Additive Colour
Additive colour mixing refers to the mixing of different coloured lights and can be easily demonstrated by the superposition of lights (primaries) on a white projection screen. When this is done using red, green, and blue primaries, the colours yellow, cyan, and magenta are produced where two of the primaries overlap. Where all three primaries overlap the sensation of white is produced if the spectral distributions and intensities of the three primaries are carefully chosen.
Additivity is not a special property of any particular set of three primaries. The range of colours that can be matched with any three primaries is called the gamut of those primaries. It turns out that no three real primaries can be chosen so that their gamut includes all possible colours. If the primaries are chosen to be red, green, and blue, however, a very large number of colours can be matched. Red, green, and blue are therefore usually the colours of the primaries in an additive colour reproduction system such as colour television.
The CIE primaries are often called imaginary primaries . It is impossible to choose three real primaries such that all possible colours can be matched with additive mixtures of those primaries. Thus, in a real additive colour reproductive system such as colour television only a limited gamut of colours can be displayed. In 1931, when the CIE system was specified, it was decided to use three imaginary primaries such that the tristimulus values X, Y, and Z, are always positive for all real colour stimuli. The concept of imaginary primaries is complex but it is not strictly neccessary to understand this concept in order to understand and use colour specification. In fact, the CIE could have used three real primaries, such as red, green, and blue lights, in which case the tristimulus values would be represented by R, G, and B.
There were several reasons for the adoption of imaginary primaries. Firstly, the primaries were chosen such that X, Y, and Z would be positive for all possible real stimuli. Although this might not seem particularly important today, the elimination of negative tristimulus values was an important consideration in pre-computer days. Secondly, the coefficients were chosen such that the Y tristimulus value was directly proportional to the luminance of the additive mixture. Thirdly, the coefficients were chosen such that X=Y=Z for a match to the equi-energy stimulus SE (a stimulus that has equal luminance at each wavelength).