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
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.
Increased Depth of Field