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.

The impact of woven shade fabrics on correlated colour temperature and illuminance with daylighting

“…In future studies, vertical CCT records on the façade are planned as the spectral characteristics and the resulting CCTs depend on the orientation of the part of the sky being analysed, 65,66 and therefore, vertical records would better represent the spectral power distribution of daylight striking the window.…”

“…6 It is also necessary to extend the study by considering different facade orientations, as daylight availability and chromaticity depend not only on the latitude but also on the facade orientation. 6,54,73 As stated by Diakite et al , 66,74 orientation-dependency is an important part of understanding daylight performance. Indoor surface spectral reflectance (walls, ceiling and floor) is also an important factor and has implications for CCT and E h .…”

Spectral sky models for advanced daylight simulations

“…For detailed review of the existing spectral sky models refer to Diakite and Knoop (2019a). A review of existing data-driven sky models, investigating which of the models performs best when validated against a large dataset of spatially, spectrally and temporally resolved measurements in the context of the NIF effectiveness of daylight is presented in Diakite-Kortlever and Knoop (2021).…”

A framework to generate local spectral skies for spectral daylight simulations

“…The multi-spectral lighting plugin Lark [5] generates a sky model using user-defined Correlated Colour Temperature (CCT) or global spectral irradiance of skies with luminance distribution from Perez all weather or CIE sky model. The plugins gendaylit [6] and OWL [7] for Radiance utilize a luminance-based CCT model [8,9], while the software ALFA [10] uses libRadtran, a physics-based radiative transfer library [11], to pre-compute spectral radiances of the sky. ALFA's sky model is currently the only one that can capture the spatial-spectral variability of direct sun and diffuse sky radiation across the sky dome [3,4].…”