Polarized Light Microscopy
Section Overview:
The polarized light microscope is designed to observe and photograph specimens that are visible primarily due to their optically anisotropic character. In order to accomplish this task, the microscope must be equipped with both a polarizer, positioned in the light path somewhere before the specimen, and an analyzer (a second polarizer), placed in the optical pathway between the objective rear aperture and the observation tubes or camera port. Image contrast arises from the interaction of plane-polarized light with a birefringent (or doubly-refracting) specimen to produce two individual wave components that are each polarized in mutually perpendicular planes. The velocities of these components are different and vary with the propagation direction through the specimen. After exiting the specimen, the light components become out of phase, but are recombined with constructive and destructive interference when they pass through the analyzer. Polarized light is a contrast-enhancing technique that improves the quality of the image obtained with birefringent materials when compared to other techniques such as darkfield and brightfield illumination, differential interference contrast, phase contrast, Hoffman modulation contrast, and fluorescence.
Review Articles
Polarization of Light
When the electric field vectors are restricted to a single plane by filtration then the light is said to be polarized with respect to the direction of propagation and all waves vibrate in the same plane.
Optical Birefringence
When light enters a non-equivalent axis in a anisotropic crystal, it is refracted into two rays each polarized with the vibration directions oriented at right angles to one another, and traveling at different velocities.
Polarized Microscope Configuration
Although similar to the brightfield microscope, the polarized light microscope contains additional components unique to this class such as a strain-free objective, condenser, and a circular graduated stage capable of 360-degree rotation.
Polarized Microscope Alignment
The series of alignment steps outlined in this section are intended for general use with polarized light microscopes and should be applicable to both student and research-level instruments.
Compensators and Retardation Plates
Optical anisotropy is studied in the polarized light microscope with plates divided into two categories: retardation plates (fixed optical path difference) and compensators (variable optical path lengths).
Michel-Levy Birefringence Chart
Birefringence is determined by a family of lines that emanate radially from the origin, each with a different measured value of birefringence corresponding to thickness and interference color.
Interactive Java Tutorials
Double Refraction (Birefringence)
Explore this simulated viewing of a ball-point pen and a line of text through a crystal of Iceland spar, producing a double image from the refracted light rays in this interactive java tutorial.
Brewster's Angle
See the polarization effect on light reflected at a specific angle (the Brewster angle) from a transparent medium with the adjustable parameters including the incident beam wavelength and refractive index of the dielectric medium.
Birefringent Crystals in Polarized Light
Explore using this virtual microscope viewport presenting a crystal as it would appear in the eyepieces of a microscope under crossed-polarized illumination as it is rotated around the microscope optical axis.
Polarization of Light
Linearly polarized light transmitted through a polarizer can be either passed or absorbed by a second polarizer. Examine the effect of rotating two polarizers on an incident beam of white light.
Polarization of Light (3-D Version)
Explore the effects of two polarizers having adjustable transmission axes on an incident beam of white light, enabling the visitor to translate the optical train in three dimensions in this interactive tutorial.
Electromagnetic Wave Propagation
Explore and discover propagation of a virtual electromagnetic wave and consider the orientation of the magnetic and electric field vectors in this interactive java tutorial.
Nicol Prisms
Examine the generation of orthogonal or mutually perpendicular (ordinary and extraordinary) waves as the result of light transmission through a Nicol prism in this interactive tutorial.
The Fresnel or Refractive Index Ellipsoid
The Fresnel, or refractive index, ellipsoid describes the dielectric properties measured in all directions through a material. Explore variations in the shape and dimensions of the ellipsoid as a function of refractive index.
Birefringence Variations with Crystal Orientation
Discover variations in birefringence that result from orientational variations between the crystal optical axis and the incident light beam in this interactive java tutorial.
Interactive Michel-Levy Birefringence Chart
Visitors can determine the interference color associated with all three values by clicking on selected regions of the interactive chart.
Polarized Light Virtual Microscopy Java Tutorials
Polarized Light Virtual Microscopes
When a birefringent material is placed between crossed polarizers in an optical microscope, light incident upon the material is split into two component beams whose amplitude and intensity vary depending upon the orientation angle between the polarizer and permitted vibration directions of the material. Use this link to explore our tutorials on polarized light microscopy.
Digital Image Gallery
Polarized Light Image Gallery
As a contrast-enhancing optical technique, polarized light is unsurpassed in the magnificent array of colors generated by interference of orthogonal light waves at the analyzer. Useful for observation of mineral thin sections, hairs, fibers, particles, bones, chemical crystals, polymers, and a wide variety of other specimens, polarized light can be employed both quantitatively as well as qualitatively. Visit this gallery to observe how polarized light can be employed to observe specimens that would otherwise be difficult to distinguish from the background.
Selected Literature and Web Resources
Polarized Light Literature References
The section provided contains periodical information about these articles, as well as a listing of selected original research reports and books describing the techniques of optical crystallography and polarized light microscopy.
Polarized Light Microscopy Web Resources
The following section leads to a compiled list of helpful web resources focused on all aspects of polarized light microscopy, optical crystallography, and other related techniques.
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