In transmission electron microscopy contrast generates due to difference in the electron densities in the image plane, which can be caused by different microscopic features inside the sample. Due to the scattering of the incident beam by the sample features, different types of contrast such as amplitude contrast, phase contrast of diffraction contrast. Diffraction contrast occurs because of a grain's specific crystallographic orientation. In this case, the crystal is orientated in a way that increases the likelihood of diffraction. Diffraction contrast reveals information about the orientation of crystals in a polycrystalline sample, as well as other details such as defects. Diffraction contrast exhibits when electrons are diffracted from the sample and are dispersed into distinct locations in the back focal plane as a result of diffraction of the electron beam. The intended reciprocal lattice vectors (g) can be chosen (or excluded) by positioning apertures in the back focal plane, or the objective aperture. As a result, only the portions of the sample that are causing the electrons to scatter to the chosen reflections will ultimately be projected onto the imaging device. It is possible to ascertain not only the location of defects but also their type by carefully choosing the orientation of the sample. Any distortion of the crystal plane that locally tilts the plane towards the Bragg angle will result in particularly strong contrast variations if the sample is orientated so that one specific plane is only slightly tilted away from the strongest diffracting angle (referred to as the Bragg Angle). Weaker contrast will result from defects that only cause atoms to move in directions parallel to the crystal plane, which do not tilt the crystal towards the Bragg angle. Under kinematic bright field conditions (Bragg condition met almost, but not quite), the dislocation is imaged as a dark line on a bright background. The magnitude of the contrast depends on the scalar product between the reciprocal lattice vector g and the Burgers vector b, g · b. Dislocations are invisible or exhibit only weak contrast if g · b = 0.
Additionally, internal structure defects of the deformed parts, which are primarily microscopic and involve the deformed microstructure of materials can provide us with lot of information about deformation mechanisms which is directly related to property and the performance of the materials. In a deformed specimen several defect structures can be observed such as: perfect and partial dislocation structures, twins, dislocation loops, deformed grains etc. In order to observe the defect structure a combination of bright field imaging and dark field imaging is used. As the electron beam passes through the deformed specimen, some part of the beam is heavily diffracted due to presence of these defects, hence appears with the dark contrast. As during deformation, dislocation motion becomes active, regions with dark contrast can be observed in the micrograph. This dark contrast region may represent strangled dislocations or dislocation loops. This often reflects in the SAD pattern as due to presence of dislocation alters the shape of the diffraction spots. The diffractions spots may become elongated or stretched or sometimes double-diffraction spot can also be observed if twins are present.

Figure 1. Imaging conditions for dislocations with maximum and minimum gb product.

Figure 2. (a) Bright field TEM images of strangled dislocations (b) Defect structures in Cu (c) Defect structures in Fe (d) Defect structures in Zn