spectrum hardening. Because of spectrum hardening, the mean free path of the X-ray spectrum changes with distance.

Sometimes one measures the thickness of a material in the number of mean free paths. Material with the thickness of one mean free path will attenuate to 37% (1/''e'') of photons. This concept is closely related to half-value layer (HVL): a material with a thickness of one HVL will attenuate 50% of