Confidential documents or security papers normally need a special, and mostly expensive, technique in order to ensure authenticity and safety. Still might change in the future with the invention of Ajay Nahata, professor of electrical and computer engineering from the University of Utah and his team. The engineers developed a method, which uses standard inkjet printers and readily available ink to print hidden images that are visible only when using a specific illumination.

Silver and Carbon Inks for Inkjet Printers

In order to print hidden images, the researchers used special silver and carbon inks, which can be ordered online by everybody and used in a simple inkjet. In contrast to time-consuming and expensive lithography, the engineers from Utah managed to create various conductivities. In this process, they discovered the possibility of hiding information in the rods of that ink: “As we were printing these rods we saw that, in many cases, we couldn’t visually tell the difference between different conductivities. This led to the idea of using this to encode an image without the need for standard encryption approaches”, explains professor Nahata. To hide information the scientists print an array of rods that all look nearly identical. They used the technique to conceal grayscale and 64-color QR codes. They even managed to hide two QR codes in one image; each of them can be seen through a different illumination.

Invisible Images Become Visible

With the help of terahertz radiation, the scientists can read the coded information because the wavelengths in this region are suited for imaging the resolution available from commercial inkjet printers. The conductivity of the rods helps to make the images visible. In their demonstration, the researchers printed three different QR codes with a size of 72 by 72 pixels. In the QR codes, they created nine different conductivities, each coding for one gray level. When they illuminated it with terahertz, illumination around 2 percent of the rods gave different values. Nahata and his team even managed to create a colored QR code overlapping them corresponding to RGB color channels, resulting in 64 colors.

What can we Expect in the Future?

With otto normal inkjet printers for around 60 dollars, these images can be produced with a resolution of 100 microns. The researchers promise that with even slightly better printers one can get even a 20-micron resolution. They also experiment with much more complex and detailed images than the QR codes. Nahata now works to develop this technique further in order to make the embedded information much more secure.

What do you think how this might affect security printing? Leave us your opinion in the comment section below.