By an example of Tetragonal structure, the principle and method are discribed by which the orientation of light can be derived from interference structure. We use crystal lattce analogy to the intensity distribution of interference structure. Those crystal planes should be selected such that their Miller indices are lower and the planes have high symmetry. An analytic solution then should be obtained by setting geometric relations between the crystal planes and the incident light. The advantages of this method are obvious compared to that of numerical analysis.