Fernández-Dorado et al., American Journal of Physics 76 (December 2008)

Telltale. The brighter laser beam reflection, accompanied by parallel lines, unmasks a counterfeit disc.

How to Outsmart a Video Pirate

Researchers have developed a dirt-cheap and simple way to expose bootleg CDs and DVDs. The approach is so easy, in fact, that anyone with a laser pointer can do it.

You can't always tell a disc by its back cover. Commercial and pirated CDs and DVDs look nearly identical to the eye, even though commercial products are manufactured in large quantities via a hydraulic press and a master recording, and bootlegged versions are burned, one at a time, with a laser. Up until now it has taken an expert's eye or special equipment to detect whether, say, a DVD of Spider Man 3 is the legitimate article or a knockoff.

Five optical physicists at Spain's University of Grenada think they have found an easier way. The key is structural differences between commercial and writable disks, and how those differences affect the way light waves bend around them--a process called diffraction. Commercial disks contain microscopic pits overlain with clear plastic material; a disc player's laser reads the changes in surface elevation as audio or video signals or both. Writable disks, meanwhile, contain a compound into which a recording laser can burn tiny marks that are read by a disc player--hence the term "burning" a disc. Shine a laser--even a standard laser pointer--on both, and the difference becomes clear, the team reports. On a manufactured disc, the reflected beam shows up as a small dot on a white surface. On a copy, however, the dot is larger and is accompanied by two parallel lines above and below it (see picture).

In this month's issue of the American Journal of Physics, the researchers describe testing their technique on more than 100 CDs and DVDs of various labels with different types of content. The method "is completely reliable," says co-author Javier Hernández-Andrés.

Optical physicist Chunlei Guo of the University of Rochester in New York state agrees and says and the finding should aid efforts to detect CD piracy. It's also a great way to teach students the concept of diffraction, he says, something that is difficult to spot in everyday life.

Posted in Physics