Image processing for enhanced detectability of corrosion in aircraft structures using the magneto-optic imager

The Magneto-Optic Imaging (MOI) system is being used for the inspection of surface and sub-surface defects in aircraft aluminum structures. The technique is based on the combination of eddy current excitation and magneto-optic sensing and imaging. A current sheet is excited with AC excitation which induces eddy currents in the sample. These eddy currents generate normal magnetic fields when they encounter a defect. The magneto-optic sensor (Garnet film) has its easy axis of magnetization in the direction of the normal magnetic fields, and memory effect. Thus by imaging the reflected polarized light through the sensor, one can image defects. The sensor has an area of 60 cm(2) which makes the instrument very attractive for rapid inspection of aircraft structures. The magnetic fields from subsurface corrosion are weak and so the defect indications get hidden by the magnetic domains and other noise. The challenge is to eliminate the magnetic domain structure from the image and to enhance the image created by the defect. Thus image processing algorithms need to be evaluated to overcome this problem. A number of schemes such as large area averaging, morphological operators, correlation filter have been implemented. Results obtained from applying these methods to some corrosion samples is discussed in the next few sections.