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

• Published in: Optics and lasers in engineering 2001-09, Vol.36 (3), p.251-268
• Main Authors: Radtke, U., Zielke, R., Rademacher, H.-G., Crostack, H.-A., Hergt, R.
• Format: Article
• Language: English
• Subjects: Eddy current sensors; Exact sciences and technology; General equipment and techniques; Instruments, apparatus, components and techniques common to several branches of physics and astronomy; Laser diode applications; Magnetic components, instruments and techniques; Magnetic fields; Magneto-optic techniques; Magnetometers for magnetic field measurements; Nondestructive testing; Physics; Polarimetry; Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
• ISSN: 0143-8166; 1873-0302
• DOI: 10.1016/S0143-8166(01)00052-5

In this paper we describe a new, laser supported, eddy current microscope for the real-time visualization of eddy current distributions. In the experimental set-up, the induction of eddy currents is conventionally performed by an alternating current excitation coil above the object surface. The magnetic field induced by the eddy currents is detected by the use of the Faraday effect. For that purpose, a laser beam is passed through a suitable thin crystal integrated in the excitation coil. The polarization direction of the laser beam is rotated in this thin crystal depending on the local magnetic field. The area distribution of the rotation angle is transformed into a grey value picture using an optical set-up, which comprises a conventional microscope, a lighting technique, an analyzator, and a CCD sensor. By choosing 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 physics 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 eddy current sensors.