Bright field, dark field, and diffraction images
The most common image generated using a TEM is a bright field image. Some areas of the sample scatter or absorb electrons and therefore appear darker. Other areas transmit electrons and appear brighter. In simple terms the bright field image appears as a shadow of the specimen. In the bright field image the objective aperture is used to select the unscattered electron beam. In doing so, the scattered electrons are excluded from forming the image. This aperture enhances the contrast in the image.
Dark field images are produced by excluding, using the primary aperture, the primary (unscattered) beam from the image collected below the sample. The image is produced by scattered electrons (i.e. only selected electrons are used to form the image). Regions where no scattering occurs, such as where the primary electron beam passes straight through the sample, appear black (e.g. in areas around the sample). This kind of imaging is useful in studying crystal defects, and for the imaging of specific crystallographic phases.
Diffraction images are the result of Bragg scattering as the beam passes through a crystalline sample. If a “selected area diffraction aperture” is inserted to delimit the region of interest, then an image created below the sample (in the region called the back focal plane) is seen as an array of dots (or a set of diffuse rings). This informs the viewer about the crystal structure of the sample.