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As shown in Figure 3356, in TEM bright-field (BF) mode, an objective aperture is placed in the back focal plane of the objective lens which allows only the transmitted beam to pass. In this case, mass-thickness and diffraction contrast contribute to image formation, that is thick areas, areas with heavy atoms, and crystalline areas in most crystalline orientations appear in dark contrast. The mixed contrasts make the interpretation difficult.

Figure 3356. TEM bright field (BF) mode.