As compact discs have become the primary means of storing and distributing digital information around the world, they have also become a favorite of bootleggers, leading to global economic concern about the peddling of illegally obtained albums and movies.
But scientists at the University of Granada in Spain have discovered a new optical technique — which they describe as fast, economical and effective — to determine whether a compact disc, DVD or Blu-Ray is an original or a possibly illegal copy. "We have tested more than 100 CDs of different makes with different types of audio and video data and have found that the method is completely reliable," write the study authors, led by professor Javier Hernández Andrés of the Department of Optics of the University of Granada.
The key to the technique is the difference between replicated CDs - mass-produced using a hydraulic press - and duplicated CDs, or CD-Rs, which are used for smaller amounts of material and are "burned" using a laser. Although you can't tell them apart with the naked eye, the phenomenon of light diffraction distinguishes between the two types of CDS.
In this case, in a very dark room, the researchers directed a helium-neon laser beam towards any part of the surface of the CD where data is stored (this excludes the center or edge, which are blank). The CD was placed about a meter from the laser and oriented perpendicularly; the diffraction pattern of the laser light, reflected from the microstructures on the CD surface that store information, was then projected onto a screen. (You can conduct a similar at-home experiment using a laser pointer, but be careful.)
The diffraction pattern for a "burned" CD-R - no matter whether it's blank or full, old or new - will show two parallel lines on the screen; in contrast, "pressed" CDs do not have these two parallel lines. This is because the two methods of production result in microscopic differences on the CDs. Copied CDs display a series of miniscule marks on the organic material - including pigment, dyes, reflective metals such as gold and silver, and a protective lacquer coating -- that comprises their surface.
Replicated discs also have a "pregroove," used for timing and tracking, that runs from the inner to the outer diameter of the CD.
"The two parallel lines that appear on the diffraction experiment on CD-Rs are due to the pregroove," write the study's authors. "Because the pregroove is only slightly modified by the burning and melting process on duplicated CDs, which act only on the surface of the pigment layer, the two lines are not affected by the burning process. In contrast, pressed CDs are fabricated without a pregroove and consequently the two lines are not present in its diffraction pattern."
Although the paper makes no mention of how this technology could be applied in the fight against bootleggers