PHOTON METROLOGY WITH ELECTROMAGNETIC-RADIATION FROM ACCELERATED RELATIVISTIC ELECTRONS - PRESENT STATE-OF-THE-ART ACHIEVED WITH THE ELECTRON STORAGE-RING BESSY-I AND FUTURE-PROSPECTS WITH BESSY-II

In the past, fundamental radiometry with primary radiator standards wa s based on the realization of Planck's radiation law by high-temperatu re black-body radiators, and was therefore restricted to the spectral regions of the IR, the visible, and the near air UV. Today, radiometry is developed in a broadly extended spectral region using synchrotron radiation which is calculable, according to Schwinger, for well-optimi zed electron storage rings. This overview is presented on the double o ccasion of the 10th anniversary of the PTB Radiometry Laboratory at th e Electron Storage Ring BESSY I in Berlin, and of the decision to real ize BESSY II. Spectral photon fluxes generated by the primary radiator standard BESSY I have relative uncertainties from 0.6 % in the x-ray region (photon energy 15 keV) down to 0.2 % in the visible and near IR (photon energy 1 eV). Undulator radiator standards with calculable tu nable monochromatic photon flux will result in further progress in the radiometry of vacuum UV and soft x-ray radiation for photon energies below 1 keV. Techniques are being developed to realize uniformly radio metric units from a few eV to about 100 keV by means of BESSY I and II up to the end of this decade. There are prospects that radiometry may be extended into the MeV spectral range using BESSY II and the Europe an Synchrotron Radiation Facility (ESRF).