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As I understand, the CIExy graphic maps "greenness", or rather middle-wavelengthness, to the Y axis and "redness", or rather long-wavelengtsness, to the X axis. The trapping used to reduce the 3d gamut of the human eye to 2d is that the gamut is specified for a certain constant luminosity, and long-wavelength + middle-wavelength + short-wavelength equals the said luminosity, thus the third coordinate can be extrapolated from the first two.

An example of this plot can be seen here, on wikipedia.

The curved edge of the human vision gamut gives the single wavelength spectrum. If this is the case, how come the RGB gamut (triangular) does not intersect the curved edge of the human vision gamut? The light coming off the 3 LED categories in the display should be of a nigh-single wavelength (as LEDs have a single, very sharp emission peak).

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The corners of the triangle represent some standard definition of red, green, and blue. There's no particular reason that they should correspond to single wavelengths, although admittedly the closer the standard colors are to the single-frequency curve, the larger the visible gamut would be. The standard colors were undoubtedly chosen as a compromise based on dyes and phosphors that existed when the standards were created.

A new gamut can be created using frequencies that LEDs make available. But such a gamut would not be consistent with accepted standards, with the result that the color specified by a particular RGB triplet would be different on an LED device than, say, a CRT display. In order to make the same RGB triplet appear as similar as possible on various devices, displays adhere to one of the standard gamuts.

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Yes, RGB predates LED monitors, but still, LED monitors are based on LEDs, which are single-wavelength. So, why does the gamut not reach the single-frequency spectrum? –  TheChymera Apr 11 at 0:49
    
Are you saying that those corners represent LEDs? Normally, the diagram you cite references standard colors, not LEDs. What's the original source of that diagram? Are you certain that they represent LEDs? –  garyp Apr 11 at 0:57
    
Perhaps some confusion arises because there is a CIE RGB color space in which the primaries are single frequencies. But this color space is not used in computer displays. There is a very brief discussion of history along with a tabulation of the currently accepted HDTV definitions of red, blue, and green here –  garyp Apr 11 at 1:11
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