Frequency Modulators

Rotating prisms and diffraction gratings as monochromators which disperse the radiations from the source falling on it and can be used to allow the radiations of different wavelengths to come out of the exit slit. These are then passed through the sample and the interaction is monitored. In dispersive IR spectrometers we use similar components, the only difference being that the monochromator is placed after the sample. You would recall that the monochromator is placed before the sample in UV-VIS spectrometry.
In FT-IR instruments  used quite extensively now a days, the radiations are not dispersed before or after passing through the sample. Instead, the response of the sample to all the wavelengths in the range is measured simultaneously and the signal so obtained is modulated and collected in time domain. This is then Fourier transformed to get the desirable frequency domain signal.

The modulation of the signal is achieved by Michelson interferometer. It  modulates the frequency of the IR radiation into the audio frequency range. The fluctuations of A monochromator is an optical device that transmits a selectable narrow band of wavelengths of radiation chosen from a wider range of wavelengths available at the input. The radiation power in the audio frequency range are then followed by suitable transducers. This results into a modulated time domain signal which contains all the information about the spectrum. This is then be Fourier transformed to retrieve the information in the frequency domain.