Biomedical Engineering Reference
In-Depth Information
Electron Gun
Electron Gun
Condensor Lenses
Condensor Lenses
Sample
Sample
Energy Filter
Projector Lenses
Projector Lenses
Viewing Screen
Magnified Image
Viewing Screen
Magnified Image
Energy Filter
Outside the Column
Energy Filter in
the Column
(a)
(b)
Fig. 3.4
(
a
) Conventional (CTEM) microscope and (
b
) Zeiss TEM microscope with an Omega
energy filter in the column
In a conventional microscope, the image is formed by the recombination of
diffracted beams coming from the object through the objective lens. Image forma-
tion occurs in the Gaussian plane, and the diffraction pattern is located at the Abbe
plane (focal plane or diffraction plane) of the lens as shown in Fig.
3.5.
Plane wave
incidence
-
Reticular
Planes
hkl
object
Focal Plane
Image
Abbe Plane
Diffraction
Image Plane
Magnified Image
Objective
Fig. 3.5
The electron beam's path through a convergent lens
The transmission electron microscope's particularity, compared to the optical
microscope, is that the set of magnification lenses can serve to magnify the image
plane or the diffraction plane. Therefore, it is possible to change from image
mode to diffraction mode by means of a simple commutation that corresponds
to different currents of the projector lenses. Thus, there are three image modes:
low-magnification mode, high-magnification mode, and selected area diffraction
mode, corresponding to intermediate magnification. The diffraction modes include
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