This is how you calibrate the polariser of a polarisation microscope.
In practical operation, the vibration directions of the upper and lower polarizers of a polarizing microscope should be orthogonal to each other, or in the east-west and north-south directions, 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 calibration of the polarizer is necessary during operation.
(1) Detection of cross hairs in eyepieces
Generally, it is necessary to check whether the cross hairs of the eyepiece are orthogonal and whether they are in the same direction as the vibration of the upper and lower polarizers. At the same time, select a piece of completely cleaved biotite and move it to the center of the cross hairs of the eyepiece. Move the cleavage seam parallel to one thread of the cross hairs and record the number of scales on the loading platform. The difference between the two scales is 90 °, indicating that the cross hairs are orthogonal.
(2) Determination and correction of the vibration direction of the lower polarizing mirror
Black mica is generally used to check the direction of polarization mirror vibration, as it is a widely distributed transparent mineral with distinctive characteristics under single polarized light. Firstly, find a piece of clear and cleavage oriented biotite, move it to the center of the crosshair of the eyepiece, push out the upper polarizing mirror, rotate the stage once, and observe the color change of the biotite. Because biotite has a strong absorption of vibration light in the cleavage direction, when the color of the biotite reaches its deepest point, the direction of the cleavage seam is the vibration direction of the lower polarizing mirror.
(3) Correction of orthogonality between upper and lower polarizers
After adjusting the direction of downward polarization, remove the thin film and push it onto the upper polarizer to 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 corrected by rotating it until the field of view reaches its darkest point. When rotating, it is necessary to first loosen the stop screw of the polarizer, align it, and then tighten it.
