The accuracy of this measurement system means that the IR frequency scale is accurate and precise. In the FT-IR instrument, the sample is placed between the output of the interferometer and the detector. The sample absorbs radiation of particular wavelengths. Therefore, the interferogram contains the spectrum of the source minus the spectrum of the sample . Pyroelectric detectors or liquid nitrogen cooled photon detectors must be used.
Thermal detectors are too slow.
To acheive a good signal to noise ratio, many interferograms are obtained and then averaged. This can be done in less time than it would take a dipersive instrument to record one scan. You should be aware of the various sources and detectors used in IR spectroscopy.