  Citation: Žiljak Grši´ c, J.; Jureˇ ci´ c, D.; Tepeš Golubi´ c, L.; Plehati, S. Security Graphics with Multilayered Elements in the Near-Infrared and Visible Spectrum. Information 2022, 13, 47. https://doi.org/10.3390/info13020047 Academic Editor: Gholamreza Anbarjafari (Shahab) Received: 10 November 2021 Accepted: 15 January 2022 Published: 20 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). information Article Security Graphics with Multilayered Elements in the Near-Infrared and Visible Spectrum Jana Žiljak Grši´ c 1,2, *, Denis Jureˇ ci´ c 3 , Lidija Tepeš Golubi´ c 1, * and Silvio Plehati 1 1 Department of Informatics & Computing, Zagreb University of Applied Sciences, 10000 Zagreb, Croatia; splehati@tvz.hr 2 Department of Multimedia, Design and Application, University North, 48000 Koprivnica, Croatia 3 Faculty of Graphic Arts, University of Zagreb, 10000 Zagreb, Croatia; djurecic@grf.hr * Correspondence: jana@tvz.hr (J.Ž.G.); ltepes2@tvz.hr (L.T.G.) Abstract: In this paper, the fusion of four graphics into one integrated graphic is selectively observed in the visible and infrared spectrum. Each graphic represents its own information derived from the following sources: vector graphics, drawing, photograph and textual information. One graphic will be visible to the naked eye after the print. The other graphics will be observed with an NIR surveillance camera. These other graphics are nested into the selected visible graphics. All the graphics together make up a security print product with the characteristics of an individual solution with multilayered elements. Reprinting is possible only for the person in possession of the solution created according to the algorithm based on the INFRAREDESIGN ® method. When these graphics are printed on paper, it is impossible to produce an identical graphic prepress (C, M, Y, K) to produce forgery with the same dual properties in the visible and NIR spectrum. Keywords: NIR spectrum; dual graphics; spectroscopy of twin dyes; nested picture; CMYKIR VZ separation 1. Introduction In this paper, we extend the domain of computer graphics to include two light spectra—the visible and the near-infrared spectrum. The final solution and dyes are visually observed with a double ZRGB camera (Figure 1) in the “Z”-near-infrared and “V”-visible (RGB) spectrum [1]. In this paper, we develop and present the procedure of merging computer graphics with the INFRAREDESIGN ® idea that will manifest itself separately in two light spectra: the visible (V) and near-infrared (NIR-Z). State-of-the-art INFRAREDESIGN ® is a field of security graphics with the idea of fusing two pictures that will manifest themselves separately in the near-infrared (NIR) and visible spectrum after integration. A one-way hierarchy of pictures was published, where the first picture hides the second picture. Previous solutions have shown the merging of only two graphics for two light spectra [2]: a dress with a hidden portrait or code and clothes with hidden textual information, printed with inkjet inks on a plotter for textiles. The clothing was designed with computer graphics, with the theme “camouflage clothing”. ZRGB cameras are used to observe and photograph double solutions [1]. The Z camera, which photographs the NIR wavelength range, has a filter for 1000 nm. The visible and NIR condition were recorded in daylight without an additional IR source. Spectral analyses of light absorbance for process dyes in two light spectra—the visible (400 to 750 nm) and the beginning of the near-infrared (750 to 1000 nm)—have been performed for several printing techniques [3]. Since our eye does not register the NIR range, it can be said that the graphics in this range are “hidden from the naked eye” [4]. Several models have been published that combine the colors and dyes in the visible spectrum and the coverage of the dyes for those parts of the image that present two spectral ranges. The implementation of the dual image is suitable for the fast creation of highly protected individualized documents [5]. Information 2022, 13, 47. https://doi.org/10.3390/info13020047 https://www.mdpi.com/journal/information