It is shown how to represent the relation between a spectral power distribution (SPD) and its general colour rendering index by means of three spectral parameters By, Gy and Ry. Using this parametric approach, the combination of two illuminants and its effect on colour rendering are analysed. The main conclusion is that the usual illuminants (whose chromaticity lies near the black-body locus) behave as if they could be classified into two classes. If two SPDS are mixed in a proportion α, the law a(α) is concave without maximum if they belong to the same class and with maximum if not. Some variations to this law may occur when the illuminants differ greatly in chromaticity.
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