Polarization Microscopy - Biomedical Optical Phase Microscopy and Nanoscopy - page 321

15.7 Glossary of Polarization Optical Terms
The following is a brief introduction to terms that are relevant for observations with a
polarized light microscope. A more detailed explanation of these and other terms that
describe physical phenomena or optical devices can be found in Refs. [36 39] .
15.7.1 Analyzer
An analyzer is a polarizer that is used to analyze the polarization state of light (see
Section 15.7.13 ).
15.7.2 Azimuth
The azimuth is an angle that refers to the orientation of the slow axis of a uniformly
birefringent region. The azimuth image refers to the array of azimuth values of a
birefringent specimen imaged with the LC-PolScope. The azimuth is typically measured
from the horizontal orientation with values increasing for counterclockwise rotation.
Angles range between 0 and 180 , with both endpoints indicating horizontal orientation.
15.7.3 Birefringence
Birefringence is a material property that can occur when there is molecular order, that is,
when the average molecular orientation is nonrandom, as in crystals or in aligned polymeric
materials. Molecular order usually renders the material optically anisotropic, leading to a
refractive index that changes with the polarization of the light. Birefringence is defined as
the difference in refractive index for light of different polarization.
Many types of molecular order lead to what is called uniaxial anisotropy, i.e., light
propagating through a uniformly aligned material suffers phase retardation that depends on
the polarization of the light, except when the light propagates along one unique direction,
which is called the optic axis. Light propagating through the anisotropic material along the
optic axis experiences the so-called ordinary refractive index ( n o ), which is the same for all
polarization directions. However, light that propagates in any other direction through the
material experiences differences in the refractive index, depending on the light's
polarization. Light that propagates perpendicular to the optic axis experiences the largest
difference in refractive index when polarized parallel versus perpendicular to the optic axis.
When polarized perpendicular to the optic axis, it is the ordinary refractive index n o , and
when polarized parallel to the optic axis, the light experiences the extraordinary refractive
index n e . Birefringence is the difference n e 2 n o 5 Δn and characterizes the anisotropy of
refraction of the uniaxial material.