The optical unit is mounted on the bottom of the helicopter. It consists of a mirror, a laser, a reference channel and a digital video recording camera. During the flight the laser beam is directed towards the wind direction on the pipeline. The reflected and scattered reflections will be collected from the mirror at the ALMA G2.
A photo detector receives the signal which is compared to the signal from a reference channel. (A laser beam through a curette with a standardized methane sample.) When methane is present the laser light will be absorbed, and the deformation of the signal is proportional to the concentration of methane along the laser beam path.