![]() ![]() The technique of polarizing microscopy exploits the interference of the split light rays, as they are re-united along the same optical path to extract information about anisotropic materials.Īlthough an understanding of the analytical techniques of polarized microscopy may be perhaps more demanding than other forms of microscopy, it is well worth pursuing, simply for the enhanced information that can be obtained over brightfield imaging. More importantly, anisotropic materials act as beamsplitters and divide light rays into two orthogonal components (as illustrated in Figure 1). They demonstrate a range of refractive indices depending both on the propagation direction of light through the substance and on the vibrational plane coordinates. In contrast, anisotropic materials, which include 90 percent of all solid substances, have optical properties that vary with the orientation of incident light with the crystallographic axes. These materials have only one refractive index and no restriction on the vibration direction of light passing through them. Isotropic materials, which include a variety of gases, liquids, unstressed glasses and cubic crystals, demonstrate the same optical properties when probed in all directions. Furthermore, the contrast-enhancing technique exploits the optical properties specific to anisotropy and reveals detailed information concerning the structure and composition of materials that are invaluable for identification and diagnostic purposes. Polarized light microscopy is capable of providing information on absorption color and optical path boundaries between minerals of differing refractive indices, in a manner similar to brightfield illumination, but the technique can also distinguish between isotropic and anisotropic substances. In addition, the critical optical and mechanical components of a modern polarized light microscope are illustrated in the figure. These concepts are outlined in Figure 1 for the wavefront field generated by a hypothetical birefringent specimen. After exiting the specimen, the light components become out of phase, but are recombined with constructive and destructive interference when they pass through the analyzer. The velocities of these components, which are termed the ordinary and the extraordinary wavefronts ( Figure 1), are different and vary with the propagation direction through the specimen. Image contrast arises from the interaction of plane-polarized light with a birefringent (or doubly-refracting) specimen to produce two individual wave components that are each polarized in mutually perpendicular planes. In order to accomplish this task, the microscope must be equipped with both a polarizer, positioned in the light path somewhere before the specimen, and an analyzer (a second polarizer see Figure 1), placed in the optical pathway between the objective rear aperture and the observation tubes or camera port. The polarized light microscope is designed to observe and photograph specimens that are visible primarily due to their optically anisotropic character. Figure 1 - Polarized Light Microscope Configuration However, steady advances made over the past few years have enabled biologists to study the birefringent character of many anisotropic sub-cellular assemblies. In contrast, the quantitative aspects of polarized light microscopy, which is primarily employed in crystallography, represent a far more difficult subject that is usually restricted to geologists, mineralogists, and chemists. Qualitative polarizing microscopy is very popular in practice, with numerous volumes dedicated to the subject. Polarized light microscopes have a high degree of sensitivity and can be utilized for both quantitative and qualitative studies targeted at a wide range of anisotropic specimens. Polarized light is a contrast-enhancing technique that improves the quality of the image obtained with birefringent materials when compared to other techniques such as darkfield and brightfield illumination, differential interference contrast, phase contrast, Hoffman modulation contrast, and fluorescence. ![]()
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