Double-beam fourier spectroscopy with two inputs and two outputs

A theoretical treatment of double-beam Fourier spectroscopy, with two inputs and two outputs measured by a single detector, is presented. One of the novel features is the use of the radiation that returns to the source in a conventional Fourier spectrometer, i.e., the anti-interferogram beam. The double beam feature allows simultaneous measurement of the difference in the spectra of the sample and a reference standard with optical cancellation of common features. Several versions of spectrometers suited for specific applications are discussed as special cases. Applications include the ability to measure small absorptivity or reflectivity against strong and varying background, to compare the emission spectra of two nearly equal sources, or the sensitivity of two different detectors. Moreover, by suitably chopping the input or output beams, the noise due to fluctuations in source intensity may be strongly reduced.