The X-ray spectrum produced by an X-ray tube contains photons across a wide range of energies. Many of these photons have relatively low energies and are unlikely to penetrate the patient to reach the detector. Instead, they are absorbed in superficial tissues, contributing mainly to radiation dose.

When the X-ray beam passes through a filter material such as aluminium, interactions occur between the photons and the atoms of the filter. The dominant mechanism responsible for removing low-energy photons is the photoelectric effect.

The probability of photoelectric absorption depends strongly on photon energy and can be approximated as:

P_PE ∝ Z³/E³​

where $Z$ is the atomic number of the absorber and $E$ is the photon energy.

Because of the ∝1/E³ relationship, low-energy photons are far more likely to undergo photoelectric absorption. As the beam passes through the filter, these low-energy photons are preferentially absorbed, while higher-energy photons are more likely to pass through.

The overall intensity of the beam decreases as photons are absorbed according to the exponential attenuation law:

I = I₀​e^−μx

where I₀​ is the initial intensity, I is the transmitted intensity, μ is the linear attenuation coefficient of the filter material, and $x$ is the thickness of the filter.

Because lower-energy photons are removed preferentially, the average energy of the remaining beam increases. This shift toward higher photon energies is known as beam hardening.

Filtration occurs in two forms. Inherent filtration arises from materials within the X-ray tube assembly such as the glass envelope, insulating oil, and tube window. Added filtration consists of thin sheets of material, commonly aluminium, placed in the beam to further remove low-energy photons.