Collimated beam

A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates.

Thus, as the distance from a point source increases, the spherical wavefronts become flatter and closer to plane waves, which are perfectly collimated.

In scintigraphy, a gamma ray collimator is used in front of a detector to allow only photons perpendicular to the surface to be detected.

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

However, due to refraction and turbulence in the Earth's atmosphere, starlight arrives slightly uncollimated at the ground with an apparent angular diameter of about 0.4 arcseconds.

A perfect parabolic mirror will bring parallel rays to a focus at a single point.

It also refers to the process of adjusting an optical instrument so that all its elements are on that designed axis (in line and parallel).

Decollimation must be accounted for to fully treat many systems such as radio, radar, sonar, and optical communications.

In the lower picture, the light has been collimated.
An example of an optical collimating lens.