from the manual
"The BeamScope scans a slit, slit pair or pinhole in a plane orthogonal to the beam
axis. The energy transmitted by the aperture(s) falls upon a silicon detector and is
measured as a function of lateral slit position. (Germanium and Indium Arsenide
ectors are employed for the longer wavelengths.) In Slit Mode, the normal mode
of operation, the slit is much smaller than the beam diameter. In Knife
the slit must be wider than the beam diameter."