Figure 4
Optical path differences [{\Delta _1}=x\,\cos {\theta _1} + z\sin {\theta _1}] and [{\Delta _2}= - x\,\cos {\theta _2}] [ +\, z\sin {\theta _2}] for an arbitrary point (x,z) in a lateral crystalline structure. Note that the angular deviations are not to scale.  [article HTML]

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