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

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 a 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.