the mechanisms responsible for color opponency receive signals from the three types of cones and process them at a more complex level.
Human trichromatic color vision is a recent evolutionary novelty that first evolved in the common ancestor of the Old World Primates.
Our trichromatic color vision evolved by duplication of the long wavelength sensitive opsin, found on the X chromosome.
Human red–green color blindness occurs because the two copies of the red and green opsin genes remain in close proximity on the X chromosome.
