The CIE XYZ 1931 Tristimulus system is a modified version of the RGB colour tristimulus system and uses non real or imaginary stimuli. This innovation allows all colours within its gamut to be expressed as positive numbers, unlike the RGB system, where some colours fall outside its gamut and are expressed as negatives.  The XYZ chromaticity co-ordinates are defined as x,y,and z.

Tristimulus values are percentages of the components in a three-colour additive mixture necessary to match a colour; in the CIE system, they are designated as X, Y and Z. The illuminant and standard observer colour-matching functions used must be designated; if they are not, the assumption is made that the values are for the 1931 observer (2º field) and illuminant C.

The values obtained depend on the method of integration used, the relationship of the nature of the sample and the instrument design used to measure the reflectance or transmittance. Tristimulus values are not, therefore, absolute values characteristic of a sample, but relative values dependent on the method used to obtain them.

Approximations of CIE tristimulus values may be obtained from measurements made on a tristimulus colorimeter that gives measurements generally normalised to 100.

These must then be normalised to equivalent CIE values. The filter measurements should be properly designated as R, G and B instead of X, Y and Z.