Direct Solar Spectral Irradiance Measurements and Updated Simple Transmittance Models

A set of 509 direct solar irradiance spectra, carefully measured over one year, is checked against spectral irradiances computed from five updated transmittance models. The wavelengths under investigation range from 290 to 900 nm, with a 5- or 10-nm step. The parameters explored include the solar altitude angle, with a range from 13 degrees to 68 degrees, and the standard Linke turbidity factor, with a range from 2.0 to 6.0. Measurement devices and experimental processes are described in detail in the paper. The comparison between measured and computed values is carried our by means of the relative mean bias error and the mean absolute relative error. These coefficients are applied to ultraviolet-B and ultraviolet-A total irradiances, and to visible and near-IR spectral irradiances. No clear and systematic sensitivity of the models or measurements to the solar altitude and the turbidity parameters is observed. Of the five models tested, three of them give mean coefficient values between 7% and 16% in UV bands and between 5% and 9% in visible or near-IR bands. Adjusting factors for the elimination of the systematic differences that occur between the measurements and the computation results of the models are proposed. Comparisons with a radiative transfer code tend to prove the competitiveness of so-called updated transmittance models, which are very fast, and they are particularly suitable when large amounts of data have to be processed.