Evaluation of multiple beam interference effects in infrared gas spectroscopy

Spectral interference resulting from the tendency of infrared windows to act as low-finesse etalons is evaluated for various infrared materials. The effect's temperature dependence and implications to CO2 concentration measurements is demonstrated by modelling the etalon's influence on the CO2 nu(3) absorption band. It is shown that semiconductor and zinc chalcogenide windows exhibit a very strong temperature sensitivity, with lower refractive index windows, such as fluorides, having a lesser but still significant sensitivity which can lead to spurious results. Elimination of any spectral distortion or changes in concentration due to etalon effects requires the windows to be wedged with a minimum angle of approximately 1.0-0.7 mrad depending on the window material.