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Polarization

The polarized light microscopy or polarization is a technique of contrast enhancement that improves the image quality of birefringent materials such as starch granules, crystals, minerals, polymers, compared to other techniques such as darkfield and brightfield illumination, DIC, phase contrast and Fluorescence.
The polarized light microscope is designed to observe and photograph specimens that are visible primarily due to their optically anisotropic character. In order to accomplish this task, the microscope must be equipped with a polarizer, positioned in the light path before the sample, and a analyzer (second polarizer) placed in the optical path between the rear objective diaphragm and the viewing tube or the camera. Generally, the polarizer is positioned in the condenser, the analyzer above the objective and has an angle of 90 ° to the polarizer, this angle can be changed in the devices that have ability to rotate the analyzer filter.
The image contrast arises from the interaction of polarized light with a birefringent subject that will lead to the formation of two individual wavelength, both polarized in planes perpendicular to each other. The speed of these waves are different and vary depending on the direction of propagation through the sample. After levaing of the sample, the waves are "out of phase, but are recombined with constructive and destructive interference as they pass through the analyzer. The samples will appear more luminous, the objects will be colored, on a dark background. The colors may change when the filters are rotated. If, on the microscope stage, there is no preparation or there is only one empty and clean glass slide, the image is completely dark, because the analyzer does not allow the passage of light having a polarization perpendicular to its axis.
Another component of the polarized light microscope is the lambda plate, this plate converts the contrast in color using the phase differences in the lamina itself, the result is the extinction of certain wavelengths of light, those that remain are only about certain colors, the result is the formation of spectacular images.

For this technique are suitable dedicated microscopes (polarizing) that have rotating polarizer and analyzer, rotative stand and tension free objective. This last feature is very important because mechanical tensions in the glass produce the so-called induced birefringence which affects the polarized light. For this reason, quantitative analysis in polarized light microscopy require the use of condensers and objectives free from internal tensions. These objectives can be identified by name written in red and the symbol "Pol.