there is disclosed a fourier transform spectrometer using a pulsed light source which can be selected from various types of light sources . the spectrometer fourier - transforms the interferogram obtained by an interferometer to give rise to a spectrum . the spectrometer is characterized in that it includes a means for impulsively lighting up the light source for the interferometer at predetermined intervals of time and a means for extracting the envelope of the output from the interferometer as an interferogram . the spectrometer can further include a means for periodically applying a stimulus to the sample . in this case , time - resolved fourier transform spectroscopy can be effected . the differential method can also be applied by lighting up the light source for the interferometer at intervals half the intervals at which the stimuli are given and by extracting the output from the interferometer via a band - pass filter and via a lock - in amplifier .