Polarized Microscopes

PM: Alignment Procedure

polarized microscopes — Sat, 19 May 2007 05:04:12 +0000

In polarized light microscopy, proper alignment of the various optical and mechanical components is a critical step that must be conducted prior to undertaking quantitative analysis between crossed polarizers alone, or in combination with retardation plates and compensators. Several essential components must be correctly positioned with respect to both the microscope optical axis and to other mechanical and optical components. The series of alignment steps outlined below are intended for general use with polarized light microscopes and should be applicable to both student and research-level instruments. At a minimum, these microscopes must be equipped with two linear polarizing elements. One of the polarizers termed the Polarizer is placed beneath the stage inside or below the condenser, or covering the microscope light source exit port in the base. The second polarizer termed the Analyzer is installed above the objective rear aperture, either in a nosepiece slot or within an intermediate tube positioned between the frame and the observation tubes. (more…)

PM: Microscope Stage

polarized microscopes — Sat, 19 May 2007 04:58:03 +0000

After the stage has been centered with respect to the microscope optical axis with the 10x objective installed in the optical train, the other objectives (4x, 20x, 40x, and 100x) should be sequentially centered to the stage. Microscopes having centerable objectives contain a pair of Allen (or a similar drive) setscrews in the nosepiece that translate the objective laterally within its seat. Each objective can be centered using the procedure described above for the circular stage by using the objective centering tools provided by the manufacturer. Accurate centering of the stage and all objectives is paramount to quantitative analysis in polarized light, and avoids the operator losing sight of specific features in the viewfield during stage rotation. After completing the centering procedure, review the steps to ensure all of the objectives on the microscope are accurately centered before continuing. Establishing Kohler illumination and centering the microscope stage and objectives is easily accomplished in brightfield mode but can also be undertaken with a birefringent specimen in polarized light mode, if necessary. (more…)

PM: Orientation Plate Polarizer

polarized microscopes — Sat, 19 May 2007 03:54:45 +0000

Calibration of the polarizer and analyzer axis alignment can be accomplished by one of two methods. Many manufacturers provide a polarized light microscope orientation plate with their research-level microscopes to assist in alignment of the crossed polarizers. These plates contain a perfectly cut rectangular thin birefringent crystal having an optical axis that is parallel to the long edge of the crystal. The crystal is mounted flat on a microscope slide and designed specifically for use on circular polarized light microscope stages. When the long axis crystal is oriented at a 45-degree angle to the polarizer and analyzer transmission azimuths, maximum birefringence is observed in polarized light. Rotating the crystal towards the polarizer axis progressively reduces the level of birefringence until it is eliminated when the long axis is exactly parallel to the polarizer transmission azimuth and the horizontal eyepiece crosshair. At this point, the crystal birefringence is completely extinguished and should be indistinguishable from the background. Note that the outline of the crystal is artificial. (more…)

Polarized light microscopy

polarized microscopes — Sat, 19 May 2007 03:52:21 +0000

Typical modern polarized and brightfield microscopes have a lamphouse, which contains a 50 to 100-watt high-energy tungsten-halogen lamp, attached to the base of the microscope. A transformer providing direct current (DC) voltage to the lamp is usually built directly into the microscope base and is controlled by a potentiometer positioned near the lamp switch in bottom of the base, the lamp voltage control. Between the lamphouse and the microscope base is a filter cassette that positions removable color correction, heat, and neutral density filters in the optical pathway. Also built into the microscope base is a collector lens, the field iris aperture diaphragm, and a first surface reflecting mirror that directs light through a port placed directly beneath the condenser in the central optical pathway of the microscope. These components control the size, intensity, and distribution of light in the illumination field. (more…)

Modern Polarized Microscope

polarized microscopes — Sat, 19 May 2007 03:50:44 +0000

Introduction to Polarized Microscopes

polarized microscopes — Sat, 19 May 2007 03:49:10 +0000

Polarized light microscopes often also referred to as petrographic microscopes, asbestos microscopes, crystal microscopes, forensic microscopes, polymer microscopes, ore microscopes, and more, utilize the contrast-enhancing technique known as polarized light to improve image contrast and quality, especially on samples containing anisotropic or birefringent material. Polarizing microscopes are often called petrographic microscopes since their main application is in studying rocks and minerals under polarized light. The study of optical mineralogy uses petrographic microscopes to produce polarized light for analysis and identification of rock and mineral thin sections on microscope slides. All of our microscopes have the full Bertrand lens system, complete with a polarizer and an analyzer. Cross polarization can be obtained with our polarized light microscopes by turning the polarizer and the analyzer at 90 degrees to each other, which is the extinction angle. Polarized light microscopy positions two polarizing filters, called polarizers, in the light path. (more…)