Filtration, beam hardening & HVL
An x-ray energy spectrum plotted as relative photon number against photon energy in keV, shown before and after added aluminium filtration. Filtration preferentially removes the low-energy (soft) photons on the left, so the shaded removed band is widest at low energy. The filtered curve is lower and its peak is shifted to higher energy: the mean energy and half-value layer rise (the beam hardens) and the total number of photons falls, while the maximum photon energy at the kVp end-point is unchanged. Notes state that total filtration is inherent plus added, that filtration lowers skin dose, and that HVL measures beam quality.
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