The CO2 filter A with an lpeak = 4.265 μm is the filter with the best possible signal and simple linearization (FWHM = 110 nm).
Water vapour is in almost every gas mixture, which often leads to a disturbing background noise. An attempts to measure this with a 2.94 μm filter, for example, lead to problems of cross-sensitivity with CO2. The M filter has proven more reliable in such practical applications: lpeak = 5,78 μm, FWHM = 180 nm.
Methane is primarily measured at 3.33 μm; however, this is not particularly specific. An excellent alternative is the S filter, which operates at a longer-wave band: lpeak = 7,91 μm, FWHM = 160 nm.
Breath alcohol measurement is becoming more and more popular. The new standard filter has the following specifications: lpeak = 9,50 μm, FWHM = 450 nm.