Non dispersive infrared spectroscopy (ND-IR) is often used to detect gas & measure the concentration of carbon oxides (e.g. carbon monoxide, carbon dioxide). An infra-red beam passes through the sampling chamber and each gas component in the sample absorbs some particular frequency infra red. In parallel a reference gas, typically nitrogen, is used in another chamber. By measuring the amount of absorbed infer red at the necessary frequency, the concentration of the gas component can be determined. It is called nondispersive because the wavelength which passes through the sampling chamber is not pre-filtered, and instead the optical filter is in front of the detector to eliminate all light except the wavelength which the selected gas molecules can absorb.