Why are the CIE primaries often called imaginary primaries?
It is impossible to choose three real primaries such that all possible colors can be matched with additive mixtures of those primaries. Thus, in a real additive color reproductive system such as color television only a limited gamut of colors 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 color stimuli. The concept of imaginary primaries is complex but it is not strictly neccessary to understand this concept in order to understand and use color 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 to
