Double-beam fourier spectroscopy with interferometric background compensation

A double-beam multiplex spectrometer is described which is specially suited for measuring small variations in absorptivity or reflectivity, or narrow spectral lines, against a strong and varying background. By a modification of the conventional Fourier spectrometer, the interferogram beam and the radiation normally going back to the source — the anti-interferogram beam — are combined in the sensitive area of a single detector. If a sample and a reference standard are placed in the interferogram and the anti-interferogram paths respectively, the Fourier transform of the interferogram of the combined intensity directly yields the difference between the spectra of the sample and the reference standard with the optical cancellation of the common features. The advantages and possible applications of this technique are discussed.