Why Do We See Any Contrast in the Images?

Abstract

If we draw two white points on a white paper we cannot see these points. The white pencil does not create any contrast on the white paper. In transmission electron microscopy we need not only a good resolution limit but also an image contrast. This contrast is the result of the interaction between the beam electrons (primary electrons) und the transmitted specimen. Different to the light optical microscopy where the attenuation of the amplitude of the light wave is mainly responsible for the contrast, the absorption of electrons within the sample (i.e. the attenuation of the amplitude of the electron wave) plays only a secondary role in transmission electron microscopy. Here, the different scattering (deflection) of the electrons acts as the dominant reason for contrast. Later, we will see that, especially at very high magnifications, phase shifts of the electron waves can be responsible for the contrast, too.