UNIVERSITY PARK, Pa. — If you look at the back of a credit card, a driver’s license, or even a dollar bill, you will likely notice a hologram, or iridescent feature, that appears to change color when viewed at different angles. These color-shifting security features have been around for decades, allowing ample time for people to find ways of counterfeiting the effects. Even so, these types of highly visible and easily recognized color-shifting features are still among the most commonly used optical security element — and Penn State-affiliated startup Chromatir may have discovered a more secure and customizable way to implement this effect.
“The origin of Chromatir started with a discovery in our lab that developed into some really interesting applications,” said Caleb Meredith, Chromatir co-founder and CEO and doctoral candidate in the Department of Materials Science and Engineering. “Creating iridescence typically requires materials to be patterned with very small nanostructures which cause light to undergo interference when it gets reflected. We realized there is actually another way to produce interference using a different type of reflective geometry 100 or even up to 1,000 times larger in size, leading to unique color-shifting appearances.”
These microstructures can be used to create color-shifting coatings which, unlike the typical iridescent nanostructures, are more customizable due to their size and shape. By using this new class of microstructures, security features can be designed that are more tunable in their colors and patterns.
“A traditional hologram is basically a bunch of dots or lines — you can change the spacing of the structures or their width, but it can only be controlled along one or two dimensions,” said Meredith. “Whereas, with the microstructures we are designing, you’re playing with geometries in 3D, and so there are more degrees of freedom by which we can use to control the shape. You can make a structure wider or deeper, change the slope of its sides, make it into a polygon or a donut shape — varying the shapes in each of these ways lead to differences in light’s reflections that result in subtle changes in color-shifts. By controlling the shapes of the microstructures, you control the color.”