The radiation generated in an X-ray tube is not monochromatic.  Similar to the visible light of an incandescent lamp, it is a continuous radiation spectrum with a particular bandwidth (bremsstrahlung or “braking radiation” from German “bremsen”: to brake and Strahlung”: radiation), which is overlaid by a line spectrum (characteristic X-rays) depending on the tube voltage and target material. The maximum energy, and thus the frequencies of this spectrum, are proportional to the tube voltage.  When the work piece to be measured is penetrated, the X-rays are partially absorbed by interacting with the atoms of the work piece. In the frequency range that is typical for commercial X-ray tomography measuring machines, attenuation takes place substantially through the photoelectric effect and the Compton scattering. These absorption processes cause the photons of the X-rays to give up energy, partially or completely, in collisions with electrons.  The parts of the radiation spectrum with less energy (low-frequency radiation) are absorbed more strongly than the high energy high frequency radiation. This means that the frequency spectrum is shifted in the direction of higher frequencies and thus higher energies during X-ray imaging. In the vernacular, this is referred to as “hardening” the radiation or “beam hardening”.

As seen in image above: Beam hardening: a) Spectrum of X-rays. b) The entire spectrum is absorbed nearly in proportion in the center region of the work piece; the tomography is “correct”.   c) In the thicker areas of the work piece, low frequency (soft) parts are nearly fully absorbed, while the high frequency (harder) parts are attenuated as The measured object appears thicker (attenuation is shown as linear rather than exponential for simplicity).