PURPOSE:To enable a background light of a fluorescent lamp, etc., to be eliminated in the Fourier transform spectroscopy using a pulse light source. CONSTITUTION:The Fourier transform spectroscopy uses a pulse light source which emits light at a certain interval or a pulse laser for Raman excitation as a light source, obtains an interferogram from an output of a detector 4 which uses a rapid scan interferometer 3, and then performs Fourier transformation to obtain the spectrum of a measured light. A signal which exists only while a light source 1 is emitting light from the output of the detector 4 is picked up from a gate circuit 6, a component within a specified range of frequency of the signal is extracted by a low-pass filter 7, and it is Fourier transformed by sampling, thus obtaining the spectrum of the measured light.
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