Surface metrology using reflected ultrasonic signals in air

An ultrasonic signal has been generated in air, using a capacitive transducer to produce a broad bandwidth transient signal. This was then focused using an off-axis parabolic mirror onto the surface of solid materials. The reflected signal was detected in terms of received amplitude and time of arrival, and images formed of the surface profile by scanning the focal region over metallic surfaces. Measurements of small steps formed from slip gauges have shown that 5 mum vertical features can be detected. The field of the transducer assembly has been measured with a miniature detector, and the width of the focal region studied. Scans of the surfaces of coins are used to illustrate the technique for non-contact surface profiling. (C) 2002 Elsevier Science B.V. All rights reserved.