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Colour rendering and spectral power distribution—a new parametric approach

In order to obtain a better understanding of the relation between the spectral power distribution of a light source and its CIE general colour rendering index Ra, three parameters have been derived from the spectrum. These are By, Gy and Ry, respectively the. 'proportion of blue, green and red radiations with respect to yellow'. An approximate index Ra(B y, Gy, Ry) ≃ Ra has been determined together with its graphic representation and the influence of each parameter on colour rendering. The magnitude of the error |Ra — Ra| is less than 5 units for usual illuminants. As an application of this parametric method, a solution is offered for the best combination of an artificial light source with a natural one.