Now standard inkjet printers create “invisible†images

Researchers at the University of Utah have developed a way to create images which are only visible using polarised sub-millimetre electromagnetic waves. The researchers believe their new technique could be used to secretly conceal information in images that look normal to the naked eye, which could be useful in anti-counterfeiting or product authentication.

The team has used inks which are created from artificial materials that exhibit properties not found in nature (metamaterials) and commercially available inkjet printers. Building on previous research by the University of Utah team, where the team had created a relatively uncomplicated way to use a retail inkjet printer to print inks created with silver and carbon available from online specialty retailers — the researchers experimented with methods to print images with varying conductivities.

"We used silver and carbon ink to print an image consisting of small rods that are about a millimetre long and a couple of hundred microns wide," stated Professor Ajay Nahata of the University of Utah. “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 polarisation through the array allows extraction of information encoded into the conductivity.”

Although terahertz electromagnetic radiation is used to reveal the information embedded in the images due to the compatibility of the wavelength with images produced by ink-jet printers, the researchers are aiming to develop ways to utilise the technique at visible wavelengths.

The research team is also investigating the development of other methods that could further secure the embedded information by developing inks that first need to be exposed to heat or light of a particular frequency before the encoded data was made visible using sub-millimetre frequency radiation.