Penetrating Imaging of Concealed Features in Banknotes with Near-Field Scanning Microwave Microscopy

Traditional anti-counterfeiting techniques and methods are inadequate to meet the modern, sophisticated anti-counterfeiting requirement in banknotes. We applied our homemade near-field scanning microwave microscopy (NSMM) for penetrating imaging of concealed anti-counterfeiting features in banknotes. This NSMM imaged and characterized several vital concealed anti-counterfeiting features in the banknotes, including the dynamic color-shifting security threads, color-changing denomination numbers, magnetically embedded security threads, and white watermarks. At the same time, with the obstruction of red ink, near-field microwaves still penetrated the medium layer to characterize the detailed information of the security threads successfully. Additionally, the cavity perturbation method was used to obtain the effective permittivity of 3.65 and 3.62 for the windowing security thread and middle-embedded security thread. The thickness of the watermark was calculated to be about 3.3 μm with the calibrated fit method.