Transmission Electron Microscope

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Transmission Electron Microscope
Basic premise of a TEM is to project a magnified
image of the specimen onto a fluorescent screen
where it can be viewed by the operator. The
image itself is the result of beam electrons that
are scattered by the specimen vs. those that are
not.
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A simplified ray diagram of a TEM consists of an
electron source, condenser lens with aperture,
specimen, objective lens with aperture, projector
lens and fluorescent screen.
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In actuality a modern TEM consists of many more
components including a dual condenser system,
stigmators, deflector coils, and a combination
intermediate and dual projector lens.
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Total magnification in the TEM is a combination
of the magnification from the objective lens
times the magnification of the intermediate lens
times the magnification of the projector lens.
Each of which is capable of approximately
100X. Mob X Mint X Mproj Total Mag
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Under different conditions of high and low
magnification the same component can have
different functions
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Depth of Field the range of distance at the
specimen parallel to the illuminating beam in
which the object appears to be in focus.
Depth of Focus the range of distance at the
image plane (i.e. the eyepiece, camera, or
photographic plate) in which a well focussed
object appears to be in focus.
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Condenser lens
The role of the condenser lenses is to make the
beam that is striking the specimen as nearly
parallel as is possible
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As magnification increases the condenser lens
must be adjusted to properly illuminate the
specimen. When the lens is brought to its
smallest spot the beam is said to be at the
crossover point
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Holey (not Holy) Formvar is used to critically
adjust the stigmation of a TEM. When the beam is
under or over focused on the specimen a Fresnel
fringe becomes visible due to the effects of
diffraction around the edges of the whole. When
this Fresnel fringe is evenly distributed then
the beam is said to be stigmated.
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Deflection Coils
In older TEMs functions such as gun and beam
alignment were accomplished by physically moving
components in the column. Today they are achieved
by use of electromagnetic deflection coils that
are positioned throughout the column
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Using the deflection coils the beam can be
shifted so that the focused beam is centered in
the back focal plane of the lens and tilted so
that the beam is centered on the specimen.
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We call these centering spots pivot points and
as the beam is shifted or tilted back and forth
there should be no apparent movement if the scope
is properly aligned
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Likewise the position of the specimen relative to
the beam is of critical importance. To adjust
the height or Z position of the specimen it is
physically rocked back and forth until an object
in the center no longer moves
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A major problem with TEM lens design is the fact
that there is very little space between the pole
pieces of the objective lens to
accommodate the specimen and objective aperture.
This also puts severe constraints on how far the
specimen can be tilted within the lens.
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A clever solution to this problem came in the
invention of the double objective or twin lens.
In this lens design the
specimen actually lies between two separate lens
fields allowing for tilt angles as large a 60o
from perpendicular to the optical axis
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Focus Wobbler Shifts the image back and forth,
and when the movement is stable the image is
focused
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Focus Wobbler Shifts the image back and forth,
and when the movement is stable the image is
focused
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Focus Wobbler Shifts the image back and forth,
and when the movement is stable the image is
focused
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The depth of focus is so great for the projector
lens system that an image that is in focus on the
screen will also be in focus on plate film
beneath the screen or even a TV chip below the
plate film