The light from the source is split into two beams of light approximately equal in intensity. One beam is termed the reference beam. The second, which passes through the sample is called the sample beam. The two beams are then recombined and pass through the detector. The beam spliter may be a simple mirror plate into which a number of holes are drilled.
Light is reflected by the mirror plate and passes down the sample beam path.
An equal portion of light passes through the holes in the plate and forms the reference beam. Another convenient beam splitter is a disk with opposite quadrants removed. The disk rotates in front of the radiation beam. The mirrored surface reflects light into the reference path. The missing quadrants permit radiation to pass down the sample beam.