A small linear attenuation coefficient indicates that the material in question is relatively transparent, while a larger values indicate greater degrees of opacity. The linear attenuation coefficient is dependent upon the type of material and the energy of the radiation. Generally, the higher the energy of the incident photons and the less dense the material in question, the lower the corresponding linear attenuation coefficient will be.
The measured intensity of transmitted through a layer of material with thickness and density is related to the incident intensity according to the inverse exponential power law that is usually referred to as Beer-Lambert law:
where denotes the path length. The Half Value Layer (HVL) signifies the thickness of a material required to reduce the intensity of the emergent radiation to half its incident magnitude. It is from these equations that engineers decide how much protection is needed for "safety" from potentially harmful radiation. The attenuation factor of a material is obtained by the ratio of the emergent and incident radiation intensities .
The linear attenuation coefficient and mass attenuation coefficient are related such that the mass attenuation coefficient is simply α/ρ, where ρ is the density in g/cm3.
The linear attenuation coefficient is also inversely related to mean free path.
If the mass attenuation coefficient is used to calculate the attenuation factor , then the Beer-Lambert's exponential attenuation law must be modified such that the mass thickness of the material is used:
Tables of photon mass attenuation coefficients are essential in radiological physics, radiography (for medical and security purposes), dosimetry, diffraction, interferometry, crystallography and other branches of physics. The photons can be in form of x-ray, gamma-ray, and bremsstrahlung radiation.
The actual values have been thoroughly examined and are available to the general public through three databases run by National Institute of Standards and Technology (NIST):