Ultraviolet and vacuum-ultraviolet detector-based radiometry at the Metrology Light Source

The Metrology Light Source (MLS) is a low-energy electron storage ring owned by the Physikalisch-Technische Bundesanstalt. It is optimized for basic radiometry and radiometric applications in the wavelength range from the terahertz to the extreme-ultraviolet spectral regime. As one of the basic tasks, detector-based radiometry is performed with a cryogenic electrical substitution radiometer as a primary detector standard. For the spectral range between 40 and 400 nm, the detector calibration facility was moved from its former site at the BESSY II storage ring with the beginning of user operation at the MLS in early 2008. In combination with a dedicated normal-incidence monochromator beamline, the spectral responsivities of photodetectors can be determined with relative uncertainties ranging from 1.1% down to 0.2% in the ultraviolet and vacuum-ultraviolet wavelength range. The beamline has several options to minimize high-order effects and comes with reliable intensity monitoring systems. Extensive characterization of the beamline properties has led to a detailed uncertainty budget.