Evaluation of multiple beam interference effects in infrared gas spectroscopy

Spectral interference resulting from the tendency of infrared windows to ac t as low-finesse etalons is evaluated for various infrared materials. The e ffect's temperature dependence and implications to CO2 concentration measur ements is demonstrated by modelling the etalon's influence on the CO2 nu(3) absorption band. It is shown that semiconductor and zinc chalcogenide wind ows exhibit a very strong temperature sensitivity, with lower refractive in dex windows, such as fluorides, having a lesser but still significant sensi tivity which can lead to spurious results. Elimination of any spectral dist ortion or changes in concentration due to etalon effects requires the windo ws to be wedged with a minimum angle of approximately 1.0-0.7 mrad dependin g on the window material.