Polarized Light Microscopy
The de Sénarmont Compensator
The de Sénarmont compensator couples a highly precise quarter wavelength birefringent quartz or mica crystalline plate with a 180-degree rotating analyzer to provide retardation measurements having an accuracy that approaches one thousandth of a wavelength or less. The device is utilized for retardation measurements over an optical path difference range of approximately 550 nanometers (one wavelength in the green region) for the quantitative analysis of crystals, fibers, and birefringence in living organisms, as well as investigations of optical strain. In addition, de Sénarmont compensators are useful for emphasizing contrast in weakly birefringent specimens that ordinarily are difficult to examine under crossed polarized illumination. This interactive tutorial examines optical path differences in a wide range of specimens using the de Sénarmont 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 de Sénarmont Compensator radio button and simulate the appearance of the specimen when a de Sénarmont compensator is placed into the optical path. The analyzer angle in the virtual compensator can be altered with the Optical Path Difference slider, which has a working range that is dependent upon the specimen thickness. When the specimen feature of interest is completely extinguished (maximum darkness) on a green background, the analyzer 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 box (in nanometers; observed when the box turns yellow) above the slider knob. The Crossed Polarizers or Retardation Plate radio buttons can be activated to simulate removal of the de Sénarmont 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, the de Sénarmont compensator is mounted in a rectangular frame (see Figure 1) and inserted through a standardized DIN slot (6 x 20 millimeters) into the optical train of a polarized light microscope at a 45-degree angle (Northwest-Southeast) to the polarizer and analyzer. Most microscopes are designed to position the de Sénarmont compensator between the specimen and the analyzer, above the rear objective aperture, in either an intermediate tube or a slot in the nosepiece. Because the de Sénarmont compensation method requires precise measurement of the analyzer transmission azimuth, this element is typically mounted in a 180 or 360-degree rotating frame that is graduated in degrees with a vernier scale accurate to a tenth of a degree.
The de Sénarmont compensation technique is extremely sensitive, but the quantitative retardation measurement range is restricted of a single wavelength (approximately 550 nanometers). In order to achieve the highest possible accuracy, the quarter wavelength plate must introduce exactly one quarter of a wavelength retardation for the wavelength used in the measurement. Otherwise, or in the case of measurements in white light, the optical path difference produced by the compensator must be accurately known. Sensitivity of the de Sénarmont technique can be improved by the application of a half-shadow device (such as a Nakamura plate), in some cases to measure optical path differences as small as one two-thousandth of a wavelength. However, when attempting to measure retardation values to this precision, other sources of error should be carefully considered, such as the deviation of the quarter wavelength crystal thickness from that of the interference filter utilized to generate monochromatic light.
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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