A reflection on partial coherence in electron microscopy.

The concept of partial coherence, which originated in light optics, is investigated in the case of dark-field electron microscopy. Two atoms arbitrarily arranged in the object space are considered as sources of the secondary waves. The influence of lens aberrations is taken into account. The real degree of coherence gamma r is calculated for the STEM as well as the CTEM. In the case of atomic resolution, the finite size of the atoms significantly influences the degree of coherence, so that it becomes a function of the image coordinate. Analytical expressions are obtained for gamma r which are valid at present-day resolutions for both the CTEM and STEM. The dark-field intensity distribution in the detector plane of a STEM is calculated for an object consisting of two atoms in different configurations and for different positions of the scanning spot.