A large number of gases absorb infrared radiation by intramolecular vibrations. For each specific material, the absorption intensity depends on the wavelength of the infrared radiation. This principle uses a non-dispersive infrared gas analyzer (NDIR gas analyzer) for gas analysis. The analyzer consists of an electronically or mechanically modulated infrared source, a sample cell through which the gas flows to be measured, and a pyroelectric detector.
In the detector cap, a narrow band pass filter (NBP Filter) is integrated, whose pass band corresponds to the absorption wavelength of the target gas. With the thermal detector, the transmission of the measured gas mixture is determined in a defined arrangement. If the gas being searched for is not present, most of the infrared radiation will reach the detector and the signal will be at its maximum. If the concentration of the gas increases, absorption will also increase according to Lambert-Beer's law, and the signal will reduce accordingly.
This concept finds itself again in standard InfraTec infrared filters with a center wavelength (CWL) of 4.27 µm and a half power bandwidth (HPBW) of 170 nm are suitable for the measurement of carbon dioxide (CO2), for example.