Solar Ultraviolet Radiation Exposure Proxy‐estimated by Sky View Fish‐eye Photography—Potentials and Limitations from an Exploratory Correlation Study

Potentials and limitations of sky view fish‐eye photography and calculation of the percentage of free sky (sky view factor, SVF) as a proxy to estimate solar ultraviolet radiation (UVR) exposure in shade settings are investigated using controlled situations. SVF and measured solar UVR exposure correlated at high mid‐latitude (56.65°N) near autumnal equinox in September. The correlation was enhanced by splitting the sky view images into a south‐ and a north‐half and weighting the south‐half higher to account for the direct sun. Sky view images from eight different settings with SVF‐values 98.3% – 14.9% were compared to exposure measurements by polysulphone film dosimeter badges in the horizontal zenith‐, vertical‐south‐, east‐, west‐ and north‐directions and their combinations. The sky view images were un‐split and un‐weighted or split and the semi‐skies given south/north weights (3.0/1.0) or a higher weight ratio (3.5/0.5). Of all tested combinations split sky view SVFs weighted 3.0/1.0 and compared to horizontal (zenith‐oriented) dosimeters yielded the highest correlation (R² = 0.96). The weight ratio (3.5/0.5) yielded the 2^nd highest correlation (R² = 0.90) both compared to measured horizontal exposure and compared to the horizontal exposure averaged with the vertical‐south‐oriented exposure. SVF from sky view fish‐eye photography may estimate solar UVR exposure in shade settings.