Acollimated beam oflight or otherelectromagnetic radiation has parallelrays, and therefore will spread minimally as it propagates. Alaser beam is an archetypical example. A perfectly collimatedlight beam, with nodivergence, would not disperse with distance. However,diffraction prevents the creation of any such beam.[1]
Light can be approximately collimated by a number of processes, for instance by means of acollimator. Perfectly collimated light is sometimes said to befocused at infinity. Thus, as the distance from a point source increases, the sphericalwavefronts become flatter and closer toplane waves, which are perfectly collimated.
Other forms of electromagnetic radiation can also be collimated. Inradiology,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"). Inscintigraphy, a gamma ray collimator is used in front of a detector to allow only photons perpendicular to the surface to be detected.[2]
The termcollimated may also be applied toparticle beams – acollimated particle beam – where typically shielding blocks of high density materials (such aslead,bismuth alloys, etc.) may be used to absorb or block peripheral particles from a desired forward direction, especially a sequence of such absorbingcollimators. This method of particle collimation is routinely deployed and is ubiquitous in everyparticle accelerator complex in the world. An additional method enabling this same forward collimation effect, less well studied, may deploy strategic nuclear polarization (magnetic polarization of nuclei) if the requisite reactions are designed into any given experimental applications.
The word "collimate" comes from theLatin verbcollimare, which originated in a misreading ofcollineare, "to direct in a straight line".[3]
Laser light from gas or crystal lasers is highly collimated because it is formed in anoptical cavity between two parallelmirrors which constrain the light to a path perpendicular to the surfaces of the mirrors.[4] In practice, gas lasers can use concave mirrors, flat mirrors, or a combination of both.[5][6][7] Thedivergence of high-quality laser beams is commonly less than 1milliradian (3.4arcmin), and can be much less for large-diameter beams.Laser diodes emit less-collimated light due to their short cavity, and therefore higher collimation requires a collimating lens.
Synchrotron light is very well collimated.[8] It is produced by bending relativistic electrons (i.e. those moving atrelativistic 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.[9]
The light fromstars (other than theSun) 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 anapparent 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 theangular diameter of the Sun as seen from Earth. During asolar eclipse, the Sun's light becomes increasingly collimated as the visible surface shrinks to a thin crescent and ultimately asmall point, producing the phenomena of distinct shadows andshadow bands.
A perfectparabolic 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 acollimator. Since the source needs to be small, such an optical system cannot produce much optical power.Spherical mirrors are easier to make than parabolic mirrors and they are often used to produce approximately collimated light. Many types oflenses can also produce collimated light from point-like sources.
"Collimation" refers to all the optical elements in an instrument being on their designedoptical 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). Theunconditional 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 variousinterpupillary 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 aCheshire eyepiece, or with the assistance of a simple laser collimator orautocollimator. Collimation can also be tested using ashearing interferometer, which is often used to test laser collimation.
"Decollimation" is any mechanism or process which causes a beam with the minimum possibleray divergence to diverge or converge from parallelism. Decollimation may be deliberate for systems reasons, or may be caused by many factors, such asrefractive index inhomogeneities, occlusions,scattering,deflection,diffraction,reflection, andrefraction. Decollimation must be accounted for to fully treat many systems such asradio,radar,sonar, andoptical communications.