The beam which emerges from the aperture of the X-ray tube is a diverging one capable of extending to a considerable width. Most X-ray machines incorporate some means of collimating or restricting this divergence to the minimum necessary to cover the area under examination.

The purpose of this is three-fold:

To prevent unnecessary irradiation of the patient or, in veterinary radiography ,of any persons involved in restraining the patient.

To reduce scattered radiation.

To minimize geometrical distortion.

In many of the simpler and older machines the restriction of the beam is accomplished by sliding an appropriate lead plate or cone over the aperture of the tube immediately prior to making the radiographic exposure. Cones posses the advantage that they make it possible to see the approximate spread of the data.

A more satisfactory method of restricting the beam is by means of adjustable lead shutters which are permanently attached to the tube aperture. where, as is now usual, such as devices also incorporate a source of light to visualize the extent of the primary beam, it is possible to adjust and position the beam very accurately. This addition is known as a light beam diagram and is most effective when used in a semi-darkened room.