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.

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 lead. This ensures that rays are recorded in their proper place on the plate, producing a clear image.

Collimators may be used with laser diodes.