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

The Transmission Electron Microscope 79
(a)
S
PP
BFP
(b)
S
PP
BFP
u ~ f
f
objective
aperture
selected-area
aperture
specimen
image
Figure 3-12. Formation of (a) a small-diameter nanoprobe and (b) parallel illumination at the
specimen, by means of the pre-field of the objective lens. (c) Thin-lens ray diagram for the
objective post-field, showing the specimen (S), principal plane (PP) of the objective post-field
and back-focal plane (BFP).
In fact, in a modern materials-science TEM (optimized for highresolution
imaging, analytical microscopy, and diffraction analysis of nonbiological
samples), the specimen is located close to the center of the
objective lens, where the magnetic field is strong. The objective pre-field
then exerts a strong focusing effect on the incident illumination, and the lens
is often called a condenser-objective. When the final (C2) condenser lens
produces a near-parallel beam, the pre-field focuses the electrons into a
nanoprobe of typical diameter 1 – 10 nm; see Fig. 3-12a. Such miniscule
electron probes are used in analytical electron microscopy to obtain chemical
information from very small regions of the specimen. Alternatively, if the
condenser system focuses electrons to a crossover at the front-focal plane of
the pre-field, the illumination at the specimen is approximately parallel, as
required for most TEM imaging (Fig. 3-12b). The post-field of the objective
then acts as the first imaging lens with a focal length f of around 2 mm. This
small focal length provides small coefficients of spherical and chromatic
aberration and optimizes the image resolution, as discussed in Chapter 2.
In a biological TEM, atomic-scale resolution is not required and the
objective focal length can be somewhat larger. Larger f gives higher image
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