The synchrotron radiation of the 6-GeV DESY machine as a fundamental radiometric standard

The characteristics of synchrotron radiation emitted by the 6-GeV DESY machine were studied in the spectral range 2200-5500 A. For this purpose, the synchrotron radiation was compared photoelectrically with the radiation of standardized sources in a nearly differential manner. These sources (tungsten ribbon lamp, Krefft-Rössler uv normal) were calibrated with high accuracy against a blackbody. Good agreement was found between our experimental results and the values predicted by the theory of the synchrotron radiation derived by J. Schwinger. The relative spectral distribution was verified to within about +/-2%, and the absolute values of the theoretically predicted flux were ascertained to within about +/-15%. For the application of synchrotron radiation as a radiometric standard, in most cases only the relative spectral distribution is important. If one supposes the validity of the theory also in the far uv, and if the synchrotron radiation is calibrated in the visible by a standard source, one can give the abolute flux in the far uv. In this way it is possible to calibrate photometers or sources in the far uv in absolute units with an accuracy of about +/- 3% with the aid of the synchrotron radiation.