Raymond Chiang¹*, Pei-Li Sun²
¹ Central Engraving and Printing Plant, Taiwan (R.O.C.)
² National Taiwan University of Science and Technology, Taiwan (R.O.C.)
*Corresponding author: Raymond Chiang, colorful_ray@hotmail.com
Keywords: intaglio, microstructure, digital watermark, security document, color technology
Poster presentation video: https://youtu.be/1-VhREuPj9Q
Intaglio printing is the main technology for security documents, and it is also a work of art. The lines of the traditional etched intaglio can be thick or thin, but the depths of the ink grooves are the same. In this study, in addition to keeping lines of different thicknesses, we set different parameters through digital files and engraved intaglio plates with variable depths through CTiP (Computer To intaglio Plate). When printing with CTiP plate, paper is placed on the plate and compressed by a heavy roller. Then the paper is removed, and the ink has been transferred from the plate (groove) to the paper. The ink on the paper is raised with tactile effect, the color density changes richly, and we call it multi-level intaglio. To identify multi-level intaglio documents, watermark is used as an identification method. But the watermark on the paper is an image formed by observing the density of the fibers through light transmission. However, the light- transmitting method cannot be used for thick intaglio ink and paper. Therefore, in this study, the reflective-watermark method is proposed for the identification of the prints with multi-level in- taglio. Take images of different focal planes at 2um height intervals via electron microscope. Connect the plane coordinates of 2D images with different focal planes and the corresponding height parameters into a 3D structure. Then it is rendered into multi-tone digital watermark image to judge the authenticity of the artwork.
If the ink thickness of multiple layers significantly affects the visual color density, we can adjust the width of the engraved lines to keep the color appearance consistent. It can make imi- tator unaware of the location of the watermark. The 3D watermark patterns after the color con- sistency adjustment can also be retrieved using the method proposed in this study.
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