Quantitative spectral radiation measurements in the VUV and soft x-ray region

The Physikalisch-Technische Bundesanstalt (PTB), the German national institute of metrology, operates a radiometry laboratory at the electron storage ring BESSY. Now, after more than ten years of BESSY operation, six experimental stations on four beamlines optimized for radiometric calibrations and characterization of optical components are in use. The calculable undispersed spectral photon flux of the primary radiation standard BESSY with relative uncertainties of below 0.4% is used in a broad spectral range from about 1 eV to 15 keV for the calibration of energy-dispersive detectors and of monochromator-detector systems at one beamline and for the calibration of radiation sources in the spectral range from 3 eV to 1.8 keV at three experimental stations equipped with monochromators. Monochromatized synchrotron radiation, with emphasis placed on high spectral purity, is provided at three stations covering the spectral range from 3 eV to about 1.5 keV for detector calibrations and for reflectometry. Brief descriptions of the beamlines, experimental stations and associated instruments are given. In addition, examples of recent radiometric calibrations and investigations and an outlook on radiometry at the future storage ring BESSY II are presented.