The technology exploits inkjet printing, which use small dots of different colored ink to create a tonal picture. In the EPFL technique, the ink is printed along lines on metallic sheets that display different colors depending on how the metal is held. As light traverses the printed ink lines, some of the colors appear as “strong colors” due to shadowing, while others that are not in a shadow are seen as “weak colors.” When the metal sheet is rotated, the shadows change causing the weak colors to appear strong and the strong color to become weak.
This difference in the color patterns allows the researchers at EPFL to print two images on one piece of metal. The team developed an algorithm that predicts the color pattern that will appear at different viewing angles. This algorithm serves as the core for printing software that allows the researcher to print on metallic sheets using a standard inkjet printer. The technique produces a visible image and a hidden image on a piece of paper.
The printing method only works on metal, which can create directional shadows when a light hits the surface. This same technique cannot be used with paper, which diffuses light in many directions, making it impossible to have shadows. Researchers believe the technology can be used as an additional form of security for use in passports or printed money.