“Materials engineers led by Jon Kellar at the South Dakota School of Mines and Technology says their invention comprises a QR code made of nanoparticles that have been combined with blue and green fluorescent ink. The code is sprayed onto a surface — paper, plastic film, office tape, glass — using an aerosol jet printer. It remains invisible until the object is illuminated by a near-infrared laser. The nanoparticles absorb photons at a non-visible wavelength but emit them in a visible wavelength, a trick called upconversion that causes the QR code to pop up almost like magic and allow itself to be scanned.”
http://www.rawstory.com/rs/2012/09/11/new-nano-code-ups-the-fight-against-counterfeiters/
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