It has been demonstrated that beam filtration significantly reduces patient skin dose, and filtration above 3.0 mm aluminum can significantly reduce the intensity of the primary beam requiring an increase in the exposure technique to compensate for lost photons. Therefore, filtration greater than 3.0 mm aluminum equivalency is not recommended by the NCRP.

Filtration removes mainly those photons of less than 40 keV effectively hardening the beam. The quality of the x-ray beam is therefore improved because it increases the half value layer (HVL) of the beam. NCRP regulations specifying filtration are in HVL units because this is the biomedical engineering standard for measuring and certifying beam filtration as required by the FDA.

The half value layer (HVL) is defined as that thickness of a given material that will reduce the intensity of a radiation beam to one half its original value(s). For energies used in diagnostic imaging up to three half-value-layers may be required by the FDA. When testing the quality of beam filtration, measuring the HVL is the testing standard.