Modern infrared spectrometers are very different from the early dispersive instruments that were introduced in the 1940s. While there is still a need for dual-beam dispersive instruments in high-precision work, most instruments today use a Fourier Transform infrared (FT-IR) system.

A Fourier transform is a mathematical operation used to translate a complex curve into its component curves. In a Fourier transform infrared instrument, the complex curve is an interferogram, or the sum of the constructive and destructive interferences generated by overlapping light waves, and the component curves are the infrared spectrum. The standard infrared spectrum is calculated from the Fourier-transformed interferogram, giving a spectrum in percent transmittance (%T) vs. light frequency (cm^-1).

An interferogram is generated because of the unique optics of an FT-IR instrument. The key components are a moveable mirror and beam splitter. The moveable mirror is responsible for the quality of the interferogram, and it is very important to move the mirror at constant speed. For this reason, the moveable mirror is often the most expensive component of an FT-IR spectrometer. The beam splitter is just a piece of semi-reflective material, usually mylar film sandwiched between two pieces of IR-transparent material. The beam splitter splits the IR beam 50/50 to the fixed and moveable mirrors, and then recombines the beams after being reflected at each mirror.

The movie below shows the optics of a simple FT-IR instrument and then demonstrates how an interferogram is generated by the moveable mirror.

The Fourier transform is named after its inventor, the French geometrician and physicist Baron Jean Baptiste Joseph Fourier, born in 1830. Today, all Fourier transforms are done by computer; working one out by hand in the 18th century must have been quite a challenge!