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

Abstract A precision solar spectroradiometer(PSR) has been designed for high precision and accurate measurements of direct normal spectral solar irradiancein 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 irradiancemeasurements 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 spectroradiometerdeployed 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 nm, 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.