In order to define the perceptual 3D space in a "standard" way, a set of experiments have been carried by having observers try and match colour of a given wavelength, lambda, by mixing three other pure wavelengths, such as R=700nm, G=546nm, and B=436nm in the following example.
Note that the phosphors of colour TVs and other CRTs do not emit pure red, green, or blue light of a single wavelength, as it is the case for this experiment.
The scheme above can tell us what mix of R,G,B is needed to reproduce the perceptual equivalent of any wavelength. A problem exists, however, because sometimes the red light needs to be added to the target before a match can be achieved. This is shown on the graph by having its intensity, R, take on a negative value.