Actinic flux determination from measurements of irradiance

The actinic flux describes the number of photons incident at a point, while the irradiance describes the radiant energy crossing a surface. An algorithm for conversion of irradiance to downwelling actinic flux has been developed and tested. The algorithm uses a simple method to distinguish between a cloudy and a cloudless sky. It is necessary to separate cloudy and cloudless situations, as the irradiance to actinic flux conversion depends on the radiance field which is rather different for cloudy and cloudless skies. Surface UV irradiance and downwelling actinic flux spectra were measured at five European locations which were representative of different climates. A total of 9015 synchronized actinic flux and irradiance spectra were available to test the proposed algorithm. The measured irradiance spectra were used to estimate downwelling actinic flux spectra. The estimated actinic flux spectra were compared with the measured actinic flux spectra for all cloud and aerosol situations encountered, a wide range of solar zenith angles, and surface conditions. The average ratio of the reproduced to measured downwelling actinic flux is 1.021 +/- 0.085 in the UV-B and 1.015 +/- 0.105 in the UV-A. In general, the performance of the algorithm is better at smaller solar zenith angles.