NDIR

Nondispersive Infrared (NDIR) sensors are simple spectroscopic devices often used for gas analysis. The main components are an infrared source (lamp), a sample chamber or light tube, a wavelength filter, and the infrared detector. The gas is pumped (or diffuses) into the sample chamber, and gas concentration is measured electro-optically by its absorption of a specific wavelength in the infrared (IR). The IR light is directed through the sample chamber towards the detector. The detector has an optical filter in front of it that eliminates all light except the wavelength that the selected gas molecules can absorb. Ideally other gas molecules do not absorb light at this wavelength, and do not affect the amount of light reaching the detector.

As many gases absorb well in the IR area, it is often necessary to compensate for interfering components. For instance, CO2 and H2O often initiate cross sensitivity in the infrared spectrum. As many measurements in the IR area are cross sensitive to H2O it is some times not possible to analyse for instance SO2 and NO2 in low concentrations using the infrared light principle.

The IR signal from the source is usually chopped or modulated so that thermal background signals can be offset from the desired signal.