Fellgett’s advantage

Multiplexing of spectral information (Fellgett s advantage) Information from all frequencies in the spectrum is gathered simultaneously. 

All frequencies of radiation fall on detector simultaneously improved S/N ratio obtained quickly (Multiplex or Fellgett s advantage)... 

The multiplexing, or Fellgett s, advantage which arises because all of the resolution elements are observed all of the time. 

In the absence of fluctuation noise, the multiplex advantage of FT-IR spectrometry over a single-channel dispersive spectrometer with the same optical throughput and efficiency (often referred to as Fellgett s advantage) is given by the square root of the number of resolution elements in the spectrum, M, where M =... 

Figure 7.2. Effect of multiplicative noise on Fellgett s advantage as a function of the number of resolution elements for a spectral SNR of 100 and several different values of the relative fluctuation amplitude. (Reproduced from [1], by permission of Academic Press copyright 1979.)...

The advantages of Fourier transform spectrometry over the use of a scanning monochromator (often referred to as dispersive spectrometry) is fully valid only when the detector noise is independent of the power of the radiation incident on the detector. When the detector is photon shot-noise limited [as it generally is for a photomultiplier tube (PMT), and often is for other sensitive detectors used in the near-infrared, visible, and ultraviolet spectral regions], the noise level is proportional to the square root of the incident power. For a boxcar spectrum, this means that shot noise is proportional to the square root of the number of resolution elements in the spectrum, This disadvantage therefore precisely offsets Fellgett s advantage when continuous broadband sources are employed. It should also be... 

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