How do we identify gases?
Remote sensing of gases using passive infrared (IR) spectrometry is based on the spectral analysis of infrared radiation that is emitted and absorbed by the molecules of a gas cloud. The infrared radiation comes from natural radiation sources, e.g. the background, the gas cloud itself or the sun. Almost all molecular air components and gaseous pollutants have characteristic spectral signature in the infrared spectral range, through energetic transitions between different vibration and rotation states of the molecules and the resulting absorption and emission of radiation. The intensity of these signatures depends on the structure of the molecules and their concentration along the optical path. Considering all signatures of all compounds that contribute to the signal and the device parameters of the sensor unit, the column density (i.e. the product of concentration and path length, ppm*m) of the target compound can be determined. At a known path length of the cloud, the concentration of the cloud can be determined (ppm). Using two or more sensor units, the depth information of a cloud can be determined by triangulation and means of tomographic reconstruction.
All target gases in the scanfeld TM spectral library (over 400 available) have a distinctive spectral signature in the infrared spectral region that is called the “fingerprint region” (approx. 500 … 1500 cm-1).