Polarization Microscopy - Biomedical Optical Phase Microscopy and Nanoscopy - page 306

Figure 15.4
Traditional polarized light microscope and image cartoons. (A) Optical arrangement of a
conventional polarizing microscope. (B) Cartoon depicting at its center the image of an aster as it
appears when located between a crossed polarizer P and analyzer A. The arrows on the polarizer
and analyzer sheet indicate their transmission directions. An aster is made of birefringent MT
arrays radiating from a centrosome (aster diameter of 15 m). MTs that run diagonal to the

polarizer and analyzer appear bright, while MTs that run parallel to polarizer or analyzer appear
dark. (C) Cartoon of aster as it appears when located between polarizer, analyzer, and a
compensator C, which is made of a uniformly birefringent plate. The arrow in the compensator
plate indicates its slow axis direction. MTs that are nearly parallel to the slow axis of the
compensator appear bright, while those that are more perpendicular to the slow axis are dark.
Therefore, the birefringence of MTs has a slow axis that is parallel to the polymer axis, as is the
case for many biopolymers.

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compensator and the polarizer and analyzer are in crossed position, so that the analyzer
blocks (absorbs) nearly all the light that has passed through the specimen. In this
configuration, the image of the specimen looks very dark, except for structures that are
birefringent or otherwise optically anisotropic and appear bright against the dark
background ( Figures 15.1 and 15.4B ). When the specimen is rotated on a revolving stage
(around the axis of the microscope), the birefringent parts change brightness, changing
from dark to bright and back to dark four times during a full 360 rotation. A uniformly
birefringent specimen part appears darkest when its optical axes are parallel to polarizer
and analyzer. This is called the extinction position. Rotating the specimen by 45 away
from the extinction position makes the birefringent part appear brightest. When rotating
the specimen, not all birefringent parts in the field of view will turn dark at the same
position, because in general, each specimen part has different axis orientations.
In summary, by rotating the specimen between crossed polarizers, one can recognize
birefringent components and determine their axis orientations.