Practical Electron Microscopy and Database

Unlike the negligible distortions in most CTEM analyses, distortions extremely affect off-axis electron holograms as the phase information is encoded in the bending of the interference fringes [1-3]. The origin of the distortions in off-axis electron holograms are projector lenses, local variations in TEM specimen thickness, charging of the biprism filament, and shear distortion of the fiber optic of the CCD camera. [3] The distortion-induced phase modulation can normally be automatically corrected within the reconstruction process by using a reference hologram, taken by removing the specimen from the field of view using the goniometer of the microscope and without changing the optical parameters. The reference phase is subtracted from the image phase in order to correct the distortion-induced phase modulations.

[1] Rau, W.-D., Lichte, H., Völkl, E. & Weierstall, U. (1991). Real-time reconstruction of electron-off-axis holograms recorded with a high pixel CCD camera. J Comput-Assist Microsc 3, 51–63.
[2] Völkl, E. & Lehmann, M. (1999). The reconstruction of off-axis electron holograms. In Introduction to Electron Holography, Völkl, E., Allard, L.F. & Joy, D.C. (Eds.), pp. 125–151. New York: Kluwer Academics/Plenum Publishers.
[3] Michael Lehmann, and Hannes Lichte, Tutorial on Off-Axis Electron Holography, Microsc. Microanal. 8, 447–466, 2002.