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. This interactive tutorial examines optical path differences in a wide range of specimens using the quartz wedge.

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 Quartz Wedge radio button and simulate the appearance of the specimen when a quartz wedge compensator is placed into the optical path. The wedge thickness can be altered with the Optical Path Difference slider, which has a working range of 0-2000 nanometers (approximately 4 wavelengths). When the specimen feature of interest is completely extinguished (maximum darkness), the wedge thickness is equal to the optical path difference and can be obtained from the wavelength value (in nanometers) above the slider knob. The Crossed Polarizers radio button can be activated to simulate removal of the quartz wedge from the microscope optical path. At any time during operation of the tutorial, new specimen can be selected using the Choose A Specimen pull-down menu.

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, as previously described. The number of interference color orders produced by a quartz wedge is a function of the wedge inclination angle. Greater angles produce a larger number of orders per unit length. By rotating the polarized microscope circular stage, a specimen is oriented in a subtractive alignment with respect to the compensator, and the quartz wedge inserted until the birefringent area of the specimen appears compensated (extinct). The specimen optical path difference can be estimated by comparing the black (zeroth order) fringe appearing in the compensated specimen area to the colored fringe in the immediately adjacent region. This interference color can be located on a Michel-Levy chart to ascertain the optical path difference with an accuracy of 100-200 nanometers. Although many quartz wedges are engraved with an optical path difference scale on the frame (see Figure 1), the compensator has a wide error range and is generally employed as a qualitative tool to determine only the approximate optical path difference.

Green interference filters (550 or 556 nanometer) should be removed from the optical path before attempting relative retardation measurements with a quartz wedge. In using low power objective (2x, 4, and 10x), ensure that the condenser aperture iris diaphragm is closed to its smallest opening size or the specimen will not be compensated (turn dark or black), even when the birefringent feature and quartz wedge are properly overlapped to achieve extinction. If the feature of interest does not become dark throughout the entire range of the quartz wedge, check to ascertain that the birefringent feature is rotated 45-degrees from the extinction position in crossed polarizers before inserting the wedge. In many cases, the specimen can also be rotated by 90 degrees with the wedge in place to achieve extinction. However, if the specimen feature is not extinguished by the quartz wedge after a 90-degree rotation, then the retardation value is outside the wedge measuring range and another technique must be implemented in order to quantitatively assess the optical path difference.