Magneto-optic microscope for visually detecting subsurface defects

A visual test method for detecting microdefects under fine surfaces is described. A new MO microscope that has a laser source, a CCD camera, and an exciting coil is developed for this work. A pulse generator supplies an intermittent square pulse to the exciting coil, which can intensify eddy currents yet reduce the working temperature of the exciting coil and sample. The magnetic field variation produced by the imbedded defect causes a rotation of the polarization plane of the reflected beam. Therefore the reflected beam carries an image of the defect, which is received by a CCD camera. The optical arrangement guarantees that no light is reflected back to the laser. The system was tested with a calibrator, which has an artificial subsurface defect; such a test attains a visual detected image. To our knowledge this is the first time an image of a subsurface defect has been distinctly detected with a MO sensor system.