“…Equation [6.2] provides the basis for many forms of long-path absorption spectroscopy using coherent light sources such as lasers; these are applicable, for instance, to atmospheric studies. They include: open-path laser absorption spectroscopy, in which a laser beam passes from a distant retro-refl ector and back to a detector that is co-located with the laser (Schiff et al , 1994;Platt, 1994Platt, , 2000Platt et al , 2012); LIDAR ( li ght d etection a nd r anging) and its dual-channel modifi cation DIAL, in which the temporal or phase characteristics of light provide an optical ranging capability (Grant and Menzies, 1983;Killinger et al , 1983Killinger et al , , 1987Grant, 1987Grant, , 1995Svanberg, 1994;Orr, 2000;Wolf, 2000;Strizik et al , 2008;Platt et al , 2012); and multi-pass absorption spectroscopy, which uses folded-path cell designs such as those of White (1942White ( , 1976 and Herriott et al . (1964) and yields optical pathlengths d eff that are typically hundreds of times the length of the absorption cell itself (Hanst and Hanst, 1994;Schiff et al , 1994;Tittel and Petrov, 2000;Tittel et al ., 2003).…”