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A collimator is a device which 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 A collimated beam of light or other electromagnetic radiation has parallel ray (optics), rays, and therefore will spread minimally as it propagates. A laser beam is an archetypical example. A perfectly collimated light beam, with no beam divergen ...
or
parallel Parallel may refer to: Mathematics * Parallel (geometry), two lines in the Euclidean plane which never intersect * Parallel (operator), mathematical operation named after the composition of electrical resistance in parallel circuits Science a ...
rays), or to cause the spatial
cross section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture and engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **A ...
of the beam to become smaller (beam limiting device).


History

The English physicist
Henry Kater Henry Kater FRS, FRAS (16 April 1777 – 26 April 1835) was a British physicist of German descent. Early life He was born at Bristol. At first he intended to study law; but he gave up the idea on his father's death in 1794. He entered th ...
was the inventor of the
floating collimator A Floating collimator was an early collimator widely used in astronomical observations. It was invented by the English physicist and astronomer Captain Henry Kater about 1825. His collimator was meant to replace a level, or plumb-line, in astrono ...
, which rendered a great service to practical astronomy. He reported about his invention in January 1825. In his report, Kater mentioned previous work in this area by
Carl Friedrich Gauss Johann Carl Friedrich Gauss (; ; ; 30 April 177723 February 1855) was a German mathematician, astronomer, geodesist, and physicist, who contributed to many fields in mathematics and science. He was director of the Göttingen Observatory and ...
and
Friedrich Bessel Friedrich Wilhelm Bessel (; 22 July 1784 – 17 March 1846) was a German astronomer, mathematician, physicist, and geodesy, geodesist. He was the first astronomer who determined reliable values for the distance from the Sun to another star by th ...
.


