In order to achieve a representation which uses only positive mixing coefficients,
the CIE ("Commission Internationale d'Eclairage") defined three new
hypothetical light sources, x, y, and z, which yield positive matching curves:

If we are given a spectrum and wish to find the corresponding X, Y, and Z
quantities, we can do so by integrating the product of the spectral power and
each of the three matching curves over all wavelengths. The weights X,Y,Z form
the three-dimensional CIE XYZ space, as shown above.