There are a variety of factors that affect the spectrum of X-rays produced in a diagnostic imaging unit. This applet shows the effect of varying the high voltage (kVp), added filtration and ripple in the high voltage supply to the X-ray tube.

The physical mechanisms by which X-rays are produced are Bremsstrahlung (in which collisions of the cathode electrons convert some of their energy into X-ray photons) and characteristic X-rays (in which the cathode electrons kick an inner shell electron out of an atom, and an X-ray photon is release when one of the atom's outer shell electron transitions to the inner shell). Changing the accelerating voltage of the cathode electrons (i.e. changing kVp) affects both mechanisms of X-ray production.

Adding filtration to the X-ray beam reduces its intensity, but does not reduce all energy X-rays equally. The details of how the filter interacts with the X-Ray beam depends upon several properties of the filter: the effective atomic number Z, the density ρ and the thickness d. Changing Z will produce a qualitatively different change in the spectrum compared to ρ and d.

Finally, voltage ripple actually causes the accelerating voltage of the cathode rays to vary below the peak voltage (kVp), so that X-ray tube is essentially averaging over a range of high voltages up to the set kVp.