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

In spectral diffuse solar irradiance measurements, when diffusing devices used are neither perfectly Lambertian nor have an ideal cosine response, significant errors may spoil the collected data. An optical method permits the determination of a spectral correction factor (SCF) which fully compensates for the diffusers' imperfection when the sky radiance 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 disk. 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 irradiance spectra measured on clear sky days with isotropic SCF is analysed by comparison with Brine-Iqbal model spectra and total diffuse measurements. A remarkable coherence is found for small solar zenith angles when using a tracking disk and a diffuser with an isotropic SCF smaller than 1.2.