The Scanning Electron Microscope (SEM) is a very useful tool for many aspects of science. A biologist might use an SEM to study the tiniest micro-structures of an insect, the geologist might use it to find out what chemicals are present in a rock specimen, and the automobile engineer might use it to find tiny imperfections in a car part. SEM images are created using electrons instead of the photons of light we use to see the world around us. Photons also help us view the microscopic world, as for example, in the familiar light microscope. Electrons are more useful than photons because they have a shorter wavelength. This permits magnified imaging of a specimen by electrons up to 200 times greater than with the light microscope. The theoretical limit of magnification for a light microscope is about 2,000X whereas the best SEM can magnify and resolve a specimen greater than 400,000X!!!

The most common imaging process begins by bombarding a specially prepared specimen with a narrow beam of electrons and scanning that beam back and forth across the surface of the specimen. When these electrons hit the specimen they "kick out" electrons that were on the surface of the specimen. The "kicked out" or secondary electrons are collected by a special detector and processed to display the magnified image on a TV-tube monitor for direct observation. The image can be captured on photographic film as a permanent record, or digitally processed for analysis and storage. The beam is focussed by a lens made from an electromagnet, and the specimen is typically magnified by changing the ratio between the display screen and the area of the specimen scanned.

You can think of the above process as if you were running on the top of a 10-story building and dropping a series of tennis balls, the electron beam, into a big basket of more tennis balls, the specimen. When the tennis balls you drop land in the basket, tennis balls in the basket are "kicked out" by the collision, secondary electrons. If you had a friend on the ground catching the balls that came from the basket, he or she would be like the detector.

The secondary electrons are not the only particles generated by beam and specimen interaction. Backscattered electrons, X-rays, and Auger electrons are also produced and can be used to image specimens for special kinds of observations.

Our high resolution Hitachi scanning electron microscope is equipped with an EDAX X-ray analysis system which includes an energy dispersive X-ray spectrometer and permits the detection of elements down to boron. In concert they provide a powerful tool for correlating the microstructures in a specimen with its elemental composition.

BGSU Home/ Biology Main