I'm trying to understand why most modern FT-IRs have three interferometers. Does this improve the signal in someway or does it have another effect?

  • $\begingroup$ Where did you see this? There is only one interferometer in standard FTIR used in chemical labs. $\endgroup$ – M. Farooq Oct 21 '20 at 20:48
  • $\begingroup$ Modern interferometers generally have 3. This is standard in modern instruments $\endgroup$ – Harley McFarlen Oct 21 '20 at 20:49
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    $\begingroup$ Could you provide a reference which says three interferometers are employed in modern ones? I have seen opened modern FTIRs all one can see is a single interferometer. $\endgroup$ – M. Farooq Oct 21 '20 at 20:51
  • $\begingroup$ This was information in a lecture that I am trying to understand. He did not provide a reference, just stated the fact. $\endgroup$ – Harley McFarlen Oct 21 '20 at 20:53
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    $\begingroup$ Most FT-IR instruments have one interferometer, as @M.Farooq says. The resolution is inversely proportional to twice the distance (in cm) that the moving mirror moves. But, for a small integrated chip, only an inadequately small mirror movement distance is feasible. So the authors of the 2020 linked paper had the nice idea of using N interferometers, driven by the same actuator, so that the desired travel distance could be divied up into N coherently scanned contiguous shorter distances. Then they process the N interferograms as necessary in order to achieve their goal spectral resolution. $\endgroup$ – Ed V Oct 21 '20 at 23:08

I use FTIR in Physics, pretty much top-of the-range commercial instruments (actually more than just IR - ours go down to the UV and would do THz if we had the detectors; this isn't uncommon so I'll mainly refer to FT rather than FTIR). There's definitely only one interferometer in each of our systems (and the ones used by our friends in Chemistry). In astronomy FT spectroscopy has been used on the ground, but the best IR work these days is done from space using dispersive instruments (e.g. Spitzer's IRS).

The only common commercial use of three series-connected spectral devices (trying to coin a deliberately vague term) is dispersive instruments, which use three diffraction gratings for high resolution spectra and, for Raman spectroscopy, laser rejection with low stray light. A good benchtop FT instrument (with one interferometer) can give just as good resolution, though slowly and without the dynamic range. I mention this because if you were giving a lecture introducing a range of systems and techniques, and skipped a slide or two, you could jump from FTIR to triple dispersive systems and cause a lot of confusion.


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