Advantage Fellgett

An improved signal/noise ratio because all signals are seen simultaneptisiy along with the instrument s own noise (called the multiplex or Fellgett advantage). 

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. 

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There are three well known advantages one gains by doing Fourier transform spectroscopy that are usually discussed in terms of their relationship to grating spectroscopy. They are called the Fellgett advantage, the Jacquinot advantage, and Connes accuracy. 

The other principal advantage which applies to Fourier transform spectroscopy is the multiplex or "Fellgett" advantage 21,64) n yas P. Fellgett who first pointed out that there is an advantage when the data in all elements of a spectrum are obtained simultaneously instead of being measured for each element successively. In Fourier transform spectroscopy, the radiation in the Michelson interferometer is not separated into spectral elements. The interferogram contains... 

Now, in order to employ a locked-in detection system, as in EMIRS, the modulation frequency of the potential at the electrode would have to be at least an order of magnitude greater than F(i). Thus, the potential modulation would have to be between 70 and 100 KHz, too great to allow sufficient relaxation time for most electrochemical processes to respond. As a consequence, lock-in detection has not been employed in in-situ FTIR studies and the sensitivity of SNIFTIRS is less than that of EMIRS. Nevertheless, the FT method does have the sensitivity necessary to detect monolayers, and submonolayers, of adsorbed species [55, 56]. This arises out of the very large improvements in S/N ratio available to FT (compared with dispersive) infrared spectrometry by the Jacquinot and Fellgett advantages. 

The Fellgett advantage is the multiplex advantage (see later) related to the simultaneous detection of all frequencies of the incident radiation. Let us assume that the FT-spectrometer and grating spectrometer have the same resolution, that the studied spectral interval is between Vi and Vj and that it contains M spectral elements ... 

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