Spectroradiometric Sun Photometry

This paper presents a method for calculating atmospheric transmittance from direct-beam solar spectral irradiance measurements under cloudless skies by treating spectral irradiance as a multichannel sun photometer. Computing the ratio of the measured spectral irradiance to the extraterrestrial spectral irradiance at the top of the atmosphere produces the atmospheric transmittance as a function of wavelength. Individual band absorber amounts and scattering parameters, based on the LOWTRAN 7 atmospheric transmittance model, are then extracted from the transmittance using iterative fitting over wavelength regions where only a few species are active. Using these parameters to extrapolate the entire terrestrial solar spectrum, the wavelength-integrated spectral irradiance is shown to be within 2% of the total irradiance measured with an absolute cavity radiometer. Instrumentation and procedures that have been used with the method at the National Renewable Energy Laboratory since 1987 are described, along with a specific application of the method.