Who doesn't love a surprise at this time of year? Well, researchers from the École Polytechnique Fédérale de Lausanne in France certainly do, so they have created a new kind of inkjet printing technique that produces images that appear different depending on the viewing angle.
The team had been playing around with halftone printing — where tiny dots of cyan, magenta and yellow make a seemingly continuous image from a distance — onto different materials. They noticed that when they used the technique on metallic sheets, the resulting colours varied slightly depending on the viewing angle. That's because, according to the researchers:
"Ink lines perpendicular to the incoming light create a large shadow and appear as "strong colours. Ink lines parallel to the incoming light do not induce a shadow and appear as "weak colours". When the print is rotated by 90 degrees, strong colours become weak and weak colours become strong."
Interestingly, it only works on metallic sheets. Paper diffuses the light too much for the effect to be noticeable.
Instead of simply giving a gallic shrug and moving on, though, the team pondered how they could use the phenomenon in a practical way. So they set to creating an algorithm that allows them to use the changing colour trick to superimpose two images that still make sense when the viewing angle changes.
The result allows them to print images using a normal inkjet printer that neatly change colour when spun 90 degrees. The team reckons that the trick could be used to manufacture new kinds of security images, for passports and credit cards.