Collimated light
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Collimated light is light whose rays are parallel and thus has a plane wavefront. Light can be collimated by a number of processes, the easiest being to shine it 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.
Laser light is often collimated due to being formed in a chamber between two such mirrors, in addition to being coherent.
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.
