Problems met in the elaboration of sky radiation measurements

Measurements of sky radiation concern direct solar flux F and diffuse sky flux E; they are used for deriving optical and physical features of the atmospheric aerosols. Sky radiation is measured by the sky-radiometer (a photometer with interferential filters annexed for the selection of the wavelengths at which to operate) and a pointing system. The wavelengths are usually selected within the main atmospheric windows, in order to reduce the radiative transfer problem to a pure scattering problem; the sky radiometer used in the authors' research program operates at 0.369, 0.500, 0.675, 0.776, 0.862 and 1.048 /spl mu/m. Measurements are usually carried out pointing along a conical surface with the same zenith angle of the Sun, or along a plane with the same azimuthal angle. The two quantities considered in their scheme of data elaboration are the direct solar flux F and the ratio R between the diffuse and the direct flux. The authors propose a procedure of calibration based on experimental data concerning both direct and diffuse radiation. A retrieval of only R data is performed for all the available measurements.