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Wiktionary
beam splitter

n. An optical device that splits a beam of light in two by reflecting a portion and transmitting the remainder.

Wikipedia
Beam splitter

A beam splitter is an optical device that splits a beam of light in two. It is the crucial part of most interferometers.

Usage examples of "beam splitter".

Each of their high-magnitude night-vision scopes was equipped with an infrared camera head/optical beam splitter attachment.