5. Phase contrast

Interference effects (phase effects) form the basis of atomic resolution imaging and electron holography. A large contrast diaphragm is used, allowing several beams to pass through (figure 1 b), and the image results from their interference. The diffracted waves are π/2 out of phase with the incident wave. However, by defocusing the lens (underfocusing, i.e. focusing above the object by a precise amount), an additional phase shift of π/2 with respect to the transmitted beam is introduced into the diffracted beams, which will compensate for the phase shift due to sphericity aberration. It is at this defocus, known as...