Ultrasound, which has a much higher frequency than audible sound, is subjected to a significant attenuation in air. Therefore coupling to the target must be done through some liquid medium like water. Also the target food must be in the condition of a liquid, paste, or airless solid. Liquid state food like milk may be inspected by coupling a sensor to the flow path like a pipe. Pouch packed food, which normally does not include a significant amount of air, may be transparent to ultrasound. Although the pouch film is often made of aluminum laminate, its lack of thickness means that in practice it can be easily penetrated by ultrasound up to 10 MHz. Therefore contaminants in such food pouches may be detected although coupling to the sensor must be retained by water. The potential for contaminant detection would be upgraded then if ultrasonic detection were added to the system, as optical and electromagnetic measures may be inadequate for detection through aluminum film.

Food processing lines routinely run as fast as 20 m/s. The detection system should be quick enough to follow the processing speed. In the case of reflection type detection using equipment similar to the medical B-mode inspector, the sensor transmits and receives ultrasound one line at a time. If the number of array elements is 128, 128 separate line scans are required to create one B-mode image. So the number of frames produced in one second may be as low as 60.

This process can realize extremely high speed detection. In the project, a prototype was created by using an array sensor composed of 512 elements. One shot of ultrasound can create 512 pixels in the image. A transmission image can be created when the target moves across the sensor.

The picture represents two hairs arranged diagonally in a rectangular wire frame that was placed in water in an aluminum laminated pouch film. The hairs can clearly be seen, suggesting the potential of detection under ideal conditions. As the ultrasonic beam is not focused, the image of the hairs is somewhat blurred. However the image may be improved if an acoustic lens using something like acrilic resin is employed. Discrimination between contaminants and solid ingredients may be realized based on the difference in transparency. Therefore it can be expected that this method will prove useful when applied to the safety control of real food.