• The purpose of X-ray tubes and apparatus described in the previous two chapter is to provide a suitably controlled beam of X-rays which can be employed to demonstrate the internal structure of patient.

• Within the X-ray tube a rapidly moving stream of electrons is caused to strike a target (the anode), where part of the kinetic energy is converted into X-rays. As each electron collides with the target its energy is librated as an X-ray photon.

• The X-ray beam is an aggregation of the photons resulting from millions of such collisions. the X-ray photons produced ate the anode of the X-ray tube are emitted in all directions but are confined by the lead shielding surrounding the tube and only emerge, in the form of a beam, through an aperture in the shielding which is known as the window.

The composition of the beam

• As mentioned in chapter 1 all X-ray beams are mixed composition, i.e.,  they consists of X-rays of a range of wave-length. Raising the kilovoltage supplied to the tube increases the proportion of short wave-length X-ray and improves the penetrating power of the beam.

• Because the beam is mixed it will also contain X-rays of such long wave-length that they are incapable of penetrating any appreciable amount of tissue . Such rays do not contribute to the production of the radiographic image but do increase the amount of potentially harmful radiation received by the patient. for this reason modern apparatus incorporates additional filtration provided by an aluminum filter in the window which prevent the passage of these useless rays.