Abstract
The aim of this project was the measurement of the radiant energy flux provided by the diffuse radiation of the ring shaped area of the sky around the unmasked sun disk. More than 200 measurements of the irradiance of this circumsolar cloudy or cloudfree sky up to an angular radius of 10° had been taken at Mainz (FRG), Deuselbach (FRG) and Adrano (Sicily). The measurements show that the irradiance of the circumsolar sky radiation may reach an amount of more then 30 % of the direct solar irradiance at the observation site. This means, optical systems combining the direct solar radiation and the circumsolar radiation may compensate — at least partly — the extinction losses of the direct solar radiation by the turbid and cloudy atmosphere. The most distinct compensation effect will be observed in the presence of thin cloud layers. A unique relationship between the irradiance of the circumsolar sky and the direct solar irradiance or an equivalent nonspectral turbidity factor could not be found.
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