Editing Collimated beam

{{Short description|Light all pointing in the same direction}}
[[Image:Collimator2.svg|thumb|In the lower picture, the light has been collimated.]]

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 divergence|divergence]], would not disperse with distance. However, [[diffraction]] prevents the creation of any such beam.<ref>{{cite book |url=http://file.lasersaur.com/docs-thirdparty/Lasers.pdf |title=Melles Griot Catalog |chapter=Introduction to Laser Technology |author=<!--Staff writer(s); no by-line--> |page=36.6 |date=n.d. |publisher=Melles Griot |access-date=25 August 2018}}</ref>

Light can be approximately collimated by a number of processes, for instance by means of a [[collimator]]. Perfectly collimated light is sometimes said to be ''focused at infinity''. Thus, as the distance from a point source increases, the spherical [[wavefront]]s become flatter and closer to [[plane wave]]s, which are perfectly collimated.

Other forms of electromagnetic radiation can also be collimated. In [[radiology]], [[X-rays]] are collimated to reduce the volume of the patient's tissue that is irradiated, and to remove stray photons that reduce the quality of the x-ray image ("film fog"). In [[scintigraphy]], a gamma ray collimator is used in front of a detector to allow only photons perpendicular to the surface to be detected.<ref>{{cite web |url=http://www.nuclearfields.com/collimators-nuclear-medicine.htm |title=Collimators for Nuclear Medicine |publisher=Nuclear Fields}}</ref>

The term ''collimated'' may also be applied to [[particle beam]]s – a '''collimated particle beam''' – where typically shielding blocks of high density materials (such as [[lead]], [[woods metal|bismuth alloys]], etc.) may be used to absorb or block peripheral particles from a desired forward direction, especially a sequence of such absorbing [[collimator]]s. This method of particle collimation is routinely deployed and is ubiquitous in every [[particle accelerator]] complex in the world. An additional method enabling this same forward collimation effect, less well studied, may deploy strategic nuclear polarization ([[magnetization|magnetic polarization]] of nuclei) if the requisite reactions are designed into any given experimental applications.

==Etymology==
The word "collimate" comes from the [[Latin]] verb ''collimare'', which originated in a misreading of ''collineare'', "to direct in a straight line".<ref>{{cite encyclopedia|encyclopedia=A Latin Dictionary|first1=Charlton T. |last1=Lewis|first2=Charles|last2=Short|location=Oxford; Medford|publisher=Clarendon Press; Perseus Digital Library|year=2010|orig-year=1879|title=collimo|url=https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0059:entry=collimo}}</ref>

==Sources==
===Lasers===
[[Laser]] light from gas or crystal lasers is highly collimated because it is formed in an [[optical cavity]] between two parallel [[mirror]]s which constrain the light to a path perpendicular to the surfaces of the mirrors.<ref>{{cite web |url=http://www.worldoflasers.com/laserproperties.htm |title=Properties of Lasers |author=<!--Staff writer(s); no by-line.--> |date=2015 |website=World of Lasers |access-date=5 August 2015 }}</ref> In practice, gas lasers can use concave mirrors, flat mirrors, or a combination of both.<ref>{{cite book|title=Engineering Physics|last=Joshi|date=2010|publisher=[[Tata McGraw-Hill Education]]|isbn=9780070704770|page=517}}<!--|access-date=6 August 2015 --></ref><ref>{{cite book|title=Engineering Physics 1: For WBUT|author=<!--Staff writer(s); no by-line.-->|date=n.d.|publisher=Pearson Education India|isbn=9788131755938|location=India|pages=3–9}}<!--|access-date=6 August 2015 --></ref><ref>{{cite book|title=Elementary Modern Physics|last=Tipler|first=Paul|date=1992|publisher=MacMillan|isbn=9780879015695|page=149}}<!--|access-date=6 August 2015 --></ref> The [[Divergence (optics)|divergence]] of high-quality laser beams is commonly less than 1 [[milliradian]] (3.4 [[arcmin]]), and can be much less for large-diameter beams. [[Laser diode]]s emit less-collimated light due to their short cavity, and therefore higher collimation requires a collimating lens.

