An Evaluation of the Practical Performance of a Digilab FTS-14 Fourier Transform Infrared Interferometer Working in the Region of 4000 to 400 cm−1

Abstract

The performance of a Digilab FTS-14 Fourier transform infrared interferometric spectrometer has been evaluated in terms of (1) sensitivity and signal/noise ratio (in comparison with a high quality infrared grating instrument), (2) the attainment of very high absolute signal/noise ratios by use of double-precision arithmetic, (3) photometric accuracy, (4) wavenumber accuracy, (5) resolution, (6a) water vapor cancellation and double-beam operation, (6b) ratioing facilities against stored reference spectra, (7) the measurement of very high optical densities, and (8) miscellaneous other operational considerations. It is shown that, compared with a diffraction grating spectrometer of good performance, over most of the spectral region the interferometer enables the attainment of more than an order of magnitude improvement in signal/noise ratio (S/N) for a given time of scan and resolution. This improvement in S/N was found to reach a maximum of about 40 times at 2000 cm−1 when the full wavenumber range (3600 cm−1) is measured with the highest resolution (0.5 cm−1). This corresponds to the measurement of 7200 spectral elements, n. The S/N improvement is proportional to n and is correspondingly reduced for more limited ranges of spectral measurement or for more limited resolutions. The enhanced S/N can alternatively be traded for rapid scanning. The performance of the interferometer is shown to be very good in relation to the other operational parameters listed above. Substantial additional advantages – particularly in relation to automated operation, ratioing of related spectra, and flexibility in modes of plotting wavenumber or intensities – occur from the use of a dedicated minicomputer with, or as part of, a spectrometer.