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A 'collimating lens' is a lens used to collimate light, that is, to gather it together in a parallel beam.
In the original spectroscope design in the early 19th century, light entered a slit and a collimating lens transformed the light into a thin beam of parallel rays. A prism then refracted the beam into its spectrum.
Collimators are used to calibrate the pointing and focus of optical imaging systems.