An air-coupled reflected ultrasound technique has been developed for the examination of surface characteristics, in collaboration with Microacoustic Instruments Inc. The general approach is to generate a transient ultrasonic pulse that is focused onto the surface of interest which is then reflected back and the returning signal is detected. By using a raster scanning technique an image of the surface can be constructed. In addition, thin materials can be imaged by scanning a focussed beam across the surface, and detecting the through-transmitted signal.

 

Surface reflection imaging 

Schematic diagram of a focusing transducer using an external mirror Measured ultrasonic intensity map from a focusing transducer.

 

In it's simplest form just the amplitude of the returned signal can be used. However it is also useful to detect transit time, as using the velocity of sound in air allows quite accurate spatial information to be recorded. As can be seen below analysis of the recorded waveforms allows good 3D images of a surface to be produced.

Image of a coin obtained with scanning focussed transducer

 

High resolution through-transmission imaging

Using focussed devices, it is possible to produce high-resolution air-coupled images. Below is an example of where ultrasound has been used to image the watermark area, the blank area to the right of the Queen’s head on this £20 note.

 

Actual Banknote	Ultrasonic image of the watermark

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Related Papers
D.A. Hutchins, T.J. Robertson, D.R. Billson and P. Solanki. " A conical air-coupled capacitance transducer for surface imaging", Ultrasonics 41, 163-173 (2003).

T.H. Gan, D.A. Hutchins, D.R. Billson and D.W. Schindel, “High resolution air-coupled imaging of thin materials”, IEEE Trans. Ultras. Ferr. Freq. Contr. 50, 1516-1524 (2003).