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.