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An electron microscope is an instrument, which like any other microscope, is used to magnify the object or specimen to be observed. However, an electron microscope has the capability of magnifying the specimen up to 10,000,000 times the actual size, whereas the ordinary microscopes can magnify only up to 2000 times the actual size. Hence it is most commonly used to examine microorganisms, organic cells, molecules, viruses etc, which otherwise are not visible through ordinary microscopes.
The principle that this microscope uses to magnify the image also is very different. Unlike ordinary microscopes that use light to illuminate the specimen, an electron microscope uses electrons (sub atomic particles that carry negative charge) beam to illuminate the specimen. Since the wavelength of electrons are much shorter than that of light, it strikes most objects in its path and thus increases the resolution. The resolution of an electron microscope can go beyond 50 picometre as compared to an ordinary microscope, whose resolution is just about 200 nanometre.
The lenses used in electron microscope are also different from that used in ordinary microscopes. Since electron beams are used for illumination, electromagnetic and electrostatic lenses control these beams and focus it. The electron beams transmitted hit the specimen and get diffracted. These lenses re-focus the diffracted beams in order to form an image. This image is the magnified form of the specimen. This image can, not only be viewed on a detecting screen, but can also be recorded using photographic films or digital camera and can be viewed later. The uses of electron microscope is not only limited to scientific research, but can also be used in pathology like observing biopsy samples and can be used in industries to observe fabrication of semiconductor devices.
Types of electron microscope:
1. Transmission Electron Microscope (aka TEM): This is an original form of an electron microscope. In this type, there is a high voltage electron beam used to create the magnified image. This high voltage beam is used to shoot electrons through the object. This beam passes through a condenser lens that helps in concentrating the beam on the object. Some of these electrons pass completely through the object and the others hit the object and get diffracted. This beam then passes through an objective lens, which magnifies it. This is then projected on to a fluorescent viewing screen. The magnified image hence produced is two dimensional in nature.
2. Scanning Electron Microscope (aka SEM): Unlike the TEM, this microscope does not use high voltage electron beam to go through the object. Instead, the beam just scans across the object. Once this electron beam hits the object, it loses different amounts of energy depending on the surface of the object it hits. The display of the SEM shows this variance in the intensity of electrons as an image. The magnified hence produced is three dimensional in nature. However, the image resolution of an SEM is relatively poorer to that of the TEM.
3. Reflection Electron Microscope (aka REM): As is the TEM, an electron beam hits the surface of the object. The reflected beam is then projected on to the viewing screen.
4. Scanning Transmission Electron Microscope (aka STEM): As in the TEM, the high voltage electron beam is made to hit the specimen. However this beam is thinned and is scanned over the specimen in a raster. This helps in attaining the high resolution as that of a TEM.
5. Low Voltage Electron Microscope (aka LVEM): This is a combination of the first three types mentioned. However, unlike TEM, this uses a low voltage electron beam, which hit the specimen with relatively low force and hence increases the image contrast. Although this type of microscope is used in many disciplines of study, it can never replace the conventional high voltage electron microscopes.
The electron microscopes are mostly used in studies related to Biology or Life Sciences, Gemology, Medical or Forensic Sciences, Metallurgy, Semiconductor Inspection, Clinical Microbiology etc.
Some of the manufacturers of electron microscopes are Phenom World, Hitachi, Carl Zeiss, Cordouan, Nikon, Nano Scientific, MTI Instruments, JEOL, Agilent Technologies, Omicron, Applied Materials, ASPEX Corporation, KLA-Tencor Corporation, FIB and many others.
You can also purchase brand new and used electron microscopes of different makes from www.oxfordlabs.co.uk