Scanning Electron Microscope
This article discusses the Scanning Electron Microscope or SEM. This microscopy instrument consists of an inverted column or an electron gun at the bottom, three electrostatic lenses and electromagnetic scan coils placed between the second and third lenses. The intensity of the image forming beam of this microscope is proportional to the scattering or secondary emission of the specimen where the probe strikes it. The more electrons a particular region emits, the brighter the image at that point. Scanning Electron Microscope images typically contain a good deal of topographical detail. A photo multiplier tube detects the scintillations on a phosphor screen with an addition of a light pipe. The light pipe allows direct optical coupling between the scintillator and the photo multiplier tube. By utilizing this microscopes inverted column, electromagnetic lenses, double deflections scan system and stigmation coils, these improvements combined in one instrument can greatly improve the efficiency caused by the secondary electron emissions.
This type of microscope utilizes an electron beam, which is used to produce the image on a screen. This microscope uses electromagnets making the observer achieve more control during his experiments in how much magnification he or she obtains. The electron beam of this certain microscope also provides greater clarity in the image produced. Rather than using lenses and light, a high voltage electron beam is scanned across the surface of a sample and detectors to form an image acquire scattered electrons. The opportunity to examine samples at high magnification generated by electron microscopes provides information that would not be detected by light microscopes.
Nowadays the Scanning Electron Microscope is one of the most versatile and widely used tools and microscopy instruments of modern science as it allows the study of both morphology and composition of a large range of materials. By scanning an electron probe across a specimen, high resolution images of the morphology or topography of a specimen, with great depth of field, at very low or very high magnifications can be obtained.
The scanning electron microscope generates a beam of electrons in a vacuum. That beam is collimated by electromagnetic condenser lenses, focused by an objective lens, and scanned across the surface of the sample by electromagnetic deflection coils. The primary imaging method is by collecting secondary electrons that are released by the sample. The secondary electrons are detected by a scintillation material that produces flashes of light from the electrons. The light flashes are then detected and amplified by a photo multiplier tube.
One of the main advantages that Scanning Electron Microscope has brought to us is that it takes us into a microscopic world beyond the reach of visible light. Just like light waves, into a narrow beam, electrons from a metal filament are collected and focused, the beam scans across the subject, synchronized with a spot on a computer screen. Electrons scattered from the subject are detected and create a current, the strength of which makes the spot on the computer brighter or darker. This creates a photograph like image with an exceptional depth of field. Magnifications of several thousand times are possible. Generally Scanning Electron Mocroscopes are black and white, and are later colored digitally, but special coloring techniques involving multiple electron detectors are possible.
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