Physical Principles of Electron Microscopy: An Introduction to TEM ...
Physical Principles of Electron Microscopy: An Introduction to TEM ...
Physical Principles of Electron Microscopy: An Introduction to TEM ...
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80 Chapter 3<br />
contrast (for a given objective-aperture diameter), which is usually <strong>of</strong> prime<br />
concern because the contrast in tissue-sample images can be very low.<br />
Objective aperture<br />
<strong>An</strong> objective diaphragm can be inserted located at the back-focal plane<br />
(BFP) <strong>of</strong> the post-field <strong>of</strong> the objective lens, the plane at which a diffraction<br />
pattern <strong>of</strong> the specimen is first produced. In this plane, distance from the<br />
optic axis represents the direction <strong>of</strong> travel (angle relative <strong>to</strong> the optic axis)<br />
<strong>of</strong> an electron that has just left the specimen.<br />
Although in practice the objective behaves as a thick lens, we will discuss<br />
its properties using a thin-lens ray diagram in which the focusing deflection<br />
is considered <strong>to</strong> occur at a single plane: the principal plane <strong>of</strong> the lens.<br />
Accordingly, an electron that leaves the specimen parallel <strong>to</strong> the optic axis is<br />
deflected at the principal plane and crosses the axis at the BFP, a distance f<br />
below the principal plane, as illustrated by the solid ray in Fig. 3-12c.<br />
Assuming parallel illumination and correctly-adjusted tilt controls, such an<br />
electron will have arrived at the specimen parallel <strong>to</strong> the axis and must have<br />
remained undeflected (unscattered) during its passage through the specimen.<br />
The dashed ray in Fig. 3.12c represents an electron that arrives along the<br />
optic axis and is scattered (diffracted) through an angle � by interaction with<br />
one or more a<strong>to</strong>ms in the specimen. It therefore leaves the specimen on the<br />
optic axis but at angle � relative <strong>to</strong> it, arriving at the principal plane at a<br />
radial distance from the axis equal <strong>to</strong> R = u tan ��f tan �. After deflection<br />
by the objective, this electron crosses the optic axis at the first image plane, a<br />
relatively large distance (v � 10 cm) from the lens. Below the principal<br />
plane, the dashed ray is therefore almost parallel <strong>to</strong> the optic axis, and its<br />
displacement from the axis at the back-focal plane is approximately R . By<br />
inserting an aperture <strong>of</strong> diameter D (centered around the optic axis) at the<br />
BFP, we can therefore ensure that the electrons that pass through the rest <strong>of</strong><br />
the imaging system are those with scattering angles between zero and �,<br />
where<br />
�� tan� � R/f = D/(2f ) (3.9)<br />
<strong>Electron</strong>s scattered through larger angles are absorbed by the diaphragm<br />
surrounding the aperture and do not contribute <strong>to</strong> the final image. By making<br />
� small, we can ensure that almost all scattered electrons are absorbed by the<br />
diaphragm. As a result, regions <strong>of</strong> specimen that scatter electrons strongly<br />
will appear as dark areas (due <strong>to</strong> fewer electrons) in the corresponding final<br />
image, which is said <strong>to</strong> display scattering contrast or diffraction contrast.