Abstract
The illumination of the environment undergoes both intensity and spectral changes during the 24 h cycle of a day. Daylight spectral power distributions are well described by low-dimensional models such as the CIE (Commission Internationale de l’Éclairage) daylight model, but the performance of this model in non-daylight regimes is not characterised. We measured downwelling spectral irradiance across multiple days in two locations in North America: One rural location (Cherry Springs State Park, PA) with minimal anthropogenic light sources, and one city location (Philadelphia, PA). We characterise the spectral, intensity and colour changes and extend the existing CIE model for daylight to capture twilight components and the spectrum of the night sky.
Highlights
The illumination of the environment undergoes both intensity and spectral changes during the 24 h cycle of a day
The spectra change shape systematically: there is a relative enrichment of short-wavelength light that grows stronger with decreasing solar elevation (Fig. 1a,b) from daylight through to nautical twilight
These changes in spectral composition occur at both the Rural and City locations; the average spectral power distributions are quite similar between locations for both daylight and civil twilight
Summary
The illumination of the environment undergoes both intensity and spectral changes during the 24 h cycle of a day. Intensity and colour changes and extend the existing CIE model for daylight to capture twilight components and the spectrum of the night sky. Judd, et al.[2] subjected a set of 622 measured daylight spectral power distributions to a dimensionality reduction technique and derived three basis functions (termed S0, S1 and S2 in the original work and below) which account for much of the variance in the dataset These were later accepted as the Commission Internationale de l’Éclairage (CIE) daylight model ( called the ‘CIE daylight model’), and are widely used for modelling and synthesizing the spectral power distributions of daylight[3]. There is evidence that twilight exhibits statistical regularity[1,10,11], but no efforts have been made to extend the CIE daylight model to capture twilight illumination
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