IMPROVEMENT OF DIFFUSE SOLAR IRRADIANCE MEASUREMENTS WITH NONCOSINE INSTRUMENTS

As a result of sky radiance anisotropy, significant errors spoil diffu se solar irradiance data obtained by means of noncosine-true instrumen ts, which happens in spite of knowledge of the relative cosine respons e. To reduce these errors we recommend a method that involves an addit ional measurement of diffuse irradiance from a horizon band. The effic iency of the so-called double-measurement method is found to be optimu m for a 25 degrees horizon band elevation. We confirm the feasibility of the method with relevance tests that use diffuse solar spectral mea surements carried out on cloudless skies by means of an UV spectroradi ometer. (C) 1996 Optical Society of America