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 Scherzer...