Principles of TEM Image Formation (II)
Abstract: The flexibility in transmission electron microscopy (TEM), required when converging the beam or switching modes between imaging and diffraction, is provided by a combination of lenses. This paper extends the previous discussion based on geometrical optics to the two-lens system in order to understand the principle of the operation, as well as some practical applications. It also introduces possible consequences of the correction of spherical aberration of the objective lens on the resolution of phase contrast images. That is, it is no longer the contrast-transfer function but the information limit itself that determines the ultimate resolution. It is also pointed out, however, that one needs to select an optimal defocus in order to achieve a better resolution with a reasonable contrast.
Key words: Transmission Electron Microscopy, Two lens system, Convergent Beam Electron Diffraction, Aberration Correction