Cutoff effect

As mentioned before the high energy photons can be removed from the primary beam before it reaches the sample by using a first reflector acting as a low-pass or cut-off filter.

Such a filter can be realized by a simple quartz glass block. It represents an ideal low-pass filter if applied as a totally reflecting mirror. At a given angle of incidence, low energy photons are totally reflected whereas high energy photons are absorbed or scattered. This angle is called the cut-off angle.