CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

electrons (i.e. diffraction mode). In TEM, contrast formation depends greatly on the mode of
operation. In conventional TEM, contrast is obtained by two modes namely the massthickness
contrast and the diffraction contrast and both are based on amplitude contrast. In
high resolution transmission electron microscopy (HRTEM), image contrast is due to phase
contrast. Mechanism of all these types of contrasts is briefly discussed below.
Mass-thickness contrast: This is the common mode of operation in TEM and it is called as
bright field imaging. In this mode, the contrast formation is obtained directly by occlusion
and absorption of electrons in the sample. Thicker regions of the sample or regions with a
higher atomic number will appear dark, while the regions with no sample in the beam path
will appear bright – hence the term "bright field". The image is in effect assumed to be a
simple two-dimensional projection of the sample. The effect of thickness and mass number of
the sample on the brightness of the image can be seen in the Fig.13 (b).
(a)
(b)
Fig.13. (a) Simplified ray diagram of TEM, (b) Mass-thickness contrast.
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