The peak position for a lidar return signal is calculated and measured for the horizontal path with variation of the laser beam divergence angle (theta), and the inclination angle (delta) between the telescope and laser axes. This work shows that theta and delta are very important parameters to use in the design or alignment of a lidar system receiving a good lidar signal. This paper describes an experimental determination of geometrical form factors in the lidar equation. We receive the signals and determine the geometrical form factors by slope method in a homogeneous atmosphere. The differential absorption lidar equation is evaluated for the dual-pulse lidar system. A method using a geometrical form factor determined by the experiment is introduced to correct the error in C2H4 measurement. This method shows good correction of measurement error in lidar dual-pulse operation, especially in the short range.