Collimated light
Overview
Collimated light is light whose rays are nearly parallel, and therefore will spread slowly as it propagates. The word is derived from "collinear" and implies light that does not disperse with distance. A perfectly collimated beam with no divergence cannot be created due to diffraction, but light can be approximately collimated by a number of processes, for instance by means of a collimator. Collimated light is sometimes said to be focused at infinity. Thus as the distance from a point source increases, the spherical wavefronts become flatter and closer to plane waves, which are perfectly collimated.
Sources
Lasers
Laser light from gas or crystal lasers is naturally collimated because it is formed in an optical cavity between two mirrors, in addition to being coherent. The divergence of high-quality laser beams is commonly less than 1 milliradian, and can be much less for large-diameter beams. It should be noted that diode lasers do not naturally emit collimated light, and therefore collimation into a beam requires a collimating lens.
Distant sources
The light from stars (other than the Sun) can be considered collimated for almost any purpose, because they are so far away and have almost no angular size.
Lenses and mirrors
A perfect parabolic mirror will bring parallel rays (from stars or distant objects) 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.
Synchrotron Light
Synchrotron light is very well collimated. It is produced by bending relativistic electrons around a circlular track.
Collimation and decollimation
"Collimation" refers to the process of tweaking an optical instrument for the best possible image quality. With regards to a telescope 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. Collimation can be done simply via inspection by looking down the drawtube 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 inhomogeneities, occlusions, scattering, deflection, diffraction, reflection, and refraction. Decollimation must be accounted for to fully treat many systems such as radio, radar, sonar, and optical communications.