TEM: Imaging Methods:

• We shall briefly cover the different imaging modes to understand the microstructure of the sample. For simple analysis, there are four different imaging modes: (i) Bright-field imaging, (ii) Dark-field imaging, (iii) Diffraction mode and (iv) High-resolution mode.

• In TEM, imaging methods utilize the information contained in the electron waves exiting from the specimen to form an image. The projector lenses allow for the correct positioning of this electron wave distribution onto the viewing system. The observed intensity of the image (I) can be approximated as proportional to the time-average amplitude of the electron wave functions.

• Different imaging methods, therefore, attempt to modify the electron waves exiting the sample in a form that is useful to obtain information with regards to the sample, or beam itself.

• To improve the contrast in the image, the TEM may be operated at a slight defocus to enhance contrast, owing to convolution by the contrast transfer function of the TEM, which would normally decrease contrast if the sample was not a weak phase object.