High contrast images of defects in food package seals

Previous work to detect channels in packaging seals using pulse-echo ultrasound inspired the backscattered amplitude integral (BAI) imaging technique, which could reliably identify channels with diameters larger than 15 μm at a center frequency of 17.3 MHz (λ=86 μm). However smaller channels (≈6 μm in diameter) can be easily revealed by processing the data according to a new imaging technique that displays a single time-gated pressure value from the received (not envelope-detected) rf waveform at each transducer position, that is, an rf sample image (RFS). The applicability of this technique for detecting channel defects is demonstrated for plastic and aluminum foil trilaminate films with 6, 10, 15, 38, 50, 75, and 100 μm channels filled with water or air. The images are formed with a focused ultrasound transducer (17.3 MHz, 6.35 mm in diameter, f/2, 173 μm -6 dB pulse-echo lateral beamwidth at the focus) scanned over a rectangular grid, keeping the package in the focus. Subwavelength channel defects as small as 6 μm can be easily detected but appear larger than 150 μm wide, according to the focal point size of the transducer. The time-gate used to create an image is chosen based on where the maximum reflection from the back surface of the material is expected. Images created with the RFS technique demonstrate higher contrast than images formed using the BAI or ultrathin C-mode (UTC) techniques. However, RFS imaging also has higher probability of not detecting a channel that is present