A reference filter should not have any gas absorption. Filter B can be used when SO2 is part of the gas mixture (peak = 3.86μm, FWHM = 90 nm). The CO2 filter A with a peak = 4.265μm is the filter with the best possible signal and simple linearisation (FWHM = 110nm).
Water vapour is in almost every gas mixture, which often leads to a disturbing background noise. An attempt to measure this with a 2.94μm filter, for example, leads to problems of cross-sensitivity with CO2. The M filter has proven more reliable in such practical applications: peak = 5.78μm, FWHM = 180nm.
Methane is primarily measured at 3.33μm; however, this is not particularly specific. An excellent alternative is the S filter, which operates at a longer-wave band: peak = 7.91μm, FWHM = 160nm. Breath alcohol measurement is becoming more and more popular. The new standard filter has the following specifications: peak = 9.50μm, FWHM = 450nm.