Numerical Evaluation of Electron Image Phase Contrast

The resolving power and general quality of electron microscope images are determined basically by image plane contrast. There are two distinct contrast mechanisms: (1) amplitude contrast -- produced by removal from the image plane of electrons scattered outside the objective half-angle, /30bj (2) phase contrast -- produced by suitable recombination at the image plane of waves scattered within the objective aperture. The first mechanism, amplitude contrast, is the one commonly operating in images of objects which are greater than about 10 A in size. The contrast between two image points for this type of object is either mass thickness or diffraction and is given approximately by A I / I = A (Qt) = QAt or tAQ. (la) for amorphous materials. (See Ref. 1, Ch. IX, for diffraction contrast.) Here Q is the cross section for scattering outside the objective aperture and t is the object thickness. In turn, the cross section for scattering outside the aperture is Q = ornp A (lb) with No = 6.02 X 1023 being Avogadro's number. Here, A is the atomic weight and p the density. The cross section per atom for scattering outside the aperture is o-at0m and consists of an elastic and an inelastic part Oatom = fel + O"inol · (lc)