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衍射计算是设计光盘光学系统的基础。由于光盘信息符（坑点）尺寸可以和波长相比较或更小，标量衍射理论的有效性必需进行检验。首先分析了标量衍射理论的能量计算误差。对于典型的坑点尺寸，标量理论的能量计算误差高达１９％。然后用耦合波理论分析了入射波偏振态的影响。计算表明ＴＭ波和ＴＥ波的衍射特性与标量理论的结论相差较大，ＴＭ波和ＴＥ波的衍射行为也有差别。这种差别是由电磁场边界条件引起的，且会随着坑宽的缩小而增大。对于ＤＶＤ，标量理论的误差已经很大，要得到准确的结果就必须采用矢量衍射理论。
Diffraction calculations are the basis for designing a disc optical system. The validity of the scalar diffraction theory has to be examined because the size of the pit information (pit) can be compared with the wavelength or less. First, the energy calculation error of scalar diffraction theory is analyzed. For typical pit sizes, scalar theory has an energy calculation error of up to 19%. Then the coupled wave theory is used to analyze the influence of the polarization of incident wave. The calculated results show that the diffraction characteristics of TM and TE waves are quite different from those of scalar theory, and the diffraction behaviors of TM and TE waves are also different. This difference is caused by the electromagnetic field boundary conditions and will increase as the pit width shrinks. For DVD, the error of scalar theory is already very large. To get accurate result, vector diffraction theory must be used.