CIE Colour Model

Last Updated February 11, 2009

CIE XYZ space

The CIE (Commission Internationale de l'Eclairage) conducted experiments in the 1930's to determine how primary components should be added in order to reproduce colours. The experiments revealed that to produce some colours, it was necessary to subtract a component of light. This is because when two primaries are added the saturation decreases. It was impossible to produce a maximally saturated colour by mixing two others. To create such colours a negative component is required. To avoid the problem of having negative components, the CIE devised three supersaturated (non-realizable) colours X, Y, Z, which, when additively mixed (i.e. without negative co-efficients) can produce all perceivable colours. These primaries are devised from the colour sensitivity distribution of the human eye, this allows the system to represent all human perceivable colours. In this system the colour the intensity or luminosity of the light. Applying some affine transformations and choosing an appropriate viewpoint, the CIE colour space can be viewed in 2 dimensions (figure below).

The CIE Chromacity Diagram

This colour model is difficult to use, as X,Y and Z do not correspond to real colours. Also, some CIE colours cannot be displayed on a traditional monitor as these work by adding red, green and blue light. It was not designed as colour selection tool, but as an international standard for accurate colour description, instruments are available which can measure the X,Y & Z components from any give colour sample.

Lab

A somewhat simplified and perceptually linear version of CIEXYZ used today is CIELab.

L= luminosity

a = position on Red-Green opponent channel ( -ive= red, +ive=green)

b = position on Yellow-Blue opponent channel ( -ive= yellow, +ive=blue)

The applets here allow you to play with different colour spaces.

Colour Gamuts

The CIE diagram is useful for specifying colour gamuts, the range of colours which can be produced by a device. As mentioned before, a typical computer monitor can only produce colours which are additive combinations of the RGB primaries, this limits its output to a subset of perceivable colours. The vertices of the monitor gamut represent the primaries used in the phosphor/LCD technology. Any colour falling outside this triangle cannot be produced by a CRT/LCD monitor.

The colour gamut of a printer is also shown, this gamut is not a simple triangle because of the mechanisim in which the inks are placed on the paper. Note that a printer cannot reproduce all the colours of a monitor and there are some printer colours which cannot be shown on a monitor.