Fourier transform infrared spectroscopy Fellgett

The transform from the interferogram to the spectrum is carried out by the dedicated minicomputer on the instrument. The theory of Fourier-transform infrared spectroscopy has been treated, and is readily available in the literature.21,22,166 Consequently, the advantages of F.t.-i.r. dispersive spectroscopy will only be outlined in a qualitative sense (i) The Fellgett or multiplex advantage arises from the fact that the F.t.-i.r. spectrometer examines the entire spectrum in the same period of time as that required... 

As in all Fourier transform methods in spectroscopy, the FTIR spectrometer benefits greatly from the multiplex, or Fellgett, advantage of detecting a broad band of radiation (a wide wavenumber range) all the time. By comparison, a spectrometer that disperses the radiation with a prism or diffraction grating detects, at any instant, only that narrow band of radiation that the orientation of the prism or grating allows to fall on the detector, as in the type of infrared spectrometer described in Section 3.6. 

For the three types of noise discussed above, the Fellgett advantage must be carefully evaluated. This multiplex advantage is an unquestioned benefit, for example, when detector noise dominates, as is the case in infrared Fourier transform spectroscopy. In visible/UV spectroscopy, the detector noise which is present can also be minimized with the multiplex advantage. Therefore it is not always necessary to cool photomultiplier tubes to reduce the thermionic emission for Fourier transform spectroscopy. 

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