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Steve's Microscope Central
What Are Compound Light Microscopes?
The bright field microscope is another term for the light microscope, the most basic form of microscope, and this classification includes the category of compound light microscopes. These microscopes are a bit different from other types of light microscopes, in that the light is shone up from beneath the specimen, and is intended to shine through it.
As you know, the ordinary light microscope has a source of light shining down on the specimen from above. There are a few other differences between compound light microscopes and other types, but this is the most basic difference. Compound light microscopes also have a lens to concentrate the light source onto the specimen, above the light, but still below the specimen. Above the lens is a diaphragm called an Iris diaphragm which allows you to adjust the amount of light focusing on the specimen.
What are Scanning Electron Microscopes?
The "Virtual Source" at the top represents the electron gun, producing a stream of monochromatic electrons.
The stream is condensed by the first condenser lens (usually controlled by the "coarse probe current knob"). This lens is used to both form the beam and limit the amount of current in the beam. It works in conjunction with the condenser aperture to eliminate the high-angle electrons from the beam
The beam is then constricted by the condenser aperture (usually not user selectable), eliminating some high-angle electrons
The second condenser lens forms the electrons into a thin, tight, coherent beam and is usually controlled by the "fine probe current knob"
A user selectable objective aperture further eliminates high-angle electrons from the beam
A set of coils then "scan" or "sweep" the beam in a grid fashion (like a television), dwelling on points for a period of time determined by the scan speed (usually in the microsecond range)
The final lens, the Objective, focuses the scanning beam onto the part of the specimen desired.
When the beam strikes the sample (and dwells for a few microseconds) interactions occur inside the sample and are detected with various instruments
Before the beam moves to its next dwell point these instruments count the number of interactions and display a pixel on a CRT whose intensity is determined by this number (the more reactions the brighter the pixel).
This process is repeated until the grid scan is finished and then repeated, the entire pattern can be scanned 30 times per second.
What are Transmission Electron Microscopes?
A TEM works much like a slide projector. A projector shines a beam of light through (transmits) the slide, as the light passes through it is affected by the structures and objects on the slide. These effects result in only certain parts of the light beam being transmitted through certain parts of the slide. This transmitted beam is then projected onto the viewing screen, forming an enlarged image of the slide.
TEMs work the same way except that they shine a beam of electrons (like the light) through the specimen(like the slide). Whatever part is transmitted is projected onto a phosphor screen for the user to see. A more technical explanation of a typical TEMs workings is as follows (refer to the diagram below):
The "Virtual Source" at the top represents the electron gun, producing a stream of monochromatic electrons.
This stream is focused to a small, thin, coherent beam by the use of condenser lenses 1 and 2. The first lens(usually controlled by the "spot size knob") largely determines the "spot size"; the general size range of the final spot that strikes the sample. The second lens(usually controlled by the "intensity or brightness knob" actually changes the size of the spot on the sample; changing it from a wide dispersed spot to a pinpoint beam.
The beam is restricted by the condenser aperture (usually user selectable), knocking out high angle electrons (those far from the optic axis, the dotted line down the center)
The beam strikes the specimen and parts of it are transmitted
This transmitted portion is focused by the objective lens into an image
Optional Objective and Selected Area metal apertures can restrict the beam; the Objective aperture enhancing contrast by blocking out high-angle diffracted electrons, the Selected Area aperture enabling the user to examine the periodic diffraction of electrons by ordered arrangements of atoms in the sample
The image is passed down the column through the intermediate and projector lenses, being enlarged all the way
The image strikes the phosphor image screen and light is generated, allowing the user to see the image. The darker areas of the image represent those areas of the sample that fewer electrons were transmitted through (they are thicker or denser). The lighter areas of the image represent those areas of the sample that more electrons were transmitted through (they are thinner or less dense)
Compound Light Microscopes
Scanning Electron Microscopes
Transmission Electron Microscpes
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