Direct Solar Irradiance measurements with a Cryogenic Solar Absolute Radiometer

The World Radiometric Reference (WRR) is an artefact based reference for Direct Solar Irradiance (DSI) measurements. The WRR is realized by a group of electrical substitution radiometers, the World Standard Group (WSG). In recent years, a relative difference of about -0.3% between the International System of Units (SI) scale and the WRR scale was observed with the SI scale being lower. The Cryogenic Solar Absolute Radiometer (CSAR) aims for i) providing direct traceability of DSI measurements to the SI system, ii) reducing the overall uncertainty of DSI measurements towards 0.01% and for iii) replacing the WSG in future. The latest SI-WRR intercomparisons performed with CSAR revealed a relative difference of -0.29% ± 0.064% (k = 1) between the SI and the WRR scale, a result that agrees well with previous findings. The uncertainty of corrections for the window transmittance results currently in the largest contribution to the total uncertainty for the CSAR measurements. The formal transition from the WRR t...