Foil seals such as aluminum-to-plastic or aluminum-to-glass cap closures are impossible to visually inspect because of the opacity of the metal seal, yet are extremely important to assure the integrity of the contents—such as pharmaceutical products. A nondestructive and noncontact terahertz scanner, however, from Applied Research & Photonics (ARP; Harrisburg, PA) can perform the inspection with high specificity and accuracy while providing both tabular/graphical data as well as visual data to readily identify the physical location of seal failure. This enables inline workflow assembly and sealing processes to be corrected in near-real time.
The terahertz scanner system illuminates a sample mounted on a rotary stage with a 10 mW dendrimer dipole excitation (DDE)-based terahertz source and analyzes the reflected signal level as the object is rotated. Because reflectivity is proportional to the distance between the surface under test and the detector, any raised foil regions are seen as a reflectivity increase in the tabular data compared to a reference scan that translates to a higher intensity in the visual analysis. Because delaminations and wrinkles are on the order of microns for this application, the 25 nm hardware sensitivity of the terahertz scanner readily identifies defective areas. Shorter-wavelength, visible, or infrared lasers cannot provide the higher sensitivity required for nanometer-level reflectivity data. ARP's DDE source produces a wide range of frequencies from approximately 0.1 to 30 THz, and uses a nanopositioning system and also a "molecular filtering" mechanism that allows detection of reflected terahertz intensity from a small feature area on the sample. Rapid rasterization then enables probing of 200 × 200 mm areas at standoff distances up to 6 in. at a rate of 7 mm/s. Reference: http://bit.ly/1HMEhze.