Imaging and Diffraction in TEM (schematic)

The objective lens forms a diffraction pattern in the back focal plane with electrons scattered and diffracted by the sample and combines them to generate an image in the image plane (1. intermediate image). Thus, diffraction pattern and image are simultaneously present in the TEM. It depends on the focussing of the intermediate lens which of them appears in the plane of the second intermediate image and magnified by the projective lens system on the viewing screen. Switching from real space (image) to reciprocal space (diffraction pattern) is easily achieved by changing the strength of the intermediate lens.

In imaging mode, an objective aperture can be inserted in the back focal plane to select one or more beams that contribute to the final image (BF, DF, HRTEM). For selected area electron diffraction (SAED), an aperture in the plane of the first intermediate image defines the region of which the diffraction is obtained.