| Conventional electron diffraction
techniques use a parallel beam of incident radiation,
these techniques are called Selected Area Diffraction
(SAD).
In contrast Convergent Beam Electron
Diffraction (CBED) uses a convergent beam of
electrons to limit the area of the specimen which contributes
to the diffraction pattern.
Each spot then becomes a disc within
which variations in intensity can usually be seen. Such patterns initially seem more
difficult to interpret but they contain a wealth of information about the symmetry and
thickness of the crystal and are widely used in TEM.
The big advantage of CBED over SAD techniques is that most of the information is
generated from small regions beyond the reach of other techniques.
 In this simulation the
relationship between a spot pattern and a convergent beam pattern can be explored by
altering the beam convergence angle.
A convergent beam of electrons creates a
Fresnel
(near field) diffraction pattern.
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