Improving the Empirical Formulae for CIE Daylight Illuminants

ABSTRACT

Daylight illuminants are widely used in the lighting industry as reference illuminants for evaluating test light sources using the color rendering index ( R _a ) or the color fidelity index ( R _f ). In both metrics, the test source is compared with a reference illuminant that has the same correlated color temperature (CCT). The spectral power distribution (SPD) of the CIE daylight illuminants (the D‐Series) is constructed from three basis functions, S ₀ , S ₁ , and S ₂ , derived by the Commission Internationale de l'Éclairage (CIE) from empirical measurements of natural daylight. The combination coefficients for these basis functions depend on the specified CCT. It has been observed, however, that the CCT of the resulting daylight SPD often differs from the intended CCT, with the discrepancy increasing as the specified CCT becomes higher. This paper introduces a new method for computing the combination coefficients used to generate daylight SPDs. With the proposed coefficients, the generated SPD attains the exact CCT specified. This improved method has practical applications in assessing light‐source quality using both the color rendering index ( R _a ) and the color fidelity index ( R _f ).