Infrared devices are equipped with a laser, usually He-Ne laser ( λ = 632.8 nm). Its beam has the same path as that of the source. It passes through the interferometer and the sample before reaching the detector. The role of this laser is first to define very precisely the position of the movable mirror in the interferometer. Second, it is used as a reference to calibrate the frequency of the system when applying a Fourier transform.