Matching CIE illuminants to measured spectral power distributions: A method to evaluate non-visual potential of daylight in two European cities

Abstract

The evaluation of non-visual effects of light is a crucial topic in lighting design research and practice. Performing such analysis requires precise information about the spectral distribution of the tested light source. Assessing non-visual effects of daylight is complicated due to its spectral composition continuously changing, depending on many factors. Currently there are no available databases with spectral and spatial radiation patterns of the sky vault for locations spread all over the world, and an easy and common method to evaluate non-visual effects of daylight is lacking.The goal of the paper is to provide a simple method to evaluate the non-visual potential of daylight, accounting for its variability. In order to present it, spectral measurements were conducted in two European cities in spring and summer. Horizontal and vertical illuminance toward the four cardinal directions was measured. Daylight variability was analysed in terms of illuminance, Correlated Colour Temperature (CCT) and Melanopic to Photopic (M/P) ratio. A comparison between the measured spectra and the CIE standard illuminants was performed and it was found that the usefulness of this method to evaluate non-visual effects of light in terms of M/P is adequate and provides reliable results. Therefore, a simple method to estimate the non-visual potential of daylight based on the use of D series of illuminants was presented and validated by comparing the results with the measured data. Using this method to calculate M/P always achieves RMSPEs below 6%.