Forecast accuracy of existing luminance-related spectral sky models and their practical implications for the assessment of the non-image-forming effectiveness of daylight

Abstract

This paper analyses the forecast accuracy of current state-of-the-art, data-driven, spectral sky models. The aim is threefold: (i) to determine the forecast accuracy of existing spectral sky models based on a large dataset of spatially, spectrally and temporally resolved measurements, (ii) to investigate the practical implications of spectral forecast accuracies for the assessment of spectrally selective responses (here non-image-forming effects are expressed through melanopic irradiance) and (iii) to study if the use of spectral sky models is more appropriate to predict the non-image-forming effectiveness of daylight than the currently assumed CIE standard illuminant D65. The forecast analysis for CIE Standard Overcast Skies (CIE Sky Type 3) showed that the model published by Chain and colleagues in 1999 performed best, whereas the correlated colour temperature distribution can also be represented with the CIE standard illuminant D65. The analysis showed substantial discrepancies in the forecast for clear skies with low luminance turbidity (CIE Sky Type 12) depending on the correlated colour temperature range. Our findings suggest that for CIE 12 skies, even when simulating with the best performing spectral sky model, forecast inaccuracies affect the estimated non-image-forming effectiveness. Nonetheless, the assumption that the spectral distribution of daylight from a CIE 12 sky corresponds with the CIE standard illuminant D65 underestimates the non-image-forming effectiveness to a greater extent. The results advance the understanding of spectral characteristics of daylight and suggest that considering realistic spectral distributions instead of D65 will lead to a difference in the non-image-forming effectiveness assessment.