Line of purples

In color theory, the line of purples or the purple boundary is the locus on the edge of the chromaticity diagram between extreme spectral red and violet. Except for the endpoints, colors on the line of purples are not spectral. Line-of-purples colors and spectral colors are the only ones which are considered fully saturated in the sense that for any given point on the line of purples there exists no color involving a mixture of red and violet that is more saturated than it. There is no monochromatic light source able to generate a purple color. Instead, every color on the line of purples is produced by mixing a unique ratio of fully saturated red and fully saturated violet, at the extreme points of visibility on the spectrum of pure hues.

Unlike spectral colors (which may be implemented, for example, by nearly monochromatic light of laser, with precision much finer than human chromaticity resolution), colors on the line of purples are more difficult to implement practically. The sensitivity of cones to both of the spectral colors at the opposite extremes of what the human eye can see is quite low (see luminosity function), so commonly observed purple colors do not achieve a high level of brightness.

The line of purples, a theoretical boundary of chromaticity, should not be confused with "purples", a more general color term which also refers to less than fully saturated colors (see variations of purple and variations of pink for possible examples) which form an interior of a triangle between white and the line of purples in the CIE chromaticity diagram.

In 3-dimensional color spaces the "line", if present, becomes a 2-dimensional shape. For example, in CIE XYZ it is a planar sector bounded by black–red and black–violet rays. In systems focused on pigment colors, such as Munsell and Pantone, boundary purples might be absent, because maximally possible lightness of a pigment vanishes when its chromaticity approaches the line of purples, such that near-boundary purple pigments become indistinguishable from black.

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