High-Resolution, Air-Coupled Ultrasonic Imaging of Thin Materials.

This paper describes the use of a focused aircoupled capacitance transducer combined with pulse compression techniques to form high-resolution images of thin materials in air. The focusing of the device is achieved by using an off-axis parabolic mirror. The lateral resolution of the focused transducer, operating over a bandwidth of 1.2 MHz, was found to be less than 0.5 mm. A combination of the focused transducer as a source and a planar receiver in through-transmission mode has been developed for the measurement of different features in paper products, with a lateral resolution in through-transmission imaging of ∼0.4 mm. Images in air of thin samples such as bank notes, high-quality writing paper, stamps, and sealed joints were obtained without contact to the sample. [ABSTRACT FROM AUTHOR]