Fringe-contrast analysis

The offset ds introduced by a phason plane can be interpreted as the vertical component of the phason-plane strain field. Since the phason plane turns hexagon columns into their alternate orientation, its strain field has in addition a lateral component corresponding to a translation of half a lattice constant along the [10 0] direction. The total phason-plane strain field can then be described by the displacement vector 7 =1/2(1 0 - 1/r ), which was experimentally verified by means of fringe-contrast analysis in TEM [37]. 

Obviously, the analysis of the correlation between the two fields emerging from the telescope and related devices makes necessary to avoid dissymmetry between the interferometric arms. Otherwise, it may result in confusion between a low correlation due to a low spatial coherence of the source and a degradation of the fringe contrast due to defects of the interferometer. The following paragraphs summarize the parameters to be controlled in order to get calibrated data. 

Figure 7.1(a-f), being fringe-images produced by diffraction interference effects, is essentially a virtual image. Another approach to a structure analysis is to use transmission methods in which the image is created by absorption effects (thickness contrast). This... 

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