OPSIS laser diode gas analyser emits laser light in the near infra-red section of the wavelength spectrum. The measurement is made by rapidly scanning the laser over the absorption line in the gas absorption spectrum.

The laser operates continuously, and it is tunable, so the laser wavelength can be slightly changed. This is achieved by applying an electric voltage across the semiconductor diode. The voltage applied is precisely controlled, and varies according to a ramp function during a scan.

During a measurement, the TDL analyser averages a large quantity of scans. The measurement interval is in the order of 1-20 seconds, and the scanning rate is in the kilohertz range.

In the end of the measurement interval, the averaged spectrum enters an evaluation procedure. The result is compared through a least squares fitting procedure with the known absorbance cross section of the gas. The cross section relates to the strength of absorption in the gas, at specific wavelengths. Knowing the monitoring path length, the concentration of the gas can then be evaluateddpuf