An additive color space defined by three primary colors has a chromaticitygamut that is a color triangle, when the amounts of the primaries are constrained to be nonnegative.[1][2]
After the development of the CIE system, color triangles were used as chromaticity diagrams, including briefly with the trilinear coordinates representing the chromaticity values.[4] Since the sum of the three chromaticity values has a fixed value, it suffices to depict only two of the three values, using Cartesian co-ordinates. In the modern x,y diagram, the large triangle bounded by the imaginary primaries X, Y, and Z has corners (1,0), (0,1), and (0,0), respectively; color triangles with real primaries are often shown within this space.
Maxwell's disc
Maxwell was intrigued by James David Forbes's use of color tops. By rapidly spinning the top, Forbes created the illusion of a single color that was a mixture of the primaries:[5]
[The] experiments of Professor J. D. Forbes, which I witnessed in 1849… [established] that blue and yellow do not make green, but a pinkish tint, when neither prevails in the combination…[and the] result of mixing yellow and blue was, I believe, not previously known.
Maxwell took this a step further by using a circular scale around the rim with which to measure the ratios of the primaries, choosing vermilion (V), emerald (EG), and ultramarine (U).[6]
Initially, he compared the color he observed on the spinning top with a paper of different color, in order to find a match. Later, he mounted a pair of papers, snow white (SW) and ivory black (Bk), in an inner circle, thereby creating shades of gray. By adjusting the ratio of primaries, he matched the observed gray of the inner wheel, for example:[7]
To determine the chromaticity of an arbitrary color, he replaced one of the primaries with a sample of the test color and adjusted the ratios until he found a match. For pale chrome (PC) he found . Next, he rearranged the equation to express the test color (PC, in this example) in terms of the primaries.
A color triangle attributed to Fick in 1892, based on imaginary primaries corresponding to the three primary sensations of the human eye. In such a triangle, all real colors fall within the curved outline defined by the "pure sensations".
This article uses material from the Wikipedia article Maxwell's_discs, and is written by contributors.
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