|A simple and commonly accepted way
of characterizing the hardness or penetrating power of an X-ray
beam is determining and stating the thickness of aluminum
filtration required to cut the intensity of the beam in half.
This filtration is called the half-value layer (HVL). The
higher the half-value layer is, the harder is the beam. The
penetration power increases with increasing half value layer,
but at the same time the achievable contrast decreases. The
latter, however, is not true, if edge filtration is used, as
then increased filtration by the edge filter will increase the
half-value layer of the primary beam hitting the object, but at
the same time narrow the spectrum and thus improve the contrast
in the image.
The half-value layer can also be used for an indirect
determination of the total filtration an X-ray beam has been
subjected to (cf. aluminum equivalent).
The actual value of the filter thickness will depend somewhat
on the type of aluminum used. Pure aluminum will yield slightly
higher values than aluminum alloys, e.g. 1100 aluminum, which
usually contain some copper. For the spectra used in general
radiography, this difference is practically irrelevant, but in
mammography with acceleration voltages between approximately 25
and 30 kV, the results differ appreciably and the aluminum type
used is of importance.