Transmitting-receiving telescope

Where Pq(A) is the transmitted power, T(k) is the transmission of the clean air from the Lidar to target and back to the Lidar, li(A) is the reflection-coefficient of the target, D is the solid angle of the Lidar telescope, absorption coefficient of the investigated gas, P(A) is the received power. 

The principle of long-path absorption techniques is illustrated in Fig. 10.17. A laser beam is transmitted continuously into the atmosphere against a corner-cube retro-reflector (Fig. 6.21) that is placed at a distance of up to 10 km. The reflected beam is received by an optical telescope that is placed at the site of the laser and is directed towards the retro-reflector. The received light intensity is measured photo-electrically as a function of the laser wavelength. The absorption spectrum of the atmosphere between the laser and the retro-reflector is then recorded and the mean concentrations Nj of pollutant molecules can be determined using the Beer-Lambert relation... 

Figure 22.4. Monostatic OP/FT-IR instruments with (a) separate telescopes for transmitting and receiving the beam and a lateral transfer retroreflector, and (h) with a single telescope for transmitting and receiving the beam and optics similar to those shown in Figure 22.5 to distinguish between the transmitted and reflected beams.

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