Fundamental Spectral Boundaries of Circadian Tunability

Abstract

The discovery of melanopsin and the non-visual impact of light, lead to a new era in lighting design. Using a melanopic action spectrum, the CIE introduced the melanopic efficacy of luminous radiation (MELR), which quantifies the influence of light on melanopsin. A significant amount of research has been conducted on the possible variation of this MELR and other related metrics for various practical lighting settings, but the fundamental spectral boundaries have not yet been disclosed. Without these limits, it is difficult to assess if a certain lighting system really achieves high or low MELR. This paper determines these fundamental MELR boundaries for different CCT and certain minimal TM-30 $R_f$ values, using a a flexible parametrization of the light source spectrum that is optimized with Differential Evolution. The obtained results show that with increasing CCT, the interval between the theoretical MELR extrema increases slightly, while the opposite occurs when increasing the minimal TM-30 $R_f$ value. The same parametric model and optimization approach is also used to determine the maximal Luminous Efficacy of Radiation (LER) within the obtained MELR limits.