2000) Horizontal and sun normal spectral biologically effective ultraviolet irradiances

(*To whom correspondence should be addressed) 1 This paper has determined the dependence of the spectral biologically effective solar UV irradiance on the orientation of the receiver with respect to the sun for relatively cloud free days at a sub-tropical Southern Hemisphere latitude for the solar zenith angle range of 35o to 64o. For the UV and biologically effective irradiances, the sun normal to horizontal ratio for the total UV ranged from 1.18±0.05 to 1.27±0.06. The sun normal to horizontal ratio for biologically effective irradiance was dependent on the relative effectiveness of the relevant action spectrum in the UVA waveband. In contrast to the total UV, the diffuse UV and diffuse biologically effective irradiances were reduced in a sun normal compared to a horizontal orientation by a factor ranging from 0.70±0.05 to 0.76±0.03