Determination of the spectral radiance of transfer standards in the spectral range 110 nm to 400 nm using BESSY as a primary source standard

The Physikalisch-Technische Bundesanstalt (PTB) operates at the electron storage ring BESSY a calibration facility for the normal incidence spectral region which can be used for measurement of the spectral radiance of transfer standards such as deuterium lamps and tungsten ribbon lamps. Because BESSY is not a standard of spectral radiance, radiation fluxes are compared and the area of the transfer source under investigation is determined by imaging the source on to a precisely measured aperture which acts as an entrance slit of the monochromator. Polarization effects are determined by rotating the complete calibration facility around the axis of the incoming radiation. Relative uncertainties of 2 % (1 σ) for the spectral radiance are achieved, with the main contributions arising from inhomogeneity of the focusing mirror, geometry and alignment.