Collimator

[kol-uh-mey-ter] /ˈkɒl əˌmeɪ tər/

noun
1.
Optics.

2.
Physics. a device for producing a beam of particles in which the paths of all the particles are parallel.
/ˈkɒlɪˌmeɪtə/
noun
1.
a small telescope attached to a larger optical instrument as an aid in fixing its line of sight
2.
an optical system of lenses and slits producing a nondivergent beam of light, usually for use in spectroscopes
3.
any device for limiting the size and angle of spread of a beam of radiation or particles
collimator
(kŏl’ə-mā’tər)
A device that turns incoming radiation, such as light, into parallel beams. Simple collimators consists of a tube having a narrow, variable slit at one end and a convex lens at the other. Radiation entering the tube through the slit exits the lens in the form of parallel beams. Collimators are used to establish focal lengths of lenses and to measure the distance of distant objects whose position is known. See illustration at spectroscope.