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Upper Boundaries to the Extent of the Jacquinot or Throughput Advantage in Fourier Transform Infrared Spectroscopy

Volume 31, Number 5 (Oct. 1977) Page 471-472

Hirschfeld, T.


The typical Fourier transform infrared (FT-ir) spectrometer has a huge light gathering power advantage over grating spectrometers of similar resolution. This advantage, called the throughput or Jacquinot advantage, is given by

TFT/TG = 2πF/l

where TFT and TG are the FT-ir and grating instrument throughput, and F and l are the focal length and slit height of the latter. For practical instruments TFT/TG ≈ 100 to 200. Here we have

TFT = 2πAM/R = π2/2 dB2/R

where AM is the interferometer beamsplitter area, dB its diameter, and R the resolution. At first glance it would appear that TFT can be enlarged for any given R by taking dB up to its technological limit. Even higher throughputs are possible in field-widened FTS spectrometers.