Extending the spectral range of silicon-based direct-beam solar spectral radiometric measurements

A method for extending the range of solar spectral measurements that does not rely upon sunphotometer measurements of the atmospheric transmittance is reported. The procedure requires the site location, elevation, air temperature and pressure, time of day, and a spectroradiometer calibrated against a standard spectral irradiance lamp. This method calculates the atmospheric transmittance using the spectral irradiance measured by a spectroradiometer at seven selected wavelengths from the 400-1100 nm range, along with the solar zenith angle and the ambient air temperature and pressure, thereby allowing the entire 300-400 nm wavelength range to be determined. Results are also presented which agree very well with the total direct solar irradiance as measured with an absolute cavity radiometer.