If the cross terms of the spatial frequencies for the unit cell of periodical object are much smaller than the main frequency terms, the restriction of the Angle is not necessary.
当周期物体的单胞空间频率交叉项比主频率项小到可忽略的程度时，角度限制失效。
And the intensity of single frequency of the entire spectrum is contributed from emissions with wide spatial and Angle ranges.
辐射谱中某单个频率的空间方向分布和位置分布比较广。
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