Alignment Procedure for Polarizing Microscope Polarizers
In practical operation, the vibration directions of the upper and lower polarizers of a polarizing microscope must be mutually orthogonal, aligned either east-west or north-south, and consistent respectively with the horizontal and vertical crosshairs of the eyepiece. Sometimes only the lower polarizer is used for observation, and its vibration direction must be confirmed. Therefore, calibration of the polarizers is required during operation.
Inspection of Eyepiece CrosshairsCheck whether the eyepiece crosshairs are orthogonal and whether they align with the vibration directions of the upper and lower polarizers. Select a biotite specimen with perfect cleavage and move it to the center of the eyepiece crosshairs. Align the cleavage trace parallel to one crosshair and record the stage reading. Rotate the stage until the cleavage trace is parallel to the other crosshair, then record the new reading. A difference of 90° between the two readings confirms that the crosshairs are orthogonal.
Determination and Calibration of the Vibration Direction of the Lower PolarizerBiotite is widely used to verify the vibration direction of the lower polarizer, as it is a common transparent mineral with distinctive optical properties under single polarized light. First, place a biotite specimen with clear cleavage at the center of the eyepiece crosshairs. Retract the upper polarizer and rotate the stage a full circle while observing color changes of the biotite. Biotite strongly absorbs light vibrating parallel to its cleavage direction. When the biotite appears darkest, the direction of its cleavage trace corresponds to the vibration direction of the lower polarizer.
Calibration for Orthogonality of Upper and Lower PolarizersAfter calibrating the lower polarizer, remove the specimen slide and insert the upper polarizer. Observe whether the field of view becomes completely dark, indicating extinction. A fully darkened field confirms that the vibration directions of the two polarizers are orthogonal. If not, calibrate the upper polarizer: loosen its locking screw and rotate the upper polarizer until the field of view reaches maximum darkness. Retighten the locking screw once calibration is complete.
