Expanding the Angle of Incidence Tolerance of Unclonable Anticounterfeiting Labels Based on Microlens Arrays and Luminescent Microparticles

Unclonable anticounterfeiting labels can be based on micrometer-scale randomness created by stochastic processes like the distribution of luminescent microparticles in a transparent layer. Adding a microlens array to the layer can simplify the hardware needed for authentication in that magnification is no longer required. The bright point-pattern generated under light-emitting diode illumination can be captured by a standard digital camera. Shifting the angle of incidence (AOI) relocates the microlens foci changing the bright point-pattern. This provides unclonability, as several distinct bright point patterns at different AOI can be required for authentication. However, it also imposes technical requirements for the authentication setup in terms of the tolerance with which the AOI must be controlled. Herein, the AOI tolerance, the deviation of angle between reference and test image for which sufficiently similar bright point patterns are recorded that they are considered matching by the authentication algorithm, is investigated. Using microlens arrays with a focal length of 550 μm, the average size of the phosphor particle was varied from 9 ± 1 to 32.5 ± 2 μm, resulting in a relaxation of tolerance from 0.8° to 3.6°. ... mehrMethods to further increase the AOI tolerance and facilitate the practical implementation of these labels are discussed.