ABSTRACT
Diffraction is unavoidable whenever scattering takes place, it is the fundamental limit to the resolution of small objects. In a system where no image plane exists there may be a useful diffraction pattern; in many cases that is all there is. The breaking apart of out-of-focus images under strong diffraction conditions is used as a method to measure spherical aberration, since the sideways movement of the separate images is a result of this type of aberration. Although an image is easier to relate to intuitively, the diffraction pattern is often easier to describe mathematically. The mechanism of image formation is different for scanning transmission electron microscopy (STEM) and TEM. The main types of diffraction pattern used in both TEM and STEM, apart from Selected area diffraction, are of the converging and stationary beam type. The patterns are classified in terms of convergence angle relative to the diffraction angles associated with the material for the relevant wave-length.
