We’ve already mentioned in Chapters 2–4 that TEM image contrast arises because of the scattering of the incident beam by the specimen. The electron wave can change both its amplitude and its phase as it traverses the specimen and both types of change can give rise to image contrast. Thus a fundamental distinction we make in the TEM is between
. In most situations, both types of contrast actually contribute to an image, although we usually select conditions so that one will tend to dominate. In this chapter, we’ll discuss only amplitude contrast and we’ll see that there are two principal types, namely,
. This kind of contrast is observed in both TEM and STEM and in both BF and DF images. We’ll discuss the important differences between the images formed in each of these two modes of operation.