A collimator is a device that narrows a beam of particles or waves. To "narrow" can mean either to cause the directions of motion to become more aligned in a specific direction (i.e. collimated or parallel) or to cause the spatial cross section of the beam to become smaller.
Optical collimators are also used in gunsights and other pointing devices to give the viewer an image of a reticle at infinity.
The figure to the right illustrates how a Söller collimator is used in neutron and X-ray machines. The upper panel shows a situation where a collimator is not used, while the lower panel introduces a collimator. In both panels the source of radiation is to the right, and the image is recorded on the gray plate at the left of the panels.
Without a collimator rays from all directions will be recorded; for example, a ray that has passed through the top of the specimen (to the right of the diagram) but happens to be travelling in a downwards direction may be recorded at the bottom of the plate. The resultant image will be so blurred and indistinct as to be useless.
In the lower panel of the figure, a collimator has been added (blue bars). This may be a sheet of lead or other material opaque to the incoming radiation with many tiny holes bored through it or in the case of neutrons it can be a sandwich arrangement (which can be up to several feet long - see ENGIN-X) with many layers alternating between neutron absorbing material (e.g. Gadolinium) with neutron transmitting material. This can be something simple e.g. air. or if mechanical strength is needed then aluminium may be used. If this forms part of a rotating assembly, the sandwich may be curved. This allows energy selection in addition to collimation - the curvature of the collimator and its rotation will present a straight path only to one energy of neutrons. Only rays that are travelling nearly parallel to the holes will pass through them—any others will be absorbed by hitting the plate surface or the side of a hole. This ensures that rays are recorded in their proper place on the plate, producing a clear image.
For industrial radiography using gamma radiation sources such as Iridium-192 or Cobalt-60, a collimator allows the radiographer to control the exposure of radiation to expose a film and create a radiographic "negative", a.k.a., a radiograph, to inspect materials for defects. A collimator in this instance is most commonly made out of tungsten, and is rated according to how many half value layers it contains, i.e., how many times it reduces undesirable radiation by half. For instance, the thinnest walls on the sides of a 4 HVL tungsten collimator 0.52" thick will reduce the intensity of radiation passing through them by 88.5%. The shape of these collimators allows the radiographer to direct the radiation to the film and away from other workers.
New systems involving multileaf collimators (MLCs) are used to further shape a beam to localise treatment fields in radiotherapy. MLCs consist of approximately 50–120 leaves of heavy, metal collimator plates which slide into place to form the desired field shape.