Fig. 3: Multiplexed luminescent QR code based on s-modules printed on banknote specimens.

a Black QR code printed directly on top of a 100 € banknote specimen under daylight illumination. The QR code is fully visible and ready to be accessed. b, c Images of the banknotes under 254 nm and 365 nm excitation, respectively, displaying the Tb³⁺ based QR code (green) and Eu³⁺ based QR code (red). d Blue and green coordinates histogram and e blue and red coordinates histogram for QR codes displayed in (b) and (c), respectively, with the decision level marked. The shaded areas are the modules colour coordinates of interest for identification. f Decoded QR codes obtained from the decision level of (d) and (e). The red square in (f) is one misinterpreted module. The images in a–c are not real images; they are digital montages; whose only purpose is to illustrate a possible application.