Measurements of solar ultraviolet irradiance on inclined surfaces

We present pyranometer and biometer results of total solar UV irradiance (295-385 nm) and erythemal irradiance measured on inclined surfaces in Rosario, Argentina (32degrees57' S, 60degrees38' W, 25 m a.s.l.), on clear-sky days near the southern hemisphere summer and winter solstices and spring equinox. The pyranometer and a biometer were placed on a rotation device in order to modify the detector inclination in the solar plane. We determined in detail a systematic difference between the detector inclination at the highest intensity and the detector inclination in the Sun direction. The maximum intensity was always registered at a detector inclination greater than the solar elevation. This can be explained by the varying relation between the diffuse and direct contributions to the irradiance at different inclinations. For total UV irradiance, the absolute difference between the detector inclination at maximum intensity and the solar elevation is less than 120 for solar elevations greater than 30degrees and for erythemal irradiance less than 20degrees. The relative difference of the intensities is less than 2% for total UV irradiance and less than 5% for erythemal irradiance. For smaller solar elevations both differences grow rapidly.