A millimeter and sub-millimeter continuous wave system has been used to image corrosion pitting, structural defects, and heat damage in common aircraft materials such as aluminum and polyamides. In order to avoid failure of components during operation, many aircraft parts are replaced earlier than necessary leading to higher costs that could be reduced if the degree of damage in the component material could be determined non-destructively. Millimeter and Sub-millimeter waves are ideally suited for this purpose because of their ability to penetrate through dielectric substances and obscurants such as pain, oil, and epoxy commonly found on the surfaces of aircraft.