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.