Page 154 - Fundamentals of Light Microscopy and Electronic Imaging

OPTICS OF THE POLARIZING MICROSCOPE 137

Image plane

ANALYZER

LOCATION OF
COMPENSATOR

Objective

OBJECT ON
ROTATING STAGE

Condenser

POLARIZER

Source

Figure 9-2
Optical components of a polarizing microscope. Notice the presence of a polarizer and
analyzer, a rotatable stage, and a slot for accommodating a compensator. Polarization
microscopy requires an intense light source.

microscope and looking down the optic axis of the microscope; the analyzer is then
rotated at 90° to the polarizer in a north-south or up-down orientation to obtain extinc-
tion. It is desirable that the polarizer or analyzer be rotatable. The rotatable holder is
often marked in degrees to facilitate adjustment to extinction. Having one of the polar-
izers in a rotatable holder assures proper orientation of the crossed polars along east-
west and north-south axes and allows precise control for obtaining extinction. The use
of a compensator also requires that the polarizers be adjusted precisely, because it is
inserted into the optical path at a fixed orientation. It is also highly desirable that the
specimen stage be rotatable, with its center of rotation adjusted to coincide with the
optic axis of the microscope so that the fixed polarized light beam can illuminate a spec-
imen at various angles simply by rotating the stage. Finally, there is an insertion slot for
a compensator, which is located near and in front of the analyzer. For applications where
birefringence is small and object contrast is very low, it is important to use strain-free,
polarization-grade condenser and objective lenses. Strain birefringence exists in most
lenses as a result of manufacture and mounting in a lens barrel. When severe, birefrin-
gence from a lens can obscure faint signals in images of weakly birefringent objects. For