Calibration Procedure for Polarizers on a Polarizing Microscope
In practical operation, the vibration directions of the upper and lower polarizers of a polarizing microscope must be mutually perpendicular, or aligned with the east-west and north-south directions, consistent with the crosshairs of the eyepiece. If only the lower polarizer is used for observation, its vibration direction must be determined. Therefore, the polarizers must be calibrated during operation.
① Inspection of eyepiece crosshairsIt is necessary to first check whether the eyepiece crosshairs are orthogonal and whether they are consistent with the vibration directions of the upper and lower polarizers. At the same time, take a cleavage flake of a perfectly extinct biotite, move it to the center of the eyepiece crosshairs, align the cleavage trace parallel to one crosshair line, and record the reading of the rotating stage. Then rotate the stage to align the cleavage trace parallel to the other crosshair line and record the stage reading. A difference of 90° between the two readings indicates that the crosshairs are orthogonal.
② Determination and calibration of the vibration direction of the lower polarizerBiotite flakes are generally used to check the vibration direction of the lower polarizer, as biotite is a widely available transparent mineral with distinct features under plane-polarized light. First, find a biotite flake with clear cleavage and move it to the center of the eyepiece crosshairs. Insert the lower polarizer, rotate the stage for a full rotation, and observe the color change of the biotite. Biotite shows strong light absorption when its cleavage trace is parallel to the vibration direction of the lower polarizer, so the direction of the cleavage trace when the biotite appears darkest is the vibration direction of the lower polarizer.
③ Calibration of the orthogonality of upper and lower polarizersAfter the direction of the lower polarizer is calibrated, remove the thin section and insert the upper polarizer. Check whether the field of view is completely dark (i.e., in the extinction state). If it is fully dark, the vibration directions of the upper and lower polarizers are mutually perpendicular. If not, the upper polarizer needs calibration: rotate the upper polarizer until the field of view reaches maximum extinction. Loosen the set screw of the upper polarizer before rotation, and tighten it again after calibration.
