What is the X in X-mas?
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.
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.