MEASURING SPECTRAL DIFFUSE SOLAR IRRADIANCE WITH NON-COSINE FLAT-PLATE DIFFUSERS

In spectral diffuse solar irradiance measurements, when diffusing devi ces used are neither perfectly Lambertian nor have an ideal cosine res ponse, significant errors may spoil the collected data. An optical met hod permits the determination of a spectral correction factor (SCF) wh ich fully compensates for the diffusers' imperfection when the sky rad iance is isotropic. A study of the errors introduced when using such a isotropic SCF in anisotropic radiance conditions is presented for two common flat-plate diffusers fitted with a shadow-ring or a tracking d isk. The wavelength band explored is 0.29-0.90 mu m and the clear sky radiance model used is Kittler's. The relevance of several diffuse irr adiance spectra measured on clear sky days with isotropic SCF is analy sed by comparison with Brine-Iqbal model spectra and total diffuse mea surements. A remarkable coherence is found for small solar zenith angl es when using a tracking disk and a diffuser with an isotropic SCF sma ller than 1.2.