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

146 Chapter 5
just-observable loss (�r) in image resolution. As seen from Fig. 5-19b, �r �
�/�v where � is the convergence semi-angle of the probe. Taking �r as
equal to the image resolution, so that the latter is degraded by a factor ��2 by the incorrect focus (as discussed for the TEM in Section 3.7) and taking
this resolution as equal to the probe diameter (assuming SE imaging),
�v � �r / � � d / � (5.5)
Figure 5-18. (a) Low-magnification SEM image of a sea-urchin specimen, in which specimen
features at different height are all approximately in focus. (b) Light-microscope of the same
area, in which only one plane is in focus, other features (such as those indicated by arrows)
appearing blurred. From Reimer (1998), courtesy of Springer-Verlag.
D/2
�
d
image of electron source
demagnified by the
condenser lens(es)
v =WD
objective lens
specimen
(a) (b)
�v
�v
�
�r
v= WD
Figure 5-19. (a) The formation of a focused probe of diameter d by the SEM objective lens.
(b) Increase �r in electron-probe radius for a plane located a distance �v above or below the
plane of focus.