===Synchrotron light===
[[Synchrotron light]] is very well collimated.<ref>{{cite book|title=Synchrotron Radiation Research|last1=Winick|first1=Herman|last2=Doniach|first2=S|date=2012|publisher=[[Springer Science & Business Media]]|isbn=9781461579984|page=567}}<!--|access-date=6 August 2015 --></ref> It is produced by bending relativistic electrons (i.e. those moving at [[Special relativity|relativistic]] speeds) around a circular track. When the electrons are at relativistic speeds, the resulting radiation is highly collimated, a result which does not occur at lower speeds.<ref>{{cite book|title=Synchrotron Radiation: Basics, Methods, and Applications|last1=Mobilio|first1=Settimio|last2=Boscherini|first2=Federico|last3=Meneghini|first3=Carlo|date=2014|publisher=Springer|isbn=9783642553158|page=31}}<!--|access-date=6 August 2015 --></ref>

===Distant sources===
The light from [[star]]s (other than the [[Sun]]) arrives at Earth precisely collimated, because stars are so far away they present no detectable angular size. However, due to refraction and turbulence in the Earth's atmosphere, starlight arrives slightly uncollimated at the ground with an [[astronomical seeing|apparent angular diameter of about 0.4 arcseconds]]. Direct rays of light from the Sun arrive at the Earth uncollimated by one-half degree, this being the [[angular diameter]] of the Sun as seen from Earth.  During a [[solar eclipse]], the Sun's light becomes increasingly collimated as the visible surface shrinks to a thin crescent and ultimately a [[Baily's beads|small point]], producing the phenomena of distinct shadows and [[shadow bands]].

===Lenses and mirrors===
[[Image:CollimatingLensSVG.svg|thumb|150px|right|An example of an optical collimating lens.]]A perfect [[parabolic mirror]] will bring parallel rays to a focus at a single point. Conversely, a point source at the focus of a parabolic mirror will produce a beam of collimated light creating a [[collimator]]. Since the source needs to be small, such an optical system cannot produce much optical power. [[Spherical mirror]]s are easier to make than parabolic mirrors and they are often used to produce approximately collimated light. Many types of [[lens (optics)|lens]]es can also produce collimated light from point-like sources.

==Collimation and decollimation==
"Collimation" refers to all the optical elements in an instrument being on their designed [[optical axis]]. It also refers to the process of adjusting an optical instrument so that all its elements are on that designed axis (in line and parallel). The [[Binoculars#Alignment|unconditional aligning]] of binoculars is a 3-axis collimation, meaning both optical axis that provide stereoscopic vision are aligned parallel with the axis of the hinge used to select various [[interpupillary distance]] settings. With regards to a telescope, the term refers to the fact that the optical axis of each optical component should be centered and parallel, so that collimated light emerges from the eyepiece. Most amateur reflector telescopes need to be re-collimated every few years to maintain optimum performance. This can be done by simple visual methods such as looking down the optical assembly with no eyepiece to make sure the components are lined up, by using a [[Cheshire eyepiece]], or with the assistance of a simple laser collimator or [[autocollimator]]. Collimation can also be tested using a [[shearing interferometer]], which is often used to test laser collimation.

"Decollimation" is any mechanism or process which causes a beam with the minimum possible [[ray (optics)|ray]] divergence to diverge or converge from parallelism. Decollimation may be deliberate for systems reasons, or may be caused by many factors, such as [[refractive index]] inhomogeneities, occlusions, [[scattering]], [[Deflection (physics)|deflection]], [[diffraction]], [[Reflection (physics)|reflection]], and [[refraction]]. Decollimation must be accounted for to fully treat many systems such as [[radio]], [[radar]], [[sonar]], and [[optical communication]]s.

== See also ==

* [[Autocollimation]]
* [[Cross-cockpit collimated display]]
* [[Schlieren photography]]

==References==
{{reflist}}

==Bibliography==
* Pfister, J. & Kneedler, J.A. (s.d.). A guide to lasers in the OR.

[[Category:Optics]]
[[Category:Observational astronomy]]