High-resolution electron holography
The performance of an electron microscope usually is described in Fourier space by means of the wave transfer function WTF(R) = B(R) exp(iχ(R)) with the pupil function B(R) and the wave aberration χ(R). R means the spatial frequency, rotational symmetry is assumed.The exp(iχ(R)) term describes the coherent transfer of the object wave a(r)exp(iφ(r)) into the image wave A(r)exp(iФ(r)). For weak specimen (a≈1 and φ<<27π) this transfer can be sketched by means of fig. 1 which shows that a mixing occurs between the amplitudes and phases according to the respective transfer functions cosχ and sinχ. Usually, it is desirable to direct by an appropriate wave aberration x the phase of the object wave - containing the most interesting information about the object structure - into the image intensity A2. This is achieved best at Scherzer focus, however, only a comparably narrow band of spatial frequencies is evenly transferred. Lower spatial frequencies (“Large area phase contrast“) are not found in the image.