At x-ray energies, filters consist of material placed
in the useful beam to absorb, preferentially, radiation based
on energy level or to modify the spatial distribution of the beam.
Filtration is required to absorb the lower-energy x-ray photons
emitted by the tube before they reach the target. The use of filters
produce a cleaner image by absorbing the lower energy x-ray photons
that tend to scatter more.
The total filtration of the beam includes the inherent filtration
(composed of part of the x-ray tube and tube housing) and the
added filtration (thin sheets of a metal inserted in the x-ray
beam). Filters are typically placed at or near the x-ray port
in the direct path of the x-ray beam. Placing a thin sheet of
copper between the part and the film cassette has also proven
an effective method of filtration.
For industrial radiography, the filters added to the x-ray beam
are most often constructed of high atomic number materials such
as lead, copper, or brass. Filters for medical radiography are
usually made of aluminum (Al). The amount of both the inherent
and the added filtration are stated in mm of Al or mm of Al equivalent.
The amount of filtration of the x-ray beam is specified by and
based on the voltage potential (keV) used to produce the beam. The thickness of filter
materials is dependent on atomic numbers, kilovoltage settings,
and the desired filtration factor.
Gamma radiography produces relatively high energy levels at essentially
monochromatic radiation, therefore filtration is not a useful
technique and is seldom used.