Six-Color Holographic Color Print

Researchers from the Singa­pore Univer­sity of Tech­nology and Design SUTD have invented a new type of anti-counter­feiting device that can be useful for counter­feit deterrence of important documents such as identity cards, passports and bank­notes. Calling it “holo­graphic colour prints”, the research team led by Joel Yang demon­strated an optical device that appears as a regular colour print under white light, but projects up to three different images onto a distant screen when lit up with laser pointers.

Enhanced optical security provided by a six-color holographic color print. (Source: SUTD)

Unlike regular diffrac­tive optical elements that have a frosted-glass appearance and projects only single images, these new holo­graphic colour prints can be a stronger deterrent to counter­feiters while looking pretty at the same time. In addition, the prints consist of nano-3D-printed polymer structures and find parti­cular use in optical document security.

Counter­feiting is a great menace to daily life, and the global anti-counter­feiting industry will reach more than US$ 357 billion by 2026 according to a report from Credence Research. Anti-counter­feiting products are important and essential for protec­ting products and documents. The most popular anti-counter­feiting products are based on holo­grams that can be found every­where including on electronic device package boxes, medicine bottles, and surfaces of bank cards. However, these anti-counter­feiting hologram products only modu­late the phase of light and can be easily copied.

The research team developed a new type of anti-counter­feiting device, i.e. holo­graphic colour printing, which modu­lates both the phase and amplitude of light. Holo­graphic colour printing displays a coloured image under ambient white light by tuning the ampli­tude of light, while projecting up to three different holo­grams under red, green, or blue laser illumination. This feat was achieved by fabri­cating a new type of nano­structured pixel strategi­cally arranged on a plane. Each pixel acts as a speed bump (phase control) and road blocks (amplitude control) for light. The dual-function of holo­graphic colour printing increases security and deters counter­feiting.

Joel Yang shared that the device’s colour pixels are created by overlaying structural coloured filters onto phase plates. Nano­structured posts of different heights are employed as struc­tural coloured filters to modulate the ampli­tude of light. The team developed a computer algo­rithm that takes multiple images as its input, and generates an output file that deter­mines the positions of different phase and coloured filter elements. The holo­scopic print was then sculpted with the aid of a nanoscale 3D printer. The team used Luigi Russolo’s art painting Perfume (1910) as a colour print that is viewable under ambient white light. Different thick­nesses of poly­merised cuboid are used to modulate the phase plates and form three multi­plexed holograms, projected as a red thumbprint, a green key, and blue let­tering that reads “SECU­RITY”. All of these images were embed­ded within a single print.

“The relation­ship of holograms in combating counter­feiting is analogous to anti­biotics against infections. Every so often, new tech­nology is needed to deter counter­feiters as the old fashioned holograms become easier to copy,” said Joel Yang. He further added, “For the first time, multiple holo­grams that are color selective are woven into a colour­ful image using advanced nano­fabrication techniques. We are hopeful that these new holo­graphic colour prints are user friendly but counter­feiter unfriendly: They are readily verified but challen­ging to copy, and can provide enhanced security in anti-counter­feiting appli­cations.” (Source: SUTD)

Reference: K. T. P. Lim et al.: Holographic colour prints for enhanced optical security by combined phase and amplitude control, Nat. Commun. 10, 25 (2019); DOI: 10.1038/s41467-018-07808-4

Link: Nanoplasmonics and Nanofabrication, Singapore University of Technology and Design SUTD, Singapore

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