Polarization Microscopy - Biomedical Optical Phase Microscopy and Nanoscopy - page 322

There are also materials whose anisotropy is called biaxial. For their explanation, the reader
is referred to Refs. [36 39] .

15.7.4 Compensator

A compensator is an optical device that includes one or more retarder plates and is
commonly used to analyze the birefringence of a specimen. For a traditional polarizing
microscope, several types of compensators exist that typically use a single fixed retarder
plate mounted in a mechanical rotation stage. With the help of a compensator, it is
possible to distinguish between the slow and fast axis direction and to measure the
retardance of a birefringent object after orienting it at 45 with respect to the polarizers of
the microscope.

The LC-PolScope employs a universal compensator that includes two electro-optically
controlled, variable retarder plates. Using the universal compensator, it is possible to
measure the retardance and slow axis orientation of birefringent objects that have any
orientation in the plane of focus.

15.7.5 Dichroism

Dichroism is a material property that can occur in absorbing materials in which the light-
absorbing molecules are arranged in a nonrandom orientation. Dichroism refers to the
difference in the absorption coefficients for light polarized parallel and perpendicular to the
principal axis of alignment.

The measurement of optical anisotropy by the LC-PolScope is affected by the dichroism of
absorbing materials. In nonabsorbing, clear specimens, however, dichroism vanishes and
birefringence is the dominant optical anisotropy measured by the LC-PolScope. Like
absorption, dichroism is strongly wavelength dependent, while birefringence only weakly
depends on wavelength.

15.7.6 Extinction

Extinction (or extinction coefficient) is defined as the ratio of maximum to minimum
transmission of a beam of light that passes through a polarization optical train. Given a pair
of linear polarizers, for example, the extinction is the ratio of intensities measured for
parallel versus perpendicular orientation of the transmission axes of the polarizers
(extinction 5 I jj / I \ ). In addition to the polarizers, the polarization optical train can also
include other optical components, which usually affect the extinction of the complete
optical train. In a polarizing microscope, the objective and condenser lenses are located
between the polarizers and significantly reduce the extinction of the whole setup.