Rotating beam splitter for the extreme vacuum ultraviolet

We have built a rotating-mirror beam splitter at grazing incidence which was used to measure directly (with minor corrections) the transmissivity of thin films in the 20-280 eV photon energy region. The intensity fluctuations usually encountered when using electron accelerators as light sources were efficiently eliminated. The same instrument could be used to measure differential transmissivities DeltaT/T with a sensitivity of a few tenths of a percent. In another arrangement a deflecting mirror which is incorporated as the photocathode into an open electron multiplier served as a very efficient beam monitor against intensity fluctuations.