Traditional Physical Optics (PO) is able to efficiently deal with 3D electromagnetic problem involving structures with electrically large dimension in the open space, usually obtaining calculation results with good precision. However, traditional PO appears to be inapposite for special complicated structures with electrically large scattering objects embedded between two parallel metal plates, so does the full wave method because of the large computation resource consumption. In this paper, a modified PO method based on Discrete Real Mirror Image theory (DRMI-PO) is proposed. The basic idea of DRMI-PO is to expand the electromagnetic current between the parallel metal plates into the Fourier series, and then linearly combine the field solution for each term, derived with the 2D PO. In comparison with Multilevel Fast Multipole Algorithm (MLFMA), DRMI-PO is able to keep enough precision and obtain higher efficiency when analyzing specific electromagnetic structures. As an application example, two terahertz fan-beam scanning antennas with different polarization are calculated with DRMI-PO and measured. The measured radiation patterns are in well agreement with the calculated results, which indicates the effectiveness of DRMI-PO.%传统的物理光学法(PO)能够方便高效地处理开放空间中电大尺寸结构的电磁散射问题,并且通常能获得较好的精度.然而,对于电大尺寸结构内嵌在金属平行板之间的这种特殊问题,传统的PO方法不再适用,而采用全波分析方法来处理往往需要消耗巨大的计算资源.该文在离散实镜像理论的基础上,提出了一种修正的物理光学法(DRMI-PO),基本思想是先将金属平行板间的激励电磁流展开为傅里叶级数,再对级数的每一项分别应用2维物理光学法求解场,最后把所有求得的场解作线性组合.与多层快速多极子(MLFMA)的全波方法相比,DRMI-PO在解决该类特殊结构的电磁仿真问题时能在保证足够高精度的同时大幅度提高计算效率；作为应用实例,对两种不同极化方式的太赫兹波段扇形波束扫描天线分别进行了DRMI-PO数值仿真与实验测试,结果表明仿真和实验得出的天线方向图吻合较好,从而证实了DRMI-PO方法的有效性.
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