collimated

Collimated describes a beam or wave whose constituent rays are parallel, or nearly so, producing little to no angular spread as it propagates. In an ideal collimated beam, all rays share the same direction, equivalent to a plane wave. Real beams, however, have a finite divergence that can be quantified in units such as milliradians.

In optics, collimation is achieved by optical elements that convert divergent light into a nearly parallel beam. A distant or extended source can be made collimated by a lens with a sufficiently long focal length, or by a dedicated collimating optic in a laser assembly. Collimated light is important for interferometry, alignment, imaging at long range, and optical communications because it maintains cross-sectional area over distance.

In astronomy, collimation refers to aligning a telescope’s optical elements so that light from distant objects comes to a proper focus. Poor collimation degrades image sharpness and can produce elongated stars. Telescope collimation is typically performed by adjusting mirrors or lenses and using alignment aids such as Cheshire eyepieces, lasers, or the star test.

In particle accelerators and radiation physics, collimation describes devices that shape and direct beams by absorbing off-axis particles. Collimators reduce beam halo, protect detectors and optics, and define beam size and angular divergence. They use dense materials to stop particles traveling outside the desired trajectory while allowing the core beam to pass.

Etymology: from Latin collimare, to line up; the term is used across optics, astronomy, and high-energy physics.