Total reflection on low-pass filter

Since Total Reflection angle depends on the energy of the photon, one can use this effect to modify the excitation spectrum beam. By eliminating the high energy photons from the excitation spectrum, it is possible to minimize their contribution to the background in the measured spectra, thus making possible to achieve better detection limits.

In this way it is possible to use the total reflection to "filter out" of a white polychromatic X-rays beam photons with energies above certain selected value. In other words, it is possible to use a flat substrate as a low-pass filter on X-rays, without substantially affecting the intensity of the beam.