Using a natural AM1.5G spectrum to help define an AM1.5D spectrum appropriate for CPV purposes

Spectral measurements were performed of direct beam and global solar irradiance at normal incidence. The measurements were taken at Sede Boqer, during noontime, on a selection of cloudless days during the period December 2001 - March 2002. For those days having a low level of diffuse radiation, the ratios of direct beam to global spectra were found to follow a previously suggested, semi-empirical, 2-parameter formula based on Rayleigh scattering. For one of the days, with an unusually high level of diffuse radiation, this formula did not provide an adequate fit to the data. In order to provide an adequate fit it was found necessary to effect an ad hoc modification to the algebraic form of the original formula.