Collimated light

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In the lower picture, the light has been collimated.

Collimated light is light whose rays are parallel, and therefore will spread slowly as it propagates. The word is related to "collinear" with all rays lining up. In theory, collimated light does not disperse with distance. Really, collimated light will disperse a little as it travels over distance. Diffraction prevents scientists from creating a perfectly collimated beam with no divergence. Light can be approximately collimated by a number of processes, for instance by using a collimator. Some people say that collimated light is focused at infinity. So, as the distance from a point source of light increases, the spherical wavefronts become flatter and closer to plane waves, which are perfectly collimated.

Etymology[change | change source]

The word "collimate" comes from the Latin verb collimare, which originated in a misreading of collineare, "to direct in a straight line".[1]

Sources[change | change source]

Lasers[change | change source]

Laser light from gas or crystal lasers is highly collimated because it is formed in an optical cavity between two parallel mirrors, in addition to being coherent.[Note 1] The divergence of high-quality laser beams is commonly less than 1 milliradian, 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[change | change source]

Synchrotron light is very collimated. It is produced by bending relativistic electrons around a circular track.

Distant sources[change | change source]

The light from stars (other than the Sun) can be considered collimated for almost any purpose, because they are so far away they have almost no angular size.

Lenses and mirrors[change | change source]

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. 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 of lenses can also produce collimated light from point-like sources.

Collimation and decollimation[change | change source]

"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 axes (in line and parallel). With telescopes, the term refers to the fact that the optical axes of each optical component should all 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. It is done by simple visual methods such as looking down the optical assembly with no eyepiece to make sure the components are lined up, 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 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 differences, occlusions, scattering, deflection, diffraction, reflection, and refraction. Engineers watch for decollimation in many systems such as radio, radar, sonar, and optical communications.

Notes[change | change source]

  1. This is not true of all gas lasers. Most gas lasers use slightly concave mirrors, otherwise the power output would be unstable due to mirror non-parallelism due to thermal and mechanical stresses.

References[change | change source]

  1. Lewis, Charlton T. (2010). "collimo". A Latin Dictionary. Oxford; Medford: Clarendon Press; Perseus Digital Library. 

Bibliography[change | change source]

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