Novel lock-in amplifier for identification of luminescent materials for authentication

Abstract

Digital and analog design approaches are reviewed for handheld low-cost electronic signal processing boxes for close-up optical detection and identification of phosphor markers for authentication of paper money, legal documents, pharmaceuticals, clothing materials, and military friend and foe identification. For extending the range to longer distances of over a meter (several feet) we propose a novel low-cost handheld lock-in amplifier that uniquely identifies a phosphor at a distance of several feet in a noisy environment of daylight, sunlight, electronic noise and reflection of the stimulating beam. The lock-in amplifier differs from a conventional one by sampling the detector out of synchronization with the source to avoid reflections which will mask the phosphor luminescence and provide opportunities for counterfeiters. The luminescence decays slowly after stimulation is removed. We simulate the lock-in amplifier to determine the trade-off between speed of authentication and distance. Only 40ms of integration in the lock-in amplifier will block noise of frequencies differing by more than 1% from the modulation frequency to allow authentication over a meter.