The spectrum of energies produced by an X-ray tube has a very broad distribution. Each imaging requirement will have an ideal monochromatic energy or narrow band of energies which will yield good image contrast and minimum patient exposure. The energy distribution can be altered by such techniques as operating the tube at constant potential, by the use of k-edge filters which have a greater energy attenuation above a particular value, or by the use of copper foil filters which tend to have a greater absorption of lower energies to reduce patient incident exposure. Beam filtration should vary with the application, in particular, as a function of the voltage applied to the X-ray tube. It is obvious that filtration appropriate to low-voltage operation will not be optimum for high-voltage operation of the same tube. Another constraint is imposed by limitations of tube anode dissipation. As beam filtration is increased (as in the case of chest radiography), anode dissipation per exposure must also increase and becomes a practical limit to the increase of beam filtration.

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