Atmospheric visibility measurement by a modulated cw lidar.

A modulated cw lidar is described for the remote sensing of atmospheric visibility. An analysis of the system response is given, optimum design parameters are established, and the theoretical capabilities of the new technique are compared with the results of previous work with similar systems. In the new technique, the output from a low power, cw laser is amplitude-modulated and transmitted by a telescope into the atmosphere where scattering and absorption occur. Backscattered light from along the visible beam path is collected and focused on a detector whose output is filtered and fed into a phase discriminator. The phase of the return signal (relative to the source) is directly related to the visibility, wherein lies the basis of this technique.