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SPECTRAL CHARACTERISTICS INFLUENCING THE METAMERIC UNCERTAINTY INDEX

This article examines the relationship between various attributes of light source spectral power distributions (SPDs) and a recently proposed measure of colour rendition, the Metameric Uncertainty Index (Rt). For SPDs comprised of discrete spectral emissions (primaries), the maximum achievable Rt value (corresponding to a reduced likelihood of metameric mismatch) increases with both the number of primaries and their bandwidth. This work also explores how Rt relates to and interacts with other measures of colour rendition—in particular, the Fidelity Index (Rf)—and luminous efficacy of radiation (k). This is important, because there is an intrinsic tradeoff between luminous efficacy of radiation and colour rendition, which informs the application-dependent optimization of illumination. When white-light SPDs are engineered to maximize k, subject to maintaining a minimum required Rf value, the result is a small number of narrow primaries and hence a comparatively poor Rt value. This can only be mitigated by including Rt among the colour rendition optimization constraints. Therefore, from this perspective, Rt is an independent and key aspect of colour rendition.