Physical Principles of Electron Microscopy: An Introduction to TEM ...

The Transmission Electron Microscope 73
increase in the beam angle � at the electron-source image plane. In this
context, it should be noted that the convergence angle of the illumination is
always defined in terms of the variation in angle of the electrons that arrive
at a single point in the specimen.
u
v 0
d 1
D
d 0
demagnified source
� �
C2
aperture
specimen
image plane
d 1
(a) (b) (c)
�
v'
d d
�
u
u
v'
d 1
�
�
image
plane
Figure 3-8. Operation of the second condenser (C2) lens; solid rays represent electrons
emitted from the center of the C1-demagnified source. (a) Fully-focused illumination whose
diameter d0 is comparable to the diameter d1 of the demagnified source (see dashed rays) and
whose convergence angle (2�0) depends on the diameter of the C2 aperture. (b) Underfocused
illumination whose diameter d depends on the image distance v and whose convergence angle
� depends on v and on d1. (c) Overfocused illumination, also providing large d and small � .
Figure 3-9. (a) Current density at the specimen as a function of distance from the optic axis,
for illumination fully focused and for defocused (underfocused or overfocused) illumination.
(b) Convergence semi-angle of the specimen illumination, as a function of C2-lens excitation.