Abstract
The dependence of the spectral biologically effective solar UV irradiance on the orientation of the receiver with respect to the sun has been determined for relatively cloud-free days at a sub-tropical Southern Hemisphere latitude for the solar zenith angle range 35–64°. For the UV and biologically effective irradiances, the sun-normal to horizontal ratio for the total UV ranges from 1.18±0.05 to 1.27±0.06. The sun-normal to horizontal ratio for biologically effective irradiance is dependent on the relative effectiveness of the relevant action spectrum in the UV-A waveband. In contrast to the total UV, the diffuse UV and diffuse biologically effective irradiances are reduced in a sun-normal compared with a horizontal orientation by a factor ranging from 0.70±0.05 to 0.76±0.03.
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