Ultrasonic imaging of solid surfaces using a focussed air-coupled capacitance transducer

A variety of small solid objects have been imaged in atmospheric air using 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 1.5 MHz) was employed in a pulse-echo arrangement such that generated waves reflected off the surface of the object before returning to the same transducer for detection. By raster scanning the transducer in a plane and recording the detected ultrasonic echo amplitude as a function of transducer position, images of object surfaces were obtained. As the transducer had been fitted with a micromachined Fresnel zone-plate, the ultrasonic waves could be focussed to a spot-size of 680 μm so as to provide images of high lateral resolution. One of the key factors in making the Fresnel zone-plate function effectively in these imaging applications involved the inclusion of a second aperture in front of the zone-plate. This additional aperture blocked the zone-plate's side-lobes and reduced the appearance of multiple (diffracted) images.