Introduction
In electron microscopic investigations of crystalline specimens the
direct observation of the electron diffraction pattern gives additional
information about the specimen. The quality of this information depends on
the quality of the crystals or the crystal area contributing to the
diffraction pattern. By selected area diffraction in a conventional electron
microscope, specimen areas as small as 1 µ in diameter can be investigated.
It is well known that crystal areas of that size which must be thin enough
(in the order of 1000 Å) for electron microscopic investigations are
normally somewhat distorted by bending, or they are not homogeneous.
Furthermore, the crystal surface is not well defined over such a large area.
These are facts which cause reduction of information in the diffraction
pattern. The intensity of a diffraction spot, for example, depends on the
crystal thickness. If the thickness is not uniform over the investigated
area, one observes an averaged intensity, so that the intensity distribution
in the diffraction pattern cannot be used for an analysis unless additional
information is available.
