           ! " #  ! "   $ " % & $    $ 1 PhysikalischTechnische Bundesanstalt, Abbestraße 212, 10587 Berlin, Germany 2 National Institute for Standards and Technology, 100 Bureau Dr., Gaithersburg, MD 208998410, U.S.A. Email: alexander.gottwald@ptb.de ’( To compare the calibration capabilities for the spectral responsivity in the vacuumultraviolet spectral region between 135 nm and 250 nm, PTB and NIST agreed on a bilateral comparison. Calibrations of semiconductor photodiodes as transfer detectors were performed using monochromatized synchrotron radiation and cryogenic electrical substitution radiometers as primary detector standards. Great importance was attached to the selection of suitable transfer detector standards due to their critical issues in that wavelength regime. The uncertainty budgets were evaluated in detail. The comparison showed a reasonable agreement between the participants. However, it got obvious that the uncertainty level for this comparison cannot easily be further reduced due to the lack of sufficiently radiationhard and longterm stable transfer standard detectors. )* PACS: 07.60.Dq, 07.85.Qe, 85.60.Dw, 06.20.fb