Fresnel fringes in STEM

Experiments carried out with a field emmission STEM show several Fresnel fringes along specimen edges, similar to CTEM observation. Analysis of the data shows agreement with computer profiles for idealized edges and the influence of defocusing distance and collector aperture size is discussed. In very coherent detection conditions the number of visible fringes is limited maily by signal-to-noise ratio considerations. In order to observe the maximum number of fringes, a collector aperture must be chosen so that there is a compromise between the limits imposed by the coherence of the detection and the signal-to-noise ratio.