Bright and Darkfield imaging
Bright field is a common method used to create an image and it is also the most basic one. It is similar to the classic methods which are used in light microscopy. In this mode the contrast formation is formed directly by diffraction and absorption of electrons in the sample. Thicker regions of the sample or regions with a higher atomic number will appear dark, while regions with no sample in the beam path will appear bright.
The image is a simple two dimensional projection of the sample. In this mode it is also possible to make dislocations visible. To do this a crystal lattice needs to be orientated in a way close to the Bragg conditions. The dislocations bent the crystal lattice. As a result at this point the lattice fulfils the Bragg condition and the electron beam will be diffracted strongly. The diffracted beam will be scattered at an aperture. The dislocation appears dark at bright field imaging.
In dark field imaging the diffracted beam appears bright the not diffracted electrons appear dark. This is done while the specimen is at a tilt and an aperture prevents that the not diffracted beam find its path to the imaging device. So dark field is an imaging mode to observe the diffracted electron beam.