The Commission Internationale de l'Eclairage (CIE - the International Commission on Illumination) experimentally determined the color map at the right as a diagram of the only colors the normal human eye can see, not counting darker versions of the same colors (which would be under the map) and black. The lower image is the map seen with the z-axis perpendicular to the page. The upper image is an isometric projection of the map with the plane x + y + z = 1 in the plane of the page and the zero origin directly under the 6500 K white point. The pure spectral hues go around the curved edge from red through green to blue. The wavelengths in nanometers (nm) are on the curve. The straight line connects red to blue through the nonspectral hues (all mixtures of red and blue). x is the portion of the observed color that is red. y is the portion of the observed color that is green. z is the portion of the observed color that is blue. It is perpendicular to the page in the lower image. The top surface of the map (the visible portion in the diagram) is the plot of the plane: x + y = z = 1 White is in the center, at the location x = .33; y = .33; and z = .33. It is at the color temperature of 6500 K. Note that all visible light wavelengths stimulate at least two cone cell types. This is because other parts of the optic system block light with wavelengths shorter than 380 nm and longer than 770 nm. Thus, the case where one cone cell type is stimulated exclusively is not possible. This explains the fact that no part of the colored area of the map reaches a corner of the graph. The red cones are also slightly sensitive to blue and violet, explaining why the blues have higher x values than some greens (see the spectral plots below). It also explains why we see all of the hues as making a circle of pure colors, so we have a color wheel. Green cones are also partly sensitive to red, explaining the nonzero Y value at the red end of the spectrum. The regions outside the colored area of the map are impossible colors, because they would require stimulating the cone cells in the eye in ways that real light can not do. The curved line through the center is the Planckian Locus, the plot of colors produced by a black body heated to various temperatures. The temperatures are given in Kelvins (K). The color temperature of a neutral white is 6500 K. Color TV and computer monitors cannot correctly produce some of the colors in the color map (and they do not reproduce properly here) because the primary colors chosen for the NTSC system (and all color monitors) correspond to the wavelengths 610 nm (red), 540 nm (green), and 470 nm (blue). The gamut (range) of colors a color monitor can reproduce is thus limited to the colors on or inside the triangle connecting these three points on the spectral perimeter (the triangle with lines A and D as sides). Any color outside the triangle cannot be reproduced on a color monitor (including this one). These colors were chosen to sacrifice colors in the cyan area to make flesh tones come out more accurately. Two proposals for including more colors on monitors exist: One proposal adds another primary at 510 nm. This covers most of the visible colors. The extra primary could be automatically selected from existing signals, using heuristics based on the values of x and y in the existing color signal. The gamut is the convex quadrilateral including lines B and D as sides.

The other proposal adds yellow at 575 nm, and changes the green primary to 510 nm. Monitors have been produced with these primaries. These also use heuristics to select the primaries used for each color. The gamut is the quadrilateral including lines C and D as sides. The white triangle is the gamut of colors visible when illuminated with many brands of white LEDs.

Isometric Projection

(axes shown 120° apart) Orthographic Projection

(z-axis perpendicular to page)