³⁹One such analyzer I saw in industry had a path length of a quarter-mile (1320 feet), to better measure extremely low concentrations of a gas! The gas in question was ambient air inside of a large shelter housing a chemical process. The analyzer was mounted on one side of the shelter, aiming a beam of laser light all the way to the opposite wall of the shelter 660 feet away, where a reflector was mounted. The laser beam’s path length was therefore twice the length of the shelter, or 1320 feet.