Biomedical Engineering Reference
In-Depth Information
Electron
beam
E
lectron gun
Anod
e
Magnetic
lens
To TV
scanner
Scanning
coils
Backscattered
electron
detector
Secondary
electron
detector
Stage
Specimen
1. 10
Diagram of a scanning electron microscope (SEM).
The transmission electron microscope or TEM is used to study the internal
structure of biomaterials at very high levels of resolution. Since this micros-
copy technique is usually used to study the basic structure of materials it will
be briefl y discussed in this section to show how it differs from scanning electron
microscopy. The TEM utilizes a beam of electrons produced by an electron
gun and focused by a series of electron lenses to illuminate a thin specimen
usually mounted on a grid to produce images of structural details in the spec-
imen. As the electron beam passes through the specimen, some electrons are
transmitted or pass through undisturbed while other electrons interact with
the atoms of the specimen and are refracted or scattered. This mechanism is
responsible for the contrast observed in TEM images. The transmitted beam
is fi nally projected onto a viewing screen coated with a phosphor, forming an
enlarged image of the specimen.
