The electron storage ring BESSY as a primary standard source - A radiometric comparison with a cryogenic electrical substitution radiometer in the visible

The electron storage ring BESSY in Berlin is used as a primary standard source with a calculable spectral radiant power from the near-infrared to the x-ray region. In order to reduce the uncertainties of the realized radiant power scale, improvements have been made in the measurement of the electron beam current and in the instrumentation of the radiometry laboratory of the Physikalisch-Technische Bundesanstalt (PTB). Diffraction at the stray light aperture stops in the beam line, stray light and fluorescence of the exit window were investigated in the visible. Thus, the spectral radiant power can now be realized with a relative uncertainty (1 sigma level) of 3 . 10(-3) at a photon energy of 15 keV in vacuum and of 8 . 10(-4) in the visible in air behind an exit window. To investigate the agreement with other well-established radiometric primary standards, BESSY was compared in the visible with the highly accurate laser-based cryogenic radiometer of the PTB Berlin, using two filter radiometers (centre wavelengths 676 nm and 800 nm) as transfer standards. The results of four comparison runs confirm. the uncertainty of the realized radiant power scale of BESSY in the visible.