Based on the principle of wave and statistical optics, shearography is analysed thoroughly. The auto-correlation function of the transmittance of the specklegram, which is the essence of speckle interference, is calculated. The ensamble average intensity distribution of the output fringe speckle field of the filtering system is derived. We have shown that, if some conditions are satisfied by the system parameters, better contrast of the fringe field will be obtained, but it still is a function of in-plane deformation fields and the system parameters. In addtion, we have found that the out-of-plane deformation gradient is measurable only when the in-plane deformation value is smaller than a value determined by system parameters.