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Collimated light is light whose rays are parallel, thus having a planar wavefront. The word is derived from "co-linear" and implies light that does not disperse, even over an infinite distance. Light can be collimated by a number of processes, for instance to project a beam on a parabolic concave mirror with the source at the focus. Collimated light is sometimes said to be focused at infinity. A simple way to test a beam for proper collimation is the shearing interferometer. In reality a perfectly collimated beam with no divergence cannot be created due to the fundamental limitations of diffraction, but in practice sufficiently low-divergence beams are considered collimated.

Laser light is normally automatically collimated because it is formed in a chamber between two such mirrors, in addition to being coherent. This is indicated by the 'pencil beam' of laser projectors, with very little angular spread.

While the phrase "collimated optics" is often applied to the optical hardware involved in generating the representations of Out-the-Window (OTW) imagery used in training of aircraft crew, the images produced by such systems are not composed of truly collimated light. This is a consequence of the fact that the images originate inside the system, which is of strictly finite size and therefore must produce light which is at best an approximation of the zero-divergence characteristics of collimated light. The approximation is sufficient to cause the generated light to appear to be collimated to the human users.

The light from stars can be considered collimated (for almost any purpose) because they are so far away. Due to its relatively large appearance on the sky, the light from the sun deviates about half a degree to all directions when compared to a point source in the same position, giving approximately collimated light.

• A perfect parabolic mirror will bring parallel rays (from a star) to a focus at a single point. Spherical mirrors are easier to make than parabolic mirrors and they are often used to produce approximately collimated light.

To produce usefully collimated light, the light source must approximate a point; that is, it must be small relative to the optical system, like the image of the star formed by a mirror.

The necessary tradeoff is that, since the luminosity of most sources is small, such an optical system cannot produce much optical energy. Lasers are a notable exception to this general rule.

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 inhomogeneities, occlusions, scattering, deflection, diffraction, reflection, and refraction.

Decollimation occurs in applications such as radio, radar, sonar, and optical communications.