Calibration Method for Polarizers on Polarizing Microscope
In practical operation, the vibration directions of the upper and lower polarizers of a polarizing microscope must be mutually perpendicular, aligned along the east-west or north-south direction, and consistent with the horizontal and vertical lines of the eyepiece crosshair respectively. Sometimes only the lower polarizer is used for observation, and its vibration direction must be determined. Therefore, the polarizer has to be calibrated during operation.
① Inspection of Eyepiece Crosshair Check whether the eyepiece crosshair is perpendicular and aligned 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 crosshair. Align the cleavage crack parallel to one line of the crosshair and record the reading on the stage scale. Rotate the stage to make the cleavage crack parallel to the other crosshair line and record the second scale reading. A reading difference of 90° indicates that the crosshair is orthogonal.
② Determination and Calibration of the Vibration Direction of the Lower Polarizer Biotite is used to verify the vibration direction of the lower polarizer. It is a widely distributed transparent mineral and exhibits distinctive features under plane-polarized light. First, take a biotite sample with clear cleavage and move it to the center of the eyepiece crosshair. Retract the upper polarizer, then rotate the stage one full revolution and observe the color change of biotite. Biotite strongly absorbs vibrating light parallel to its cleavage direction. When the biotite appears darkest, the direction of the cleavage crack corresponds to the vibration direction of the lower polarizer.
③ Calibration for Orthogonality of Upper and Lower Polarizers After the lower polarizer direction is calibrated, remove the thin section, insert the upper polarizer, and check whether the field of view turns completely dark (extinction state). A fully dark field means the vibration directions of the upper and lower polarizers are mutually perpendicular. If not, calibrate the upper polarizer by rotating it until the field of view reaches maximum darkness. Loosen the set screw of the upper polarizer before rotation and tighten it after completing calibration.
