device for changing the diverging light or other radiation from a point source to a parallel beam. This collimation of the light is required to make specialized measurements in spectroscopy and in geometric and physical optics.

An optical collimator consists of a tube containing a convex lens at one end and an adjustable slit at the other, the slit being in the focal plane of the lens. Radiation entering the slit leaves the collimator as a parallel beam, so that the image can be viewed without parallax.

The collimator may be a telescope with a slit at the principal focal length of the lens. Light from the luminous source is focused on this slit by a lens of similar focal length, and the slit then serves as the luminous object of the optical system.

In radiology, a collimator is an arrangement of absorbers for limiting a beam of X rays, gamma rays, or nuclear particles to the dimensions and angular spread required for the specific application.