The addition and subtraction of characters can be performed in a coherent optical processing system if a grating is used as a filter.

利用光栅作为滤波器，在相干光处理系统中可对字符进行相加和相减。

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With a phase grating located at Fourier frequency plane, optical image subtraction is obtained as a result of the coherent superposition of the zeroth and the first order of the diffracted light.

置一位相光栅于相干光学处理系统频谱平面，利用其零级与一级衍射光的相干迭加实现图象相减。

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