Double-beam weighting in Fourier transform spectroscopy

This paper describes a double-beam weighting system in slow-scanning Fourier transform spectroscopy by use of a photo-conductive detector with fast response. It uses the method of amplitude modulation. The intensity of reference beam (A) and that of sample beam (B) are obtained by summing and subtracting the signals (A + B) and (A ? B), which are obtained in this form from a modulated signal by means of an electronic system. Merits arising from using a detector with fast response in double-beam weighting are also discussed. The operational performance of the system has been evaluated and it has been found that the spectra thus obtained are reproducible because they are kept within 3% bound and each measured value coincides with the averaged trace of single-beam measurements within ±2%.