Compensators and Retardation Plates

Section Overview:

Polarized light microscopy is a valuable tool for revealing the presence and nature of submicroscopic structural motifs in a wide variety of materials ranging from mineral thin sections to fibers and biological specimens. In many cases, molecular ordering in these specimens is an intrinsic material property, but order can also be induced on multiple levels by dynamic shear, stretching, concentration changes, temperature fluctuations, and force fields. When the ordered state involves structural anisotropy, the optical state usually also displays anisotropic effects in polarized light observations. Quantative measurements of optical anisotropy is therefore useful in the optical analysis of birefringent specimens. These measurements are often accomplished with the aid of specialized tools termed retardation plates and compensators.

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Optical anisotropy is studied in the polarized light microscope with accessory plates that are divided into two primary categories: retardation plates that have a fixed optical path difference and compensators, which have variable optical path lengths. The terms relative retardation, used extensively in polarized light microscopy, and optical path difference (Δ or OPD), are both formally defined as the relative phase shift between the orthogonal wavefronts, expressed in nanometers.

The Quarter Wavelength Retardation Plate

The quarter wavelength retardation plate is an optical accessory for polarized light microscopy that operates by introducing a relative phase shift of 90 degrees between the orthogonal wavefronts passing through when the plate is illuminated with linearly polarized light. Quarter wavelength retardation plates are useful for the qualitative analysis of conoscopic and orthoscopic images, and for the assessment of optical path differences in birefringent specimens.

The First Order (Full Wave) Retardation Plate

Optical path differences ranging from a fraction of a wavelength up to several wavelengths can be readily estimated using a first order or full wave retardation plate. This versatile tool is known by several names, including a red plate, red-I (red-one) plate, lambda (λ) plate, gypsum plate, selenite plate, sensitive violet, or simply a color tint plate, and adds a fixed optical path difference between 530 and 560 nanometers (depending upon the manufacturer) to every wavefront in the field.

The Quartz Wedge Compensator

The quartz wedge is a simple, semi-quantitative compensator designed around a crystalline block of quartz cut with an elongated wedge angle so that the optical axis of the quartz is oriented either parallel or perpendicular to the edge of the birefringent crystal. The optical path difference between the orthogonally polarized fast and slow wavefronts traversing the wedge is a continuously variable function of the thickness along the wedge hypotenuse.

The de Sénarmont Compensator

The de Sénarmont compensator couples a 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 for the quantitative analysis of crystals, fibers, and birefringence in living organisms.

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. 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.

The Bräce-Köhler Compensator

The Bräce-Köhler compensator is ideally suited for measuring very small phase retardations (optical path differences) that are often found in living organisms, thin films, and glasses having low strain birefringence. The device can also be employed to emphasize contrast in polarized light microscopy investigations of weakly birefringent specimens in order to enhance observation of textures that display minute retardation values.

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Contributing Authors

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.