Electron Microscopes

Electron microscopes use a focused beam of electrons instead of light to "image" the specimen and gain information as to its structure and composition.  Common types of information yielded are topography, morphology, composition and crystallographic information.  Electron microscopes can be used in one of several applications including biological or life science, gemological, medical or forensic, metallurgical, measuring or inspection and semiconductor inspection.

Electron microscopes are one of two main types: scanning electron microscopes (SEM) and transmission electron microscopes (TEM).  Scanning electron microscopes are microscopes in which the image is formed by a detector synchronized with a focused electron beam scanning the object. The intensity of the image-forming beam is proportional to the back scattered or secondary emission of the specimen where the probe strikes it.  The magnification is controlled by the length or area scanned.  Transmission electron microscopes (TEM) pass image-forming rays through the specimen being observed.  Contrast or diffracted beam images are used to analyze the sample.

Important parameters in specifying electron microscopes include accelerating voltage, total magnification and resolution.  Accelerating voltage is the range of voltage used to produce electrons in scanning and transmission electron microscopes.  Total magnification is a ratio of the size of an image to its corresponding object. This is usually determined by linear measurement.  Resolution is the fineness of detail in an object that is revealed by an optical device. Objectively, resolution is specified as the minimum distance between two lines or points in the object that are perceived as separate by the human eye. Subjectively, the images of the two resolved points must fall on two receptors (rods or cones), which are separated by at least one other receptor on the retina of the eye.

Features common to electron microscopes are digital displays, computer interfaces, image analysis processing software and environmental, low vacuum or variable pressure chambers, which allows it to maintain a pressure differential between the high vacuum levels required in the gun and column area and the relatively low pressures used in the chamber. This facility means that the microscope can be used to examine uncoated specimens such as type material, pinned insects, mineralogical specimens and fossils.  Variable pressure electron microscopes allow the pressure in the sample chamber to be maintained at a much higher level than in conventional SEMs.  The presence of molecules of air (or inert gas) in the chamber has two major effects. First, it significantly reduces outgassing from samples that are hydrated or oily. This allows these types of samples to be examined without the need for complex sample preparation, such as freeze-drying or critical point drying, etc. The second important benefit is that the air molecules also help dissipate the build up of charge on the surface of nonconducting specimens, which means that they no longer need to be coated with a conducting layer before examination.

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