Methods for Calibrating the Polarizer of a Polarizing Microscope
During operation, the vibration directions of the upper and lower polarizers of the polarizing microscope shall be mutually perpendicular, i.e., aligned along the east-west and north-south directions, and consistent with the horizontal and vertical crosshairs of the eyepiece respectively. Sometimes only the lower polarizer is used for observation, so it is necessary to determine its vibration direction. Therefore, the polarizers must be calibrated during operation.
① Inspection of eyepiece crosshairs It is generally required to check whether the eyepiece crosshairs are perpendicular and whether they are consistent with the vibration directions of the upper and lower polarizers. Select a biotite sample with perfect cleavage and move it to the center of the eyepiece crosshairs. Align the cleavage crack parallel to one crosshair and record the reading on the stage scale. Rotate the stage to make the cleavage crack parallel to the other crosshair and record the scale reading again. If the difference between the two readings is 90°, the crosshairs are perpendicular.
② Determination and calibration of the vibration direction of the lower polarizer Biotite is commonly used to verify the vibration direction of the lower polarizer, as biotite is a widely distributed transparent mineral. First, take a biotite specimen with clear cleavage and move it to the center of the eyepiece crosshairs. Retract the upper polarizer, then rotate the stage one full rotation and observe the color change of biotite. Biotite strongly absorbs vibrating light parallel to its cleavage direction. When the biotite reaches its darkest color, the direction of the cleavage crack is the vibration direction of the lower polarizer.
③ Calibration for perpendicularity of upper and lower polarizers After the direction of the lower polarizer is calibrated, remove the thin section and insert the upper polarizer. Check whether the field of view turns completely dark (extinction state). If full darkness is achieved, the vibration directions of upper and lower polarizers are mutually perpendicular. If not, the upper polarizer needs to be calibrated until the field of view becomes darkest. Loosen the locking screw of the upper polarizer before rotation, and tighten it after calibration.
