Collimator
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In X-ray and gamma ray imaging, a collimator is a device that filters a stream of photons so that only those travelling parallel to a specified direction are allowed through. The illustration below 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 (to the right of the diagram) 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.
Collimators are used in X- and gamma-ray imaging because it is not yet possible to focus such short wavelengths into an image with lenses as at optical or near-optical wavelengths.
In the bottom of the illustration to the left, a collimator has been added. This is a sheet of lead or other material opaque to the incoming radiation with many tiny holes bored through it. Only rays 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. Although collimators improve the resolution, they also reduce the intensity of the signal--most lead collimators let less than 1% of incident photons through. For this reason, attempts have been made to replace collimators with electronic analysis.
Collimators are also used with radiation detectors in nuclear power stations for monitoring sources of radioactivity.
In optics, a collimating lens is one that produces parallel rays of light. Collimators may be used with laser diodes.
Proper collimation of a laser source with long enough coherence length can be verified with a shearing interferometer.
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