Circadian metric – Computation of circadian stimulus using illuminance, correlated colour temperature and colour rendering index

Abstract

The circadian rhythm is influenced by light and other non-visual responses in humans. Recent researches have shown that the visual quantity illuminance is not appropriate for evaluating non-visual effects. Metrics such as the circadian stimulus have been developed to address this issue. One of the limitations with circadian metrics is that the determination requires a spectroradiometer instead of the standard illuminance meters. In this article, a simple circadian stimulus calculation model for white light sources is presented. The model is determined with the parameters illuminance, correlated colour temperature CCT and colour rendering index Ra. The accuracy and reliability of the model is high and is based on 7641 spectra with 19 illuminance levels, which are generated using a heuristic optimisation procedure. In addition, the model has been internally validated with the method repeated-split-data and has an error deviation of 0.058 for CCT > 3710 K and 0.045 for CCT ≤ 3710 K at 99% confidence interval in the circadian stimulus CS range between 0.2 and 0.5. Since the quantities illuminance, correlated temperature and colour rendering index are common in practice, this model enables non-visual planning of illumination installations by lighting designers or lighting design tools. Furthermore, it allows retrospective analysis of past non-visual research results in scientific studies, where illuminance and correlated colour temperature were provided.