THE ELECTRON STORAGE-RING BESSY AS A PRIMARY STANDARD SOURCE - A RADIOMETRIC COMPARISON WITH A CRYOGENIC ELECTRICAL SUBSTITUTION RADIOMETERIN THE VISIBLE

The electron storage ring BESSY in Berlin is used as a primary standar d source with a calculable spectral radiant power from the near-infrar ed to the x-ray region. In order to reduce the uncertainties of the re alized radiant power scale, improvements have been made in the measure ment of the electron beam current and in the instrumentation of the ra diometry laboratory of the Physikalisch-Technische Bundesanstalt (PTB) . Diffraction at the stray light aperture stops in the beam line, stra y light and fluorescence of the exit window were investigated in the v isible. Thus, the spectral radiant power can now be realized with a re lative 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 ex it window. To investigate the agreement with other well-established ra diometric primary standards, BESSY was compared in the visible with th e 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 visibl e.