ULTRASONIC-IMAGING OF SOLID-SURFACES USING A FOCUSED AIR-COUPLED CAPACITANCE TRANSDUCER

A variety of small solid objects have been imaged in atmospheric air u sing a focussed, micromachined air-coupled capacitance transducer. The transducer, which was capable of generating and receiving ultrasound in air over a large frequency bandwidth (< 100 kHz similar to 1.5 MHz) was employed in a pulse-echo arrangement such that generated waves re flected off the surface of the object before returning to the same tra nsducer for detection. By raster scanning the transducer in a plane an d recording the detected ultrasonic echo amplitude as a function of tr ansducer position, images of object surfaces were obtained. As the tra nsducer had been fitted with a micromachined Fresnel zone-plate, the u ltrasonic waves could be focussed to a spot-size of similar to 680 mu m so as to provide images of high lateral resolution. One of the key f actors in making the Fresnel zone-plate function effectively in these imaging applications involved the inclusion of a second aperture in fr ont of tile zone-plate. This additional aperture blocked the zone-plat e's side-lobes and reduced the appearance of multiple (diffracted) ima ges. (C) 1998 Elsevier Science B.V.