Polarization Microscopy - Biomedical Optical Phase Microscopy and Nanoscopy - page 324

the field vector describes an ellipse. The ellipse has long and short principal axes that are
orthogonal to each other and have fixed orientation.

Any type of polarization (linear, circular, or elliptical) can be transformed into any other
type of polarization by means of polarizers and retarders.

15.7.13 Polarizer

A polarizer, sometimes called a polar, is a device that produces polarized light of a certain
kind. The most common polar is a linear polarizer made from dichroic material (e.g., a
plastic film with small, embedded iodine crystals that have been aligned by stretching the
plastic), which transmits light of one electric field direction while absorbing the orthogonal
field direction. Crystal polarizers are made of birefringent crystals that split the light beam
into orthogonal linear polarization components. A polarizer that produces circularly
polarized light, a circular polarizer, is typically built from a linear polarizer followed by a
quarter wave plate.

The LC-PolScope employs a universal compensator that also serves as a universal polarizer
in that it converts linear polarization into any other type of polarization by means of two
variable retarders.

15.7.14 Retardance

Retardance is a measure of the relative optical path difference, or phase change, suffered by
two orthogonal polarization components of light that has passed through an optically
anisotropic material. Retardance is also called differential or relative retardation. Retardance
is the primary quantity measured by a polarizing microscope. Assume a nearly collimated
beam of light traversing a birefringent material. The light component that is polarized
parallel to the high refractive index axis travels at a slower speed through the birefringent
material than the component polarized perpendicular to that axis. As a result, the two
components, which were in phase when they entered the material, exit the material out of
phase. The relative phase difference, expressed as the distance between the respective wave
fronts, is called the retardance:

retardance R 5 ðn e 2 n 0

Þ U l 5 Δn U l ;

where l is the physical path length or thickness of the birefringent material. Hence,
retardance has the dimension of a distance and is often expressed in nanometers.
Sometimes, it is convenient to express that distance as a fraction of the wavelength λ ,
such as λ /4 or λ /2. Retardance can also be defined as a differential phase angle, in which

/4 corresponds to 90 and /2 to 180 phase difference.

case λ λ