| Summary: | We present a method of detecting production faults in flexible plastic packages using terahertz (THz) radiation. A focused THz beam is scanned along the sealed area of a package, and the transmitted signal is collected. Defect detection is effected through the large difference between the absorption coefficients of plastic and water for water-filled channel defects, and on the refraction index difference between plastic and air for air-filled channel defects owing to reduction of the incident signals into the detector. Compared to previous methods, such as visual and ultrasound inspection, our technique can be applied to optically opaque packages and does not require immersion in a matching liquid. The method was tested on fabricated 10∼100-μm diameter water-filled and air-filled channel defects imbedded in polyethylene films. The detection limit (the minimum size of a detectable defect), which depends on the conveying speed, was determined and analyzed. The results show that our system has potential for application in actual-production, real-time, inspection.
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