The Perkin-Elmer 21 Infrared spectrometer was the first infrared instrument to use a dual-beam optical system. In a dual beam infrared instrument, the infrared beam is split using mirrors to make two beams. One of the beams goes through a blank, or reference, cell while the other beam goes through the sample. The beams are read in fast succession by the detector and the signal from the reference beam is subtracted from the sample beam, resulting in a much more precise spectrum in a shorter amount of time.

The dual beam design of the PE 21 avoided many of the problems that kept its predecessor, the Perkin-Elmer 12, from being a tool worth using by non-spectroscopists. With the advent of the PE 21, infrared spectroscopy became a tool for chemists of all disciplines, particularly organic and polymer chemistry.

The movie below shows how the dual beam system of the PE 21 works.