diffractioncontrast

Diffraction contrast is a term used in electron microscopy to describe image contrast produced by the diffraction of electrons by crystalline lattices. It arises when the distribution of diffracted and forward-scattered electrons varies across the specimen due to changes in crystal orientation, thickness, or the presence of defects, causing local brightness differences in the image.

In transmission electron microscopy, Bragg diffraction governs which lattice planes scatter electrons into a chosen diffracted beam. By placing the objective aperture to transmit either the unscattered beam (bright-field) or a selected diffracted beam (dark-field), regions of the sample that satisfy a given orientation appear brighter or darker. The resulting contrast is strongly influenced by sample thickness, orientation relative to the diffraction vector g, and local strain or defects. A simple two-beam model captures the basic dependence, but real samples often involve many diffracted beams and dynamic scattering, which can complicate interpretation.

Diffraction contrast is especially useful for imaging crystalline defects. Dislocations create distinctive contrast fields in the nearby lattice planes, while grain boundaries, stacking faults, and phase boundaries alter the set of diffracted beams and thus the image brightness. It is also exploited to map thickness variations in thin foils and to identify crystallographic orientations in specific grains.

Limitations include sensitivity to specimen orientation, thickness, and multiple scattering, which can make quantitative interpretation challenging. Proper interpretation typically relies on selection of appropriate diffraction conditions, simulations, and complementary imaging modes such as bright-field or high-resolution imaging.