Foreign-body detection in dry food using continuous sub-terahertz wave imaging

Recently, significant attention has been paid to the food safety issue of foreign-body detection in the food industry. Commercialized X-ray imaging is extensively used for monitoring any foreign bodies that may be embedded in food products. However, foreign bodies such as low-density organic materials cannot be easily observed by conventional detection methods. The sub-terahertz and terahertz wave methods can provide not only the identification of a low-density foreign body but also a safe inspection due to the non-ionizing radiation. In this study, foreign bodies concealed in dry food were detected with a high-resolution raster scan imaging system at 210 GHz in the sub-terahertz wave range. Our raster-scan imaging system using Gaussian beam focusing demonstrated the diffraction-limited spatial resolution in the sub-terahertz wave range. Transmission images of foreign bodies showed better contrast than reflection images. Nondense materials, such as insects and plastics, were observed with the continuous sub-terahertz wave, in contrast to the difficulty in observing them in X-ray transmission imaging.