title: High-resolution, air-coupled ultrasonic imaging of thin materials.
abstract: This paper describes the use of a focused air-coupled 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 approximately 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.
authors.affiliation: School of Engineering, University of Warwick, Coventry CV4 7AL. esrig@eng.warwick.ac.uk
authors.forename: Tat-Hean
authors.initials: TH
authors.lastname: Gan
authors.forename: David A
authors.initials: DA
authors.lastname: Hutchins
authors.forename: Duncan R
authors.initials: DR
authors.lastname: Billson
authors.forename: David W
authors.initials: DW
authors.lastname: Schindel