The structure of lidar equation is analyzed in the small-angle approximation taking into account multiple scattering of light. To describe lidar return signal in this approximation, the scattering phase function is modeled as a sum of diffraction and geometric-optics components. A method is proposed for separation of the diffraction and geometric-optics components of the return signals. It is shown that, at large receiver's field of view, this allows the information about the diffraction component of the scattering phase function to be replaced by its Hankel transform at zero point.