Method for spectroscopy using two Fabry-Perot interference filters

Claim 1

Method for Fabry-Perot spectroscopy using a spectrometer the radiation path of which being provided with a radiation source, two successive Fabry-Perot interference filters through which the radiation passes, a blend of substances to be examined, and a detector for receiving radiation flux having passed through the filters and the blend, said Fabry-Perot interference filters having been adjusted to the same optical layer thickness, the optical layer thickness of one of the Fabry-Perot interference filters is continually varied by modulation, the improvement comprising producing an interferogram of an envelope (FIG. 5) of a transmission curve which envelope results from superpositioning of the Airy-functions of the two Fabry-Perot interference filters, whereby the envelope represents the difference in the optical layer thickness of the two Fabry-Perot interference filters, receiving a radiation flux having passed through the filters and the blend, by means of the detector and over a broad band in order to make use of the envelope part of the transmission curves; and mathematically converting the interferogram received by the detector into a spectrum as a function of the wave number.