The most important approximation for Fraunhofer diffraction is that the observation point must be limited close to the axis,or the diffraction angle is not too large.

• 其中最重要的条件是观测点必须限制在傍轴附近,衍射角不能太大,否则得不到简单的解析公式,也就不能称之为夫琅和费衍射。