Jacquinot’s advantage

The high luminosity of the instrument with no slits to limit the size of the beam. This is the Jacquinot s advantage also called etendue. ... 

In detector noise limited spectroscopies such as PAS it is advantageous to enhance the throughput of energy (Jacquinot s advantage) by utilizing a Michel son interferometer. One then Fourier transforms (FTs) the resulting interferogram to yield a PA spectrum that qualitatively resembles an absorption spectrum. 

The combination of Fellgett s and Jacquinot s advantage coupled with the inherent speed differential should lead to an enormous difference between FT-IR and dispersive instruments. However, in practice, part of this advantage is offset by the difference in the performance of the triglycine sulfate (TGS) and thermocouple detectors. At low modulation frequencies, the thermocouple detector is about an order of magnitude more sensitive than TGS. 

The second fundamental advantage of an FT spectrometer over a grating spectrometer is often called Jacquinot s advantage and derives from the increased optical throughput of an interferometer compared to a grating monochrometer. 

Enhanced spectral throughput (Jacquinot s advantage) Because no entrance and exit slits of the monochromator are used, the amount of energy falling on the detector is greatly enhanced. 

Much larger beam aperture used higher energy throughput (Throughput or Jacquinot s advantage)... 

For FT-IR spectrometers where the throughput is limited by the detector foreoptics (the constant throughput case discussed in Section 7.2), Jacquinot s advantage may be considerably less than the value calculated from Eq. 7.18. For example, if the resolution is changed from 2 cm to 8 cm, the throughput of the grating spectrometer can increase by a factor of 16 while / remains unchanged [9]. 

Figure 7.3. Calculated variation of Jacquinot s advantage between a Digilab FTS-14 FT-IR spectrometer and a Beckman Model 4240 grating spectrometer, both operating at 2 cm resolution. The dashed lines indicate the grating changes for the monochromator. (Reproduced from [9], by permission of the Society for Applied Spectroscopy copyright 1977.)...

Combining the values of Fellgett s and Jacquinot s advantages, FT-IR spectrometers should be about 2000 times more sensitive than grating spectrometers that operate in the mid-infrared. In practice, however, smaller values are found. To understand why this is the case, we must consider other components in these spectrometers. The same types of source are used in both types of instruments, so we will neglect any discussion of this component. This cannot be said of the detector, however. 

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