Chris Neuzil writes:

Microscopic markings that are too small to be reproduced by standard printers can give rise to optical illusions, and could be used to mark authentic banknotes or luxury-brand goods.

Juergen Brugger, Victor Cadarso and their group at the Swiss Federal Institute of Technology in Lausanne fabricated microscopic lines in rows or spirals (pictured, left) by patterning pixels in gold using ultraviolet light. When an array of tiny cylindrical lenses is laid over the lines, interference patterns produce images (pictured, right) of letters and other symbols. Copying these patterns, or moirés, would be difficult, the researchers say, because even micrometre-sized blemishes visibly distort the illusion.

To read the complete article, see:Micro-optical method thwarts counterfeiting(www.rdmag.com/news/2013/07/micro-optical-method-thwarts-counterfeiting?et_cid=3399527&et_rid=472507257&type=cta)