Abstract
Abstract 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, al...
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