Optical collimators

In
optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...
, a collimator may consist of a
curved mirror A curved mirror is a mirror with a curved reflecting surface. The surface may be either ''convex'' (bulging outward) or ''concave'' (recessed inward). Most curved mirrors have surfaces that are shaped like part of a sphere, but other shapes are ...
or
lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...
with some type of light source and/or an image at its
focus Focus (: foci or focuses) may refer to: Arts * Focus or Focus Festival, former name of the Adelaide Fringe arts festival in East Australia Film *Focus (2001 film), ''Focus'' (2001 film), a 2001 film based on the Arthur Miller novel *Focus (2015 ...
. This can be used to replicate a target focused at
infinity Infinity is something which is boundless, endless, or larger than any natural number. It is denoted by \infty, called the infinity symbol. From the time of the Ancient Greek mathematics, ancient Greeks, the Infinity (philosophy), philosophic ...
with little or no
parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...
. In
lighting Lighting or illumination is the deliberate use of light to achieve practical or aesthetic effects. Lighting includes the use of both artificial light sources like lamps and light fixtures, as well as natural illumination by capturing daylight. ...
, collimators are typically designed using the principles of
nonimaging optics Nonimaging optics (also called anidolic optics)Roland Winston et al., ''Nonimaging Optics'', Academic Press, 2004 R. John Koshel (Editor), ''Illumination Engineering: Design with Nonimaging Optics'', Wiley, 2013 is a branch of optics that is conce ...
. Optical collimators can be used to calibrate other optical devices, to check if all elements are aligned on the
optical axis An optical axis is an imaginary line that passes through the geometrical center of an optical system such as a camera lens, microscope or telescopic sight. Lens elements often have rotational symmetry about the axis. The optical axis defines ...
, to set elements at proper focus, or to align two or more devices such as
binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...
or
gun barrel A gun barrel is a crucial part of gun-type weapons such as small arms, small firearms, artillery pieces, and air guns. It is the straight shooting tube, usually made of rigid high-strength metal, through which a contained rapid expansion of high ...
s and
gunsights A sight or sighting device is any gadget, device used to assist in precise visual perception, visual alignment (i.e. ''aiming'') of weapons, surveying instruments, aircraft equipment, optical illumination equipment or larger optical instruments ...
. A surveying camera may be collimated by setting its
fiduciary marker A fiducial marker or fiducial is an object placed in the field of view of an image for use as a point of reference or a measure. It may be either something placed into or on the imaging subject, or a mark or set of marks in the reticle of an opt ...
s so that they define the principal point, as in
photogrammetry Photogrammetry is the science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring and interpreting photographic images and patterns of electromagnetic radiant ima ...
. Optical collimators are also used as
gun sight A sight or sighting device is any device used to assist in precise visual alignment (i.e. ''aiming'') of weapons, surveying instruments, aircraft equipment, optical illumination equipment or larger optical instruments with the intended target. ...
s in the
collimator sight A collimator sight is a type of optical sight that allows the user looking into it to see an illuminated aiming point aligned with the device the sight is attached to, regardless of eye position (with little parallax). They are also referred to a ...
, which is a simple optical collimator with a cross hair or some other
reticle A reticle or reticule, also known as a graticule or crosshair, is a pattern of fine lines or markings built into the eyepiece of an optical device such as a telescopic sight, spotting scope, theodolite, optical microscope or the electronic v ...
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. Elementary optics and applications to fire control instruments: May, 1921 By United States. Army. Ordnance Dept, page 84
/ref> Adding a
beam splitter A beam splitter or beamsplitter is an optical instrument, optical device that splits a beam of light into a transmitted and a reflected beam. It is a crucial part of many optical experimental and measurement systems, such as Interferometry, int ...
allows the viewer to see the reticle and the
field of view The field of view (FOV) is the angle, angular extent of the observable world that is visual perception, seen at any given moment. In the case of optical instruments or sensors, it is a solid angle through which a detector is sensitive to elec ...
, making a
reflector sight A reflector sight or reflex sight is an optical sight that allows the user to look through a partially reflecting glass element and see an aiming point or some image (helping to aim the device, to which the sight is attached, on the target) sup ...
. Collimators may be used with
laser diode file:Laser diode chip.jpg, The laser diode chip removed and placed on the eye of a needle for scale A laser diode (LD, also injection laser diode or ILD or semiconductor laser or diode laser) is a semiconductor device similar to a light-emittin ...
s 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 A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...
optics, and
neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
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 difficult to focus these types of radiation into an image using lenses, as is routine with
electromagnetic radiation In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...
at optical or near-optical wavelengths. Collimators are also used in radiation detectors in
nuclear power station A nuclear power plant (NPP), also known as a nuclear power station (NPS), nuclear generating station (NGS) or atomic power station (APS) is a thermal power station in which the heat source is a nuclear reactor. As is typical of thermal power st ...
s to make them directionally sensitive.


Applications

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 ENGIN-X is the dedicated materials engineering beamline at the ISIS Neutron and Muon Source in the UK. The beamline uses neutron diffraction to determine the spacing between layers of atoms in order to measure elastic strain, and thus residua ...
) with many layers alternating between neutron absorbing material (e.g.,
gadolinium Gadolinium is a chemical element; it has Symbol (chemistry), symbol Gd and atomic number 64. It is a silvery-white metal when oxidation is removed. Gadolinium is a malleable and ductile rare-earth element. It reacts with atmospheric oxygen or moi ...
) with neutron transmitting material. This can be something simple, such as air; alternatively, if mechanical strength is needed, a material such as 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 Industrial radiography is a modality of non-destructive testing that uses ionizing radiation to inspect materials and components with the objective of locating and quantifying defects and degradation in material properties that would lead to the ...
using gamma radiation sources such as
iridium-192 Iridium-192 (symbol 192Ir) is a radioactive isotope of iridium, with a half-life of 73.827 days. It decays by emitting beta (β) particles and gamma (γ) radiation. About 96% of 192Ir decays occur via emission of β and γ radiation, leading to ...
or
cobalt-60 Cobalt-60 (Co) is a synthetic radioactive isotope of cobalt with a half-life of 5.2714 years. It is produced artificially in nuclear reactors. Deliberate industrial production depends on neutron activation of bulk samples of the monoisotop ...
, a collimator (beam limiting device) allows the radiographer to control the exposure of radiation to expose a film and create a radiograph, to inspect materials for defects. A collimator in this instance is most commonly made of
tungsten Tungsten (also called wolfram) is a chemical element; it has symbol W and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first ...
, 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 thick will reduce the intensity of radiation passing through them by 88.5%. The shape of these collimators allows emitted radiation to travel freely toward the specimen and the x-ray film, while blocking most of the radiation that is emitted in undesirable directions such as toward workers.


Limitations

Although collimators improve resolution, they also reduce
intensity Intensity may refer to: In colloquial use * Strength (disambiguation) *Amplitude * Level (disambiguation) * Magnitude (disambiguation) In physical sciences Physics *Intensity (physics), power per unit area (W/m2) *Field strength of electric, m ...
by blocking incoming radiation, which is undesirable for
remote sensing Remote sensing is the acquisition of information about an physical object, object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring inform ...
instruments that require high sensitivity. For this reason, the
gamma ray spectrometer A gamma-ray spectrometer (GRS) is an instrument for measuring the distribution (or spectrum—see figure) of the intensity of gamma radiation versus the energy of each photon. The study and analysis of gamma-ray spectra for scientific and techni ...
on the
Mars Odyssey ''2001 Mars Odyssey'' is a robotic spacecraft orbiting the planet Mars. The project was developed by NASA, and contracted out to Lockheed Martin, with an expected cost for the entire mission of US$297 million. Its mission is to use spectro ...
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.


In radiation therapy

Collimators (beam limiting devices) are used in linear accelerators used for
radiotherapy Radiation therapy or radiotherapy (RT, RTx, or XRT) is a treatment using ionizing radiation, generally provided as part of cancer therapy to either kill or control the growth of malignant cells. It is normally delivered by a linear particle ...
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 collimator A multileaf collimator (MLC) is a collimator or beam-limiting device that is made of individual "leaves" of a high atomic numbered material, usually tungsten, that can move independently in and out of the path of a radiotherapy beam in order to ...
s (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.


Computing the spatial resolution

To find the spatial resolution of a parallel hole collimator with a hole length, l, a hole diameter D and a distance to the imaged object s, the following formula can be used R_\text = D + \frac where the effective length is defined as l_\text = l - \frac Where \mu is the linear attenuation coefficient of the material from which the collimator is made.


See also

*
Autocollimation {{more citations needed, date=November 2021 Autocollimation is an optical setup where a collimated beam (of parallel light rays) leaves an optical system and is reflected back into the same system by a plane mirror. It is used for measuring small ...
*
Autocollimator An autocollimator is an optical instrument for non-contact measurement of angles. They are typically used to align components and measure deflections in optical or mechanical systems. An autocollimator works by projecting an image onto a target mi ...
*
Collimated light A collimated beam of light or other electromagnetic radiation has parallel ray (optics), rays, and therefore will spread minimally as it propagates. A laser beam is an archetypical example. A perfectly collimated light beam, with no beam divergen ...
*
Hohlraum In radiation thermodynamics, a hohlraum (; a non-specific German word for a "hollow space", "empty room", or "cavity") is a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity. First proposed by Gustav Kir ...
*
Nonimaging optics Nonimaging optics (also called anidolic optics)Roland Winston et al., ''Nonimaging Optics'', Academic Press, 2004 R. John Koshel (Editor), ''Illumination Engineering: Design with Nonimaging Optics'', Wiley, 2013 is a branch of optics that is conce ...
*
Snoot A snoot is a tube or similar object that fits over a light used in theatrical lighting or photography, to control the direction and radius of the light beam. Snoots can be cylindrical or truncated conical in shape, different lengths and diameter ...
in lighting *
Reflector sight A reflector sight or reflex sight is an optical sight that allows the user to look through a partially reflecting glass element and see an aiming point or some image (helping to aim the device, to which the sight is attached, on the target) sup ...
in fighter cockpits


References

{{Electromagnetic spectrum Accelerator physics Neutron instrumentation Optical devices Radiology Synchrotron instrumentation X-ray instrumentation