The Precision Solar Spectroradiometer (PSR) for direct solar irradiance measurements

A precision solar spectroradiometer (PSR) has been designed for high precision and accurate measurements of direct normal spectral solar irradiance in the wavelength range 300-1000 nm. Each PSR is radiometrically characterised and calibrated in the optical laboratory of PMOD/WRC. The calibration is traceable to SI through the primary spectral irradiance standard of the Physikalisch-Technische Bundesansalt (PTB), in Braunschweig, Germany. The expanded relative uncertainty for spectral solar irradiance measurements in the 350-1000 nm range is between 1.7% and 2.0%, with larger uncertainties below 400 nm which are mainly caused by the low signal levels and the uncertainties due to the straylight correction. A solar extraterrestrial spectrum was retrieved from solar measurements using the Langley-technique at a high altitude observatory and was found to be in excellent agreement with a literature solar spectrum (mean ratio 0.998 +/- 0.019). On two clear sky days solar measurements from 3 PSR and one independently calibrated spectroradiometer deployed during the International Pyrheliometer Comparison agreed to within their combined expanded uncertainties of 3%. Aerosol optical depth retrieved from the same solar measurements of the three PSR showed median differences with respect to the World reference for AOD between - 0.010 and + 0.014 at the center wavelengths 368 nm, 412 run, 500 nm, and 862 nm. The instrument has a second auxiliary port for connecting different entrance optics for near coincident measurements such as global solar irradiance.