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

The Transmission Electron Microscope 71
through the specimen should fall within this diameter, which is equivalent to
a diameter of (15 cm)/M = 1.5 �m at the specimen. For viewing larger areas
of specimen, however, the final-image magnification might need to be as
low as 2000, requiring an illumination diameter of 75 �m at the specimen. In
order to achieve the required flexibility, the condenser-lens system must
contain at least two electron lenses.
The first condenser (C1) lens is a strong magnetic lens, with a focal
length f that may be as small as 2 mm. Using the virtual electron source
(diameter ds) as its object, C1 produces a real image of diameter d1. Because
the lens is located 20 cm or more below the object, the object distance u �
20 cm >> f and so the image distance v � f according to Eq. (2.2). The
magnification factor of C1 is therefore M = v / u � f /u � (2 mm) / (200 mm)
= 1/100, corresponding to demagnification by a factor of a hundred. For a
W-filament electron source, ds � 40 �m, giving d1 � Mds = (0.01)(40 �m) =
0.4 �m. In practice, the C1 lens current can be adjusted to give several
different values of d1, using a control that is often labeled “spot size”.
equivalent
lens action
� 1 � 1
cathode
Wehnelt
crossover
equipotential
surfaces
virtual source
anode plate
Figure 3-7. Lens action within the accelerating field of an electron gun, between the electron
source and the anode. Curvature of the equipotential surfaces around the hole in the Wehnelt
electrode constitutes a converging electrostatic lens (equivalent to a convex lens in light
optics), whereas the nonuniform field just above the aperture in the anode creates a diverging
lens (the equivalent of a concave lens in light optics).