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Oblique Illumination
Oblique illumination is a lighting technique that uses an illuminating beam of
light that is directed onto the stage of the microscope at an
oblique angle rather than a normal angle to the stage. It has a number of very
useful applications that result from the fact that the
numerical aperture (NA) of the light source is greater than that of the
objective in one direction and may be less than that of the objective
at right angles to the plane containing the incident beam axis and the normal to
the stage. It can result in increased resolution, increased
depth of field, greater contrast, and rapid determination of the refractive
index relative to the mounting medium under the right conditions.
A few examples are given below. A simple explanation can be made by an
application of Abbe's theories and Snell's Law. Those will be alluded
to below.
Refractive Index Application
If the illuminating beam traverses the stage at an oblique angle from right to
left then and the object on the stage is mounted in a medium of
lower refractive index then the light at the right side of a transparent object
will be refracted toward the center of the object. The result
is that the beam is refracted at an angle that the objective can't collect. That
side of the object will appear dark. The other side of the
object will appear bright because refraction toward the center of the object
brings more of the oblique light into an angle that can be collected
by the objective. This is a simple explanation but it is a good one for most
situations, such as the examples below.
Contrast Application
Increased Resolution
Increased Depth of Field
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