LOOK: Mystery Images Made with Commercial Inkjet Printers

Researchers from the University of Utah have found a way to print hidden images using a very inexpensive product: commercial inkjet printers. By using an invisible ink to hide messages in seemingly regularly printed images, it is a revolutionary way to detect counterfeit items in the future.

Ajay Nahata, a professor of electrical and computer engineering at the University of Utah and the Associate Dean for the Academics College of Engineering, is the leader of the study that was published in The Optical Society's journal for high impact research, Optica, and his team were able to print hidden images that are made visible when illuminated with appropriately polarized waves in the terahertz region of the electromagnetic spectrum.

"We used silver and carbon ink to print an image consisting of small rods that are about a millimeter long and a couple of hundred microns wide," said Nahata. "We found that changing the fraction of silver and carbon in each rod changes the conductivity in each rod just slightly, but visually, you can't see this modification. Passing terahertz radiation at the correct frequency and polarization through the array allows extraction of information encoded into the conductivity."

The researchers were able to showcase their new method of concealing image information in a set of printed rods that all look almost identical by using the technique to conceal both grayscale and 64-color QR codes, and even embedded two QR codes into a single image, with each code viewable using a different polarization. Though the images look identical when first studied, the embedded QR code image becomes visible when viewed with terahertz radiation.

"Our very easy-to-use method can print complex patterns of rods with varying conductivity," shared Nahata. "This cannot easily be done even using a multimillion dollar nanofabrication facility. An added benefit to our technique is that it can performed very inexpensively."

The researchers are expanding their technique so the images can be interrogated with visible, rather than terahertz, wavelengths. Nahata and his team of researchers are also looking at the feasibility of developing additional capabilities that could make the embedded information even more secure.

"As we were printing these rods we saw that, in many cases, we couldn't visually tell the difference between different conductivities," said Nahata. "That led to the idea of using this to encode an image without the need for standard encryption approaches."