Radiometric comparison for measuring the absolute radiant power of a free-electron laser in the extreme ultraviolet

At the new SPring-8 extreme ultraviolet free-electron laser in Japan, a radiometric comparison between a gas-monitor detector from Physikalisch-Technische Bundesanstalt/Deutsches Elektronen-Synchrotron/Ioffe Physico-Technical Institute (Ioffe) and a cryogenic radiometer from National Institute of Advanced Industrial Science and Technology was carried out. Radiant power in the range between 30 μW and 350 μW was measured at the wavelengths of 51.3 nm, 56.1 nm and 61.2 nm. The results obtained with the two different detectors agree within 2.6%, which is well below their combined relative standard uncertainty, which varies from 4.1% to 5.5%. (authors)