Polarized Light Microscopy
Compensators and Retardation Plates
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.
Introduction - 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. Addition of a retardation plate or compensator to the polarized light microscope produces a highly accurate analytical instrument that can be employed to determine the relative retardation (often symbolized by the Greek letter G) or optical path difference between the orthogonal wavefronts (termed ordinary and extraordinary) that are introduced into the optical system by specimen birefringence. The terms relative retardation, used extensively in polarized light microscopy, and optical path difference (D 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 a common optical accessory for polarized light microscopy that operates by introducing a relative phase shift of 90 degrees between the orthogonal wavefronts (ordinary and extraordinary) passing through when the plate is illuminated with linearly polarized light. A phase shift of 90 degrees between the ordinary and extraordinary components converts the incident linear polarized light vibrations into either elliptical or circularly 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 (l) 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 first order retardation plate is a standard accessory that is frequently utilized to determine the optical sign (positive or negative) of a birefringent specimen in polarized light microscopy. In addition, the retardation plate is also useful for enhancing contrast in weakly birefringent specimens.
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. A typical quartz wedge has an effective range of 4 orders (approximately 500 to 2000 nanometers) and is commonly employed for qualitative retardation measurements of petrographic specimens (rock and mineral thin sections) or other birefringent materials whose retardation value falls within the wedge limits. The quartz wedge compensator is also employed for the routine determination of the direction of anisotropy (crystalline fast and slow axes orientation) in birefringent specimens.
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.
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. Similar to many other birefringence measuring devices, the Bräce-Köhler compensator can be used to differentiate between slow and fast axes in birefringent specimens. The combined features of a Bräce-Köhler compensator render the instrument a highly sensitive and accurate device for conducting quantitative retardation measurements.
Interactive Java Tutorials
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 de Sénarmont Compensator - In order to obtain a measurement of specimen birefringence (and the related optical path difference), the de Sénarmont compensator is placed into the microscope optical train after the sign of birefringence and orientation of the specimen slow axis has been established with a first-order retardation plate. 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. This interactive tutorial examines optical path differences in a wide range of specimens using the de Sénarmont compensator.
The Quartz Wedge Compensator - In practice, the quartz wedge compensator is inserted at a 45-degree angle (with respect to the polarizer and analyzer) into the microscope optical train through a DIN slot located in the microscope nosepiece or an intermediate tube positioned between the frame and the observation tubes. As the wedge compensator is slowly translated farther into the light path, the optical path difference of wavefronts passing through increases to generate a succession of interference colors. This interactive tutorial examines optical path differences in a wide range of specimens using the quartz wedge.
Compensation Accessory Plates and Wedges - A careful examination of anisotropy as a function of specimen orientation permits identification of the refractive index difference and orientation of the extraordinary and ordinary light rays produced by birefringent materials. This tutorial explores how compensators may be employed to help determine orientation parameters of anisotropic materials.
de Sénarmont Compensator Wavefronts - A de Sénarmont compensator is composed of a linear polarizer combined with a quarter-wavelength retardation plate, and is capable of producing either linear, elliptical, or circularly polarized light, depending upon the orientation of the polarizer vibration axis with respect to the fast and slow axes of the retardation plate. This interactive tutorial explores the relationship between wavefronts emanating from the compensator as the polarizer is rotated through its useful range.
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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