Electron microscopy was first developed in the early 1930s to solve a problem. Scientists wanted to see very small structures inside cells, and light-based microscopes were unable to reach the magnification they needed. Electron microscopes can achieve magnifications greater than one million times. Their high cost restricts their use to well-funded organizations.
Both transmission and scanning electron microscopes use a high-voltage power source to drive an electron gun. It sends a stream of electrons through a vacuum to a sample being studied. Sets of magnets focus the electrons as a glass lens focuses light. A screen displays the magnified image of the specimen.
Transmission electron microscopy (TEM) is the oldest form of electron microscopy. It sends an electron beam through a thin sample, projecting an image on a fluorescent screen. In most TEMs, a camera picks up the image on the screen, sending it to a computer or other video monitor. Since the electrons are used directly to make the magnified image, the pictures are clearer than images made with a SEM.
The scanning electron microscope (SEM) focuses the electron beam to a very small point on the sample. The SEM scans the electron beam over an area, building up a larger, more useful image. Secondary effects of the electrons hitting the surface, rather than the electrons themselves, are used to make the image. Images taken with SEM show three-dimensional shapes with depth better than those made by TEM.
Samples usually require preparation for both SEM and TEM. Because the sample is in a vacuum and being exposed to an electron beam, you cannot use live biological specimens. Also, the samples need to be at least partially conductive. This means the original sample must be sliced very thin, chemically treated or coated with metal powders.
Researchers use TEMs when they need extreme magnification of samples with high clarity. They can image viruses, pollutants and dust particles. Computer chip manufacturers use TEMs to inspect semiconductors.
A forensic crime lab can use a SEM to examine tiny bits of evidence. It can help identify gunshot residues and compare clothing fibers and bullet holes. Researchers use SEMs when texture and depth of field are important.
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