On the integration of electron diffraction intensities in the Vincent-Midgley precession technique

In the Vincent-Midgley technique, where the electron beam is precessed around the optical axis, integrated electron diffraction intensities can be measured directly, similar to measurements performed in X-ray diffraction methods. When introduced into the structure determination or refinement schemes, the experimental intensities must be corrected for geometrical factors associated with the precession by a factor analogous to the Lorentz factor used in X-ray diffraction. In this paper, Lorentz factors are derived for reflections in zeroth-and higher-order Laue zones. The result differs from previously published correction factors (Vincent and Midgley, 1994 Ultramicroscopy 55 (1994) 271), the effect on structure determination or refinement being most important where reflections corresponding to a large range of lattice spacings are used. The effect of beam divergence is discussed and appropriate Lorentz factors derived.