Application of magneto-optical method for realtime visualization of eddy currents with high spatial resolution for nondestructive testing

In this paper we describe a new, laser supported, eddy current microscope f or the real-time visualization of eddy cut-rent distributions. In the exper imental set-up, the induction of eddy currents is conventionally performed by an alternating current excitation coil above the object surface. The mag netic field induced by the eddy currents is detected by the use of the Fara day effect. For that purpose, a laser beam is passed through a suitable thi n crystal integrated in the excitation coil. The polarization direction of the laser beam is rotated in this thin crystal depending on the local magne tic field. The area distribution of the rotation angle is transformed into a grey value picture using an optical set-up, which comprises a conventiona l microscope, a lighting technique, an analyzator, and a CCD sensor. By cho osing a suitable optic, a real-time measurement of the magnetic field with a high spatial resolution can be carried out. In this paper, the basic phys ics and the design of the new microscope are described. The initial results of experimental investigations concerning the resolution power of magneto- optic eddy current sensors are presented and compared with conventional edd y current sensors. (C) 2001 Elsevier Science Ltd. All rights reserved.