Linear Imaging Conditions in TEM
- Practical Electron Microscopy and Database -
- An Online Book -

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This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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Following Contrast transfer function (CTF), the wave function of the image can be given by

         perfect coherent illumination in reciprocal space the CTF ---------------------------- [3698]

where,

           A(u) -- Aperture function
           E(u) -- Envelope functions, including Es and Ec as described above
           ψo,x(u) -- Wave function of scattered electrons

From Equation 3698, we can know that when the wave of the incident electrons interacts with a TEM sample the scattered parts (the second term on the right of the equation) will be –π/2 out of phase with the direct beam. These scattered beams will then pass through the objective lens at angles which will cause an additional phase change before being combined with the direct beam in the image plane. If the objective lens also gives a –π/2 phase change (namely the phase contrast transfer function is -1) the scattered beams will be –π out of phase and thus interfere destructively with the direct beam. Since the atoms in the sample are the scattering centers, they will appear darker than the background in the image. This is so-called linear imaging conditions in TEM.

 

 

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