CIE Colour space
(Commission Internationale de l'Eclairage)

In these notes on the CIE System the primaries under consideration are additive only; that is they may be considered as coloured lights which when added together in the correct proportions give white light. The primaries of this system are red, green and blue as opposed to the subtractive primaries of pigment mixing - magenta, yellow and cyan.

Any attempt to match all colours with three real primaries ends with a somewhat restricted colour gamut or colour triangle.

This representation of the Maxwell triangle shows the effect of mixing three real primaries. As long as the triangle formed by joining the primaries, together, roughly encompasses the centre of the diagram, white light may be obtained by mixing the lights in the correct proportions, but the only wat to define all the spectrum colours in terms of the chosen three is to use negative quantities of one of the three for those colours which fall outside the triangle. The diagram will illustrate how this is done experimentally.

The cyan colour at point C cannot be matched by mixing blue and green but the colour at point B can be attained. If red is then added to the colour C until it reaches point B the amount of red added is used in the definition of C as a negative amount.

It may have taken y parts of green, z parts of blue and minus x parts of red to reach point C, i.e. y.G + z.B - x.R = Colour C.

If however use is made of imaginary primaries which lie outside the spectrum locus, while we are unable to physically mix the lights, we can define all colours by positive amounts of the three primaries. As the transformation from real to imaginary is purely mathematical to enable numerical values to be used for the definition of colours there is no objection to this manipulation.

The CIE system which defines a colour by the amounts of the three mixable stimulus colours required to match it, uses such imaginary stimuli. Thes are not random choices but are based on the sensitivity of the human eye to the colours of the spectrum. The work that led to the CIE system being adopted internationally in 1931 was carried out by Wright and Guild in 1928 with visual colorimeters in which the operator mixed three coloured lights in one half of the photometric field to match a pure spectrum colour in the other half of the field. Calibrated controls on each of the three primaries enabled the amount of each primary to be recorded. As three real primaries cannot match all spectrum colours some of the spectrum colours had to be degraded with their complimentary colours. This is in effect adding negative quantities, as has already been explained.

The exact colours of the primaries used were of little significance as the tristimulus values given by one system could be transformed into the values of another.

The observers in Wright's experiments used narrow band spectrum primaries of wavelengths 650, 530 and 460 nanometers, while Guild's observers used red, green and blue filters. The tristimulus values (the amounts of each of the three primary colours) required to match various spectrum colours were obtained.

The agreement between the two sets of tristimulus values after transformations, were so extremely good that the results were accepted by the CIE in 1931 as the basis of an international standard.

The data was further transformed to a set of unreal primaries, X, Y and Z, lying outside the gamut of spectrum colours so that any real colour could be evaluated in terms of positive tristimulus values.

The value of X is the amount of a reddish purple primary of higher saturation than any real colour. The value of Y is the amount of an unreal primary having a higher saturation than a colour of 520 nanometers wavelength and the value of Z is the amount of an unreal blue primary of higher saturation than a colour of 477 nanometers wavelength,

The curves which define the X, Y and Z tristimulus values are shown below for the equal energy spectrum.