Electron Holography and the Correction of Spherical Aberration
Division of wavefront electron holograms have been recorded using a field emission source and electrostatic biprism to get an off axis reference beam. A schematic diagram of the apparatus used is shown in Fig. 1. The electron beam is focused to a point above the specimen and biprism fiber by a specially designed electron gun. With positive potential applied to the fiber the Fresnel diffraction pattern of the specimen is superimposed on the interference pattern of the two effective sources forming the hologram. Two standard magnetic projector lenses are used to magnify the hologram which is recorded outside the vacuum system directly on film in contact with a phosphor-coated fiber optic faceplate. A He-Ne laser is used for the reconstruction as shown in Fig. 2. Following Meier's analysis it is found that when a parallel beam is used to reconstruct the wavefront, the real image will be in focus a distancein front of the hologram where m is the total magnification of the hologram and μ is the ratio of the illuminating to recording wavelengths.