Monochromators and Frequency Modulators:

Previously you have learnt about rotating prisms and diffraction gratings as monochromators that disperse the radiations from the source falling on it and could be used to permit the radiations of different wavelengths to come out of the exit slit. These are then passed by the sample and the interaction is monitored.  Within dispersive IR spectrometers we use same components, the just difference being which the monochromator is placed after the sample. You would recall that the monochromator is placed before the sample in UV-VIS spectrometry.

Within FT-IR instruments, used quite extensively now days, a radiation is not dispersed before or after passing by the sample. On the other words, the response of the sample to all the wavelengths in the range is measured at the same time and the signal so acquired is modulated and collected in time domain. This is then Fourier changed to get the desirable frequency domain signal.

The modulation of the signal is achieved through Michelson interferometer. It modulates the frequency of the IR radiation within the audio frequency range. The fluctuations of the radiation power in the audio frequency range are then followed through suitable transducers. This results within a modulated time domain signal that holds all the data about the spectrum. This is then be Fourier changed to retrieve the data within the frequency domain.