How a lead collimator filters a stream of rays. Top: without a collimator. Bottom: with a collimator.

A collimator is a device used to filter a stream of rays (such as X-rays) so that only those travelling parallel to each other in a certain direction are allowed through. The picture shows a lead collimator used in X-ray machines. The image will be recorded on the plate at the left of the picture. Without a collimator (top picture) rays from all directions will be recorded; for example, a ray that has passed through the top of the specimen but happens to be travelling in a downwards direction may be recorded at the bottom of the plate. This will not produce a readable image. Basic diagram of how a lead collimator works. ... In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz...

In the bottom picture a lead collimator has been added. Effectively, this is a thick sheet of lead with many tiny holes bored through it. Only rays travelling at 90° will pass through - any others will be absorbed by hitting the side of a passage. This ensures that rays are recorded in their proper place on the plate, producing a clear image.

Collimators may be used with laser diodes. A laser diode is a laser where the active medium is a semiconductor p-n junction similar to that found in a light-emitting diode. ...

Collimating lenses may also be used in optical systems to make rays of light parallel (see also Collimating lens). A collimating lens is a lens used to gather together a parallel beam of light. ...

Collimators are also used with radiation detectors in nuclear power stations for monitoring sources of radioactivity.