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The de Sénarmont compensator couples a highly precise quarter wavelength birefringent quartz crystalline plate with a 180-degree rotating analyzer.
This setup provides 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 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.
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The Babinet compensator is a continuously adjustable retardation plate. It is constructed of a fixed and a moveable wedge that are stacked together as shown at figure. One wedge is cut so that its optic axis is in the direction of motion, the other is cut so that its optic axis is transverse (normal) to the direction of motion. As the plate moves the retardation changes. Depending upon the construction, retardation can be adjusted over several waves.
One disadvantage of the Babinet compensator is that the retardation varies across the face of the compensator. This has been corrected in the Soleil-Babinet compensator.
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The Soleil-Babinet compensator is a continuously adjustable retardation plate. It is based on the design of the Babinet compensator, but unlike that compensator, the retardation is constant as a function of position of the beam across the surface. The compensator is constructed of two identical birefringent wedges, one fixed and one moveable, that are stacked together as shown below. A third plate is cut with the optic axis normal to the axis of the wedges, as shown in the diagram below. As the wedge moved, the combined thickness of the wedges changes, changing the retardation through the three-element system. When the thickness of the wedge pair is the same as the thickness of the fixed plate, there is no retardation.
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An optical device, rotatable birefringent plate, that is capable of quantitatively determining the wavelength retardation of a crystal, fiber, plastic film or other birefringent material. Provided the thickness of the material can be measured, a Berek compensator can be utilized to ascertain the birefringence value. Berek compensator is a plan-parallel birefringent plate from single axis crystal (quartz , calcite ,MgF2, ...) with the plate cut parallel to the crystal optical axis. The compensation is determined by phase shift with a proper plate inclination. This function is non-linear.
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Solc compensator is made from two quartz plates. One of them is cut parallel with optical axe, the other is cut in special angle to optical one. Choosing the proper azimuth orientation of plates it is possible to get practically linear function for phase shift vs. inclination of plates. It is also possible to realise the compensators for broad phase shifts.
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The ring compensator is created from plan-parallel plate composed from plan-convex and plan-concave birefringent lenses, cutting plan-parallel with crystal optical axis. It is possible to create this compensator in the differential form, when one half of plate has azimuth zero and the second half of it has 90°. The compensator is then hold in Cardan holder that provides in two axes.
The advantage for this compensator is the high accuracy measurement even in broad phase shifts.
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