The International Commission on Illumination (usually known as the CIE for its French language name Commission Internationale de l'Eclairage) is the international authority on light, illumination, color, and color spaces.
The CIE has its headquarters in Vienna, Austria.
The CIE's chromaticity diagram, developed in 1931, is still used as a standard reference for defining colours, and as a reference for other color spaces. The diagram is a two-dimensional plot of colours of constant intensity based on the visual response of the CIE-1931 standard observer, which was determined by physiological measurements of human colour vision. Since the human eye has three types of colour sensor that respond to different ranges of wavelengths, a full plot of all visible colours is a three-dimensional figure. This is inconvenient to draw on a two-dimensional sheet of paper, so for convenience the CIE transformed the three-dimensional colour space into two artificial dimensions of colour (collectively called chromaticity) and one of intensity, and then took a two-dimensional slice through this space at the level of maximum intensity. This slice became the chromaticity diagram. Incidentally, this technique of converting a three-dimensional colour space to a combination of chromaticity and intensity is also used in colour television.
The gamut of all visible colours on the CIE plot is a tongue-shaped or horseshoe-shaped figure, with the curved edge corresponding to the colours of the visible spectrum and the straight edge (the purple line) corresponding to non-spectral shades of purple. Less saturated colours appear in the interior of the figure, with white at the centre.
A three-dimensional figure can be made by plotting the CIE's chromaticity on two axes and intensity on the third axis. It is a roughly pyramidal solid that is informally called the colour bag.
