High resolution air-coupled ultrasonic imaging of thin materials

This paper describes the use of a focussed air-coupled capacitance transducer combined with pulse compression techniques to form high-resolution images of thin materials in air. The focussing of the device is achieved by using an off-axis parabolic mirror. Initial measurements were performed to determine the lateral resolution of the focussed transducer, which was found to be better than 0.5mm. A combination of the focussed transducer as a source and a planar receiver in through-transmission mode was then used to image thin materials. It will be shown that the technique is able to detect various watermarks embedded in bank notes, high quality writing paper and postage stamps. In addition, measurements of adhesive layers are possible. It will be demonstrated that a high-resolution imaging technique results, which could be used to test a range of thin materials where contact and immersion testing is not possible.