Biomedical Engineering Reference
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scattered by the object recombine to form the image and the resulting image con-
trast is very weak. Contrast is therefore directly related to variations in the number
of electrons scattered or diffracted by the sample.
Phase contrast has another origin: This involves the interference of several
diffracted beams with the incident beam in order to form the image.
As described in the preceding case, contrast is very weak or even nonexistent
(especially for a very thin sample). Yet the diffracted beams (at least in the approx-
imate framework of the weak phase) have undergone a phase shift of
/2 while
passing through the sample. Any objective lens defocus can produce an additional
phase shift of
π
π
/2, resulting in interferences in the image plane between diffracted
beams and the incident beam. These interferences correspond to the contrast in
high-resolution electron microscopy (HRTEM).
5.2 Diffraction Contrast Imaging Modes in TEM and TEM/STEM
The dual role of the objective lens, using a set of magnification lenses, is to simulta-
neously enable observation of the image plane and the back focal plane that contains
the specimen's diffraction pattern. The combination of diffracted beams coming
from the object - focused by the objective lens - forms the magnified image of the
object in the Gaussian plane, as well as its diffraction pattern in the Abbe plane.
Depending on the size and position of the objective apertures located within this
plane, phase contrasts (HRTEM) or amplitude contrasts (bright field, dark field, and
weak beam) can be made as a function of the scattered or diffracted beams selected
(Fig.
3.12)
.
a)
Abbe Plane:
Diffraction Plane
Objective
Diaphragm
Bright Field
Gaussian
Plane
Crystalline
Structure
Objective
Lens
Microcrystalline
Structure
Amorphous
Structure
Fig. 3.12
Different diffraction contrasts in the imaging modes: (
a
) bright-field and (
b
)dark-field
imaging modes. In the diffraction plane, three examples of diffraction patterns are shown: a crys-
talline structure, a microcrystalline structure, and an amorphous structure. The bright-field image
(
a
) situated in the image plane of the objective lens corresponds to monocrystalline spherules, for
which the objective aperture is centered on the optic axis. The dark-field image (
b
) is taken with
an objective aperture centered on a portion of the diffraction rings
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