Calibration Methods of Polarizers for Polarizing Microscopes
In practical operation, the vibration directions of the upper and lower polarizing mirrors of a polarizing microscope should be orthogonal to each other, or in the east-west and north-south directions, respectively, consistent with the transverse and longitudinal directions of the crosshair of the eyepiece. Sometimes only one lower polarizer is used for observation, and the vibration direction of the lower polarizer must be determined, so the polarizer must be calibrated during operation.
(1) Detection of crosshair in eyepiece
Generally, it is necessary to check whether the crosshair of the eyepiece is orthogonal and consistent with the vibration direction of the upper and lower polarizing mirrors. At the same time, select a piece of biotite with complete cleavage, move it to the center of the crosshair of the eyepiece, make the cleavage seam parallel to one of the crosshair wires, record the scale number of the stage, and then rotate the stage to make the cleavage seam parallel to the other crosshair. Record the scale number of the stage. The difference between the two degree readings is 90 °, indicating that the crosshair is orthogonal.
(2) Determination and correction of the vibration direction of the polarizing mirror
Generally, biotite is used to check the vibration direction of polarizers, because biotite is a widely distributed transparent mineral that is very characteristic under single polarization. Firstly, find a clear and cleaved piece of biotite, move it to the center of the crosshair of the eyepiece, push out the upper polarizer, rotate the stage once, and observe the color change of biotite. Because biotite absorbs the vibration light in the cleavage direction strongly, when the color of biotite reaches depth, the direction of the cleavage seam is the vibration direction of the lower polarizer.
(3) Correction of orthogonal polarization of upper and lower polarizing mirrors
After aligning the direction of the lower polarizer, remove the thin film, push the upper polarizer, and observe whether the field of view is completely black, that is, whether it is in an extinction state. If it is all black, it indicates that the vibration directions of the upper and lower polarizations are orthogonal to each other. Otherwise, the upper polarizer needs to be calibrated by rotating it until the field of view becomes dark. When rotating, the stop screw of the upper polarizer must be loosened first, calibrated correctly, and then tightened.
