The diffraction divergence of a light beam after passing through a sharp edge diaph-ragm depends on the intrinsic energy[1] of the beam. In this paper we have derived that the intrinsic energy of a uniform light beam after passing through a sharp edge diaphragm is propor-tional to the total length of the diaphram edge. We obtained a very simple result using the diaphragm edge length to describe the beam diffraction divergence.