Why are the CIE primaries 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 might have been 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 S_E (a stimulus that has equal luminance at each wavelength).