Polarized Light Microscopy
The Berek Compensator
The Berek compensator is an optical device that is capable of quantitatively determining the wavelength retardation of a crystal, fiber, mineral, plastic film or other birefringent material, including biological specimens. Provided the thickness of the material can be measured, a Berek compensator can be utilized to ascertain the birefringence value. The compensator operates by measuring the rotation angle of a calcite or magnesium fluoride optical plate cut perpendicular to the optical microscope axis. This interactive tutorial examines optical path differences in a wide range of specimens using the Berek compensator.
The tutorial initializes with a randomly chosen specimen appearing in the virtual microscope viewport under crossed polarized illumination. In order to operate the tutorial, use the mouse cursor to activate the Berek Compensator radio button and simulate the appearance of the specimen when a Berek compensator is placed into the optical path. The birefringent retardation plate angle in the compensator can be altered with the Optical Path Difference slider, which has a working range of 0-1640 nanometers (approximately 3 wavelengths). When the specimen feature of interest is completely extinguished (maximum darkness), the compensator angle can be utilized to calculate the optical path difference. In the tutorial, the optical path difference is automatically derived for each specimen and can be obtained from the wavelength value (in nanometers) above the slider knob. The Crossed Polarizers or Retardation Plate radio buttons can be activated to simulate removal of the Berek compensator from the microscope optical path or to substitute a first order retardation plate for determination of the specimen birefringence sign. At any time during operation of the tutorial, new specimen can be selected using the Choose A Specimen pull-down menu.
In practice with a Berek compensator, a birefringent specimen (usually a crystal) is placed on the microscope stage and rotated until it attains the extinction position (where the features of interest become dark). Next, the stage is rotated +45-degrees and clamped into place. The compensator drum knob is set to a position of 30-degrees (turning the knob in a single direction to avoid backlash) and inserted into the microscope intermediate tube. Finally, the compensator knob is rotated to confirm that the black fringe intersects the center of the field of view. If the fringe does not intersect the center, the stage is rotated by 90-degrees and re-clamped. An interference filter having a transmission bandwidth between 540 and 570 nanometers will improve the accuracy of measurements, but also produces multiple black fringes in addition to the one that is visible in the absence of the filter. A reference table gives the optical path differences for the tilt angle in which compensation is achieved.
John D. Griffin, Ian D. Johnson, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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