Abstract: The coherent length of a wave plane is one of the most fundamental parameters in an adaptive optics system,relating closely to the angular distribution of radiant intensity.Coherent function of sources can be regarded as Gaussian functions of the distance between the two points of interest.Characteristic lengths are the coherent lengths of source plane.There's determined relation between times of diffraction limit N and coherent length of a source with homogeneous intensity.For a beam with a Gaussian section N must be replaced by M,which we call Gaussian limit times,in order to explain the coherence of the beam.