39 - Polarization microscopy

Summary

The state of polarization of a given beam of light is modified upon reflection from (or transmission through) an object. The resulting change in polarization state conveys information about the structure and certain physical properties of the illuminated region. Polarization microscopy is a variant of conventional optical microscopy that enables one to monitor these changes over a small area of a specimen. Such observations then allow the user to identify and analyze the specimen's structural and other physical features.

Traditionally, observations with a polarization microscope have been categorized “orthoscopic” or “conoscopic.” orthoscopic observations involve direct imaging of the sample itself, thus allowing one to view the indentations, striations, variations of optical activity and birefringence, etc., over the sample's surface. conoscopic observations, however, involve illuminating a crystalline surface with a cone of light and then imaging the exit pupil of the objective lens. This mode of observation is used in characterizing the crystal's ellipsoid of birefringence and identifying its optical axes.

The polarization microscope

Figure 39.1 is a simplified diagram of a polarization microscope. The light source is typically an extended white light source, such as a halogen lamp or an arc lamp. The collected and collimated beam from the source is linearly polarized as a result of passage through a polarizer. In metallurgical microscopes, such as the one shown here, the objective lens is used both for illuminating the sample and for collecting the reflected light.