Collimator
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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., make collimated light or parallel rays), or to cause the spatial cross section of the beam to become smaller (beam limiting device).
History
An English physicist Henry Kater was the inventor of the floating collimator, which rendered a great service to practical astronomy. He reported about his invention in January 1825.[1] In his report, Kater mentioned previous work in this area by Carl Friedrich Gauss and Friedrich Bessel.
Optical collimators
In optics, a collimator may consist of a curved mirror or lens with some type of light source and/or an image at its focus. This can be used to replicate a target focused at infinity with little or no parallax.
In lighting, collimators are typically designed using the principles of nonimaging optics.[2]
Optical collimators can be used to calibrate other optical devices,[3] to check if all elements are aligned on the optical axis, to set elements at proper focus, or to align two or more devices such as binoculars or gun barrels and gunsights.[4] A surveying camera may be collimated by setting its fiduciary markers so that they define the principal point, as in photogrammetry.
Optical collimators are also used as gun sights in the collimator sight, which is a simple optical collimator with a cross hair or some other reticle at its focus. The viewer only sees an image of the reticle. They have to use it either with both eyes open and one eye looking into the collimator sight, with one eye open and moving the head to alternately see the sight and the target, or with one eye to partially see the sight and target at the same time.[5][clarification needed] Adding a beam splitter allows the viewer to see the reticle and the field of view, making a reflector sight.
Collimators may be used with laser diodes and CO2 cutting lasers. Proper collimation of a laser source with long enough coherence length can be verified with a shearing interferometer.
X-ray, gamma ray, and neutron collimators
In X-ray optics, gamma ray optics, and neutron optics, a collimator is a device that filters a stream of rays so that only those traveling parallel to a specified direction are allowed through. Collimators are used for X-ray, gamma-ray, and neutron imaging because it is not yet possible to focus these types of radiation into an image using lenses, as is routine with electromagnetic radiation at optical or near-optical wavelengths. Collimators are also used in radiation detectors in nuclear power stations to make directional sensitivity.
Limitations
Although collimators improve resolution, they also reduce intensity by blocking incoming radiation, which is undesirable for remote sensing instruments that require high sensitivity. For this reason, the gamma ray spectrometer on the Mars Odyssey is a non-collimated instrument. Most lead collimators let less than 1% of incident photons through. Attempts have been made to replace collimators with electronic analysis. [citation needed]
In radiation therapy
Collimators (beam limiting devices) are used in linear accelerators used for radiotherapy treatments. They help to shape the beam of radiation emerging from the machine and can limit the maximum field size of a beam.
The treatment head of a linear accelerator consists of both a primary and secondary collimator. The primary collimator is positioned after the electron beam has reached a vertical orientation. When using photons, it is placed after the beam has passed through the X-ray target. The secondary collimator is positioned after either a flattening filter (for photon therapy) or a scattering foil (for electron therapy). The secondary collimator consists of two jaws which can be moved to either enlarge or minimize the size of the treatment field.
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.
See also
References
- ^ The Description of a Floating Collimator. By Captain Henry Kater. Read January 13, 1825. [Phil. Trans. 1825, p. 147.]
- ^ Chaves, Julio (2015). Introduction to Nonimaging Optics, Second Edition. CRC Press. ISBN 978-1482206739.
- ^ "Collimators and Auto Collimators" by Ron Dexter
- ^ WIPO "Magnetic lightweight collimator"
- ^ Elementary optics and applications to fire control instruments: May, 1921 By United States. Army. Ordnance Dept, page 84
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