beam splitter 

https://doi.org/10.1351/goldbook.08228
Device to split a beam of electromagnetic radiation into two parts.
Note:
In an ideal two-beam Fourier-transform spectrometer, the beam splitter would transmit half of the radiation and reflect half of it to create the two beams.
Source:
PAC, 2021, 93, 647. (Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)) on page 652 [Terms] [Paper]