We analyzed the stray light source of system and drew a conclusion that non-image beam has great effect on system qualities and system ghost images caused by parallel Windows should not be ignored.

分析了系统杂散光的来源，得出非成像光束对系统影响较大、平板玻璃窗产生的鬼像不容忽视的结论。

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The straight-edge diffraction of this parallel beam superposing on the light spot constitutes the so-called M-line.

这束平行光的直边衍射花纹与光斑本底叠加的结果在观察屏上形成为通常所称的M线。

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