Recently, a new method to determine overlap factor for vertically pointing lidar is presented by lidar group in the center of atmosphere optics from Anhui Institute of Optics and Fine Mechanics (AIOFM). The finding has been published in OPTICS LETTERS（2015，40（8），1749-1752）.
The incomplete overlap between laser beam and receiver field of view significantly leads to lidar signal loss in the near-field range, and affects the lidar measurement in this range. However, lidar measurement in the near-field range is very essential. The experimental methods have been explored to determine GFF normally. A new experimental method is introduced by lidar group from AIOFM to determine GFF for vertically pointing lidar by iterative algorithm based on the difference between the particle backscatter coefficient affected by incomplete overlap (Mie lidar) and the actual particle backscatter coefficient (CCD side-scatter technique). Keep a proper distance between a CCD camera and a vertically pointing Mie lidar, which are operated at the same wavelength. The CCD side-scatter technique has no overlap problem, and profiles of particle backscatter coefficient can be retrieved from CCD picture. The lidar measurements would perform simultaneously in a cloudless night, and using iterative method, the GFF for elastic channel of vertically pointing lidar can be derived from elastic signal received by vertically pointing lidar and particle backscatter coefficient profile measured by CCD side-scatter technique.
The effect of overlap factor on Mie lidar is corrected successfully in different atmosphere conditions. Good agreement between slope method and our new method proves this method is available in practice, and GFF measurements for lidars can be determined easily in cloud-free night time by adding a CCD detector with a proper way.
Particle backscatter coefficients and corresponding returned Mie signals with (red line)/without (black line) overlap factor
GFF obtained by slope method and new method discussed in this Letter