Abstract: According to Huygens’ Principle, the computation of the second rank field of a flat plane can be solved by calling the first rank field function twice, so ray trace is the key. The method for the computation of the second rank field of the flat plane is deduced using the image method in this paper. For the ray trace of reflection-reflection, reflection-diffraction, and diffraction-reflection, the result can be easily obtained by using the image point of the source point or field point. For the diffraction-diffraction case, geometry is hybrid together with algebra to change the ray trace for the calculation of the equations system. Examples and analytical deduction show the validity of the method. Compared to the method of searching the points used before, the method derived in this paper is better in accuracy and speed.

Key words: uniform theory of diffraction, flat plane, ray trace, second rank ray