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Transmission electron microscopy (TEM) is an imaging technology in which electron beams pass through very thinly sectioned specimens. As the electrons are transmitted through the specimen and interact with its structure, an image resolves that is magnified and focused onto an imaging medium, such as photographic film or a fluorescent screen, or captured by a special CCD camera. Because the electrons used in transmission electron microscopy have a very small wavelength, TEMs can image at much higher resolutions than conventional optical microscopes that depend on light beams. Due to their higher resolving power, TEMs play an important role in the fields of virology, cancer research, the study of materials, and in microelectronics research and development.
The first TEM prototype was built in 1931, and, by 1933, a unit with a resolving power greater than light had been demonstrated using the images of cotton fibers as a test specimen. Over the next few decades, the imaging capabilities of transmission electron microscopy were refined, making the technology useful in the study of biological specimens. Following the introduction of the first electron microscope in Germany in 1939, further developments were delayed by World War II, in which a key laboratory was bombed and two researchers died. Following the war, the first electron microscope with 100k magnification was introduced. Its fundamental multi-stage design can still be found in modern transmission electron microscopy.
As TEM technology matured, a related technology, scanning transmission electron microscopy (STEM), was refined in the 1970s. The development of the field emission gun and an improved objective lens permitted the imaging of atoms using STEMs. Much of the development of STEM technology resulted from advancements in transmission electron microscopy.
TEMs usually incorporate three lensing stages: the condensing lens, the objective lens, and the projector lens. The primary electron beam is formed by the condensing lens, while the objective lens focuses the beam that passes through the specimen. The projecting lens expands the beam and projects it onto the imaging device, such as an electronic screen or sheet of film. Other specialized lenses are used to correct beam distortions. Energy filtering is also used to correct chromatic aberration, a form of distortion caused by the inability of a lens to focus all colors of the spectrum at the same point of convergence.
While various transmission electron microscopy systems differ in their specific designs, they have several components and stages in common. The first of these is a vacuum system that generates the electron stream and incorporates electrostatic plates and lenses with which the operator can direct the beam. The specimen stage includes airlocks that permit inserting the object to be studied into the stream. Mechanisms in this stage permit positioning the specimen for an optimal view. An electron gun is used to "pump" the electron stream through the TEM. Finally, an electron lens, acting similarly to an optical lens, reproduces the object plane.