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Bright field (reflected) - This is the most common
technique for parts inspection. The light passes through the
center of the objective, is reflected off the sample and transmitted
back through the objective where it can be observed. Make sure
your objectives are
made for reflected light, and are in good condition, as some
do not have the correct coatings, or have had the coatings worn
off. This, or dirt on the lens, will cause the image to appear
washed out. |
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Dark field (reflected) - This technique is great for
detecting surface contamination, scratches on wafers or
edge definitions. In this technique the light is blocked
at the illuminator from passing through the center lenses
and passes through a portion around these lenses. The
light strikes the sample at a low angle. Light may be
reflected by a particle or edge causing it to travel
through the center lenses where it can be observed. |
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DIC (reflected)- Differential Interference Contrast or
Nomarski illumination is a technique that uses
interference to bring out surface detail. There are three
things required for Nomarski: a polarizer,an analyzer and
a modified Wolliston prism. Polarized light from the
illuminator passes through the modified Wolliston prism
where it is divided into two rays that are very close
together. These two rays reflect off the sample and back
through the Wolliston prism where they are rejoined. The
light then passes thorough an analyzer before the eye
pieces. This allows small differences in the optical path,
due to height or angle of reflection, to be observed as a
change in color or intensity. One of the better techniques
for telling small changes in height or slope |
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Fluorescent ring light - The ring light provides
an even shadow-free illumination of the sample. The main difference
among different models is in the color temperature of the lamp.
Most lamps come in varying temperatures from a blue-white color
to a yellow-white color. Polarized and yellow filters are available
for photo resist coated components. |
Example image
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Fiber Optic ring light - Like fluorescencent
ring lights these provide a shadow free form of illumination.
They usually use a halogen lamp as a light source so the color
is warmer than the fluorescencent lamps. great for assembly but
poor for finding solder flaws. some come with a filter holder
so you can control the color easily. |
Example image
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Oblique - This is one of the most basic ways
to illuminate objects under the stereo microscope. It consists
of a lamp or fiber optic bundle that is placed off the optical
axis of the microscope. You will get shadows, but sometimes they
can be a good thing. Besides, it's cheap. |
Example image
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Co axial - This is similar to the compound Bright
Field technique. The one draw back is that at powers lower than
1.0x the illumination spot is smaller than the field of view.
Thus the center spot is illuminated while as much as 70% of the
surrounding field is dark. The angle of the stereo microscope
needs to be perpendicular to the work surface in order to get
the most out of this type of illumination. |
Example image
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Transmitted - Only used on transparent samples.
This technique is great for through-hole boards or masks. |
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Transmitted bright field - Most commonly used
in the biological field, this technique is the most basic for
the compound microscope. Light travels through a sub stage collecting
lens and sample before it gets to the objective and eye pieces. |
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Phase contrast - Requires two parts; the phase
ring in the illumination path (one for each power objective)
and phase contrast objectives. The light passes through the ring
where the center of the light is blocked. The remaining light
travels through the sample as a hollow cone. The light that is
undeviated as it passes though the sample passes through a thin
ring in the objective while the light that is diffracted passes
through a thicker portion of the objective. The result is that
the diffracted light and the undeviated light are now 1/2 wave
length out of phase as they approach the image plane. At the
image plane they form destructive interference so that parts
of the sample appear dark against a lighter background. |
