Real-time orientation-sensitive magnetooptic imager for leakage flux inspection

We propose a new system for magnetooptical imaging that can observe the distributions of the amplitude and phase of an ac leakage magnetic field on a steel sheet driven externally by a dipole or a quadrature (rotating dipole) magnet. The new system, which we call a three-phase correlation image sensor (3PCIS), can perform two-dimensional imaging and parallel lock-in detection of the intensity-modulated light generated by a magnetooptic film and polarimetric techniques. We apply the imaging system to the, leakage flux inspection of surface or near-surface defects of steel sheets and to the grain observation of electromagnetic steels. The imaging system allows us to observe the amplitude and phase (polarity for dipole excitation and defect orientation for quadrature excitation) of vertical flux components. The imager is also applicable to nonlinearity visualization via the harmonic responses of driving frequency, and provides rich clues for the horizontal flux components through observation of fine movements of domain walls of the magnetooptic film.