Cholesteric Spherical Reflectors as Physical Unclonable Identifiers in Anti-counterfeiting

Cholesteric Spherical Reflectors (CSRs) are made of droplets of cholesteric liquid crystals (the same material under the screen of our mobile phones) but molded in a spherical shape and hardened into a solid. CSRs have a peculiar behavior when illuminated: they reflect light and produce unique optical patterns whose full display is hardly predictable. They have been argued to behave like an optical Physical Unclonable Function (PUF), therefore finding application in anti-counterfeiting, in particular for object authentication. However, a fundamental challenge remains open: to understand what makes each optical response unique and how to extract this identifying information reliably and repeatedly. We study the problem, and we design and discuss two pivotal procedures to build authentication protocols for objects coated with CSRs. We test the quality of our procedures against large data sets of pattern images: images from CSRs are used to calculate inter- and intra-distance; simulated patterns created artificially are used to measure security in terms of false positive ratio. Our procedures successfully cluster images coming from the same CSR, distinguishing them from images of different CSRs and decoys. Our work is one of the few that has studied procedures of information extraction for materials derived from CSRs. It advances the state of the art in this area, closing the gap between the research on optical PUFs and practical applications.