Counterfeiters date back to at least the 13th Century, when watermarks were invented to authenticate original documents. Ever since then, printers and forgers have been in an arms race to out-do eachother. Now new and simple technology may give printers the upper hand.
A team of researchers from the South Dakota School of Mines and Technology and the University of South Dakota have found a way to print invisible quick response codes onto documents. More frequently referred to as QR codes, these black barcode-like stamps contain digital information in the form of square dots arranged in a square pattern. The codes are commonly used in advertising and may contain a URL to another website or other useful information that can be accessed by scanning it with a camera from a smartphone.
Instead of making the QR codes with black ink printed on a white background, the researchers found a way to make the codes with invisible ink that's still visible to a smartphone camera.
The researchers started with ink made with nanometer-sized particles that glow under a laser light. But the way the ink fluoresces is different than expected. Normally, fluorescent ink emits light of a longer wavelength. For example, shine ultraviolet light onto one of those blacklight posters and visible light colors are produced.