Beam Divergence Effects on Fresnel Fringes
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The divergence of the incident electron beam in TEM is an important factor that affects the observation of Fresnel fringes. Assuming the divergent incident beam can be split into two beams at an angel β as shown in Figure 2950, the two fringe systems generated by the two beams will be out of distance by about H·sing(β) (≈Hβ). In order to observe a given fringe, the separation distance Hβ must be much smaller than the fringe spacing. The narrower the fringe is, the smaller the divergence should be. Therefore, the visibility of the fringes is affected by the selection of condenser aperture 2 and defocus.

Schematic illustration of beam divergence effects on Fresnel fringes

Figure 2950. Schematic illustration of beam divergence effects on Fresnel fringes

 

 

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