“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.”Joel Yang
With the development of anti-counterfeiting products increasingly significant for industry, a new approach to color printing that helps to improve optical security has been developed at the Singapore University of Technology and Design. The novel holographic optical device uses four authentication images, a microscale color print and three holographic projections, that could be used for advanced security labels on banknotes, passports and credit cards, as well as in dye-free structural color printing.
As decribed in Nature Communications [Lim et al. Nat. Commun. (2019) DOI: 10.1038/s41467-018-07808-4], the device makes what the researchers call “holographic color prints”, which appear as standard color prints under white light but the images can be projected onto a screen when lit up with laser pointers, making counterfeiting more difficult and authentication easier.
Although holograms are one of the most popular measures for counterfeit documentation deterrence, those currently employed tend to use only the phase of light, and don’t exploit its amplitude, and patterned gratings to print security images, which are easily forged. While typical diffractive optical elements are only able to project single images, these new color prints bring complexity to the design to offer higher levels of security as well as being more visually appealing.
The prints are made up of nano-3D-printed polymer structures, and a colored image is displayed under ambient white light by tuning the amplitude of light, while projecting up to three different holograms under red, green, or blue laser illumination. The researchers demonstrated this by making a new type of nanostructured pixel arranged on a plane, with each pixel acting as a speed bump (phase control) and a road block (amplitude control) for light.
The color pixels were developed by overlaying structural colored filters onto phase plates, and nanostructured posts of different heights acted as structural colored filters that modulate the amplitude of light to realize a dye-free full color image. A computer algorithm that takes multiple images as its input was utilised, which then generated an output file that determines the positions of different phase and colored filter elements. The holoscopic print was produced with high-resolution nanoscale 3D printing, which offers greater functionality by incorporating nanostructures onto each phase block.
As team leader Joel Yang said “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.” The team now hope to produce large-scale and high-power hologram projections, and to develop dynamic holographic display for special applications.
A new holographic optical device that uses four authentication images