What is collimated beam of light?

How to minimize the divergence of a collimated beam?

To minimize divergence of a collimated beam two factors must be balanced: focal length of the collimating system and size of the light source. Equation 1 approximates the divergence of a collimated beam:

For beams with non-ideal beam quality, the Rayleigh length is effectively reduced by the so-called M2 factor, so that the beam waist radius needs to be larger for a beam to be collimated. When describing a collimated beam with light rays, it consists of essentially parallel rays only.

The collimation can be checked, for example, by measuring the evolution of beam radius over some distance in free space, via a Shack–Hartmann wavefront sensor, or with certain kinds of interferometers. In fiber optics, one often uses fiber collimators.