Analysis of Object-Image Relationships in Electron Microscopy by Image Processing Techniques

Citation

Horgen, Henri Michel (1975) Analysis of Object-Image Relationships in Electron Microscopy by Image Processing Techniques. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/46Q4-TE10. https://resolver.caltech.edu/CaltechETD:etd-10152002-154431

Abstract

By considering an electron microscope to be an information channel, it is shown that the correspondence between the object and the image can be linked to the electron optical characteristics of the instrument and to the statistical properties of the noise. A discussion of the image formation of a cluster of atoms is introduced in order to demonstrate the main contrast mechanisms that operate at the atomic level. A direct extension of this analysis to more complex specimens gives rise to a wave optical theory of image formation, which is used to present the concept of the amplitude transfer function. This formalism greatly simplifies in the case of weakly scattering objects, where the total object wave is linearly related to both the projected potential distribution of the specimen and the amplitude attenuation of the incident beam. For this category of specimens, there exists a linear relationship between the image intensity in bright-field and the total object wave. Phase and amplitude contrast transfer functions describe the perturbing influence of the objective lens aberrations on the phase and the amplitude of the object wave. If spatial and chromatic incoherence effects are included in this formalism, it is shown that the linearity between image intensity and object is preserved. The validity of the approximations of the wave optical theory is first checked by studying the effect of defocusing on the transfer conditions of the phase and amplitude contrast mechanisms. A medium resolution experiment is conducted on a bright-field image of bovine liver catalase. The results demonstrate a qualitative agreement between experiment and theory. A reconstruction scheme is next implemented on a through-focus series of a specimen of gold on carbon. This scheme is analyzed critically prior to a description of the experimental results. It is demonstrated that under certain conditions this technique is capable of restoring the total object wave and simultaneously achieving a selective contrast enhancement at heavy atom locations. This potential Z discrimination is tested experimentally, and the difficulties encountered during the processing are discussed. A qualitative estimate of all contrast mechanisms that contribute to a high-resolution bright-field micrograph can be inferred from this analysis. The problems which one faces in a quantitative interpretation of micrographs at the atomic level are also discussed, and possible ways to circumvent these problems are mentioned. Next, image processing schemes for improving the signal-to-noise ratio of an image are applied to micrographs of crystalline specimens. The enhancement of lattice fringe images is demonstrated for both silicon and gold specimens. Periodic images can be processed in either real space or Fourier space and an analysis of these processing modes is presented. By enhancing a weak-beam image of a dissociated-dislocation dipole in germanium, a quantitative comparison to a simulated image is rendered possible. The resolving power of the weak-beam technique is analyzed for this particular example. It is found that by selecting a diffraction geometry so that the systematic reflections are dynamically interacting, four dislocation peaks are individually resolved. Problems associated with the contrast interpretation are discussed in conjunction with a calculation of the image contrast. Finally, suggestions for further study of the atomic structure of crystalline defects by the weak-beam method are given.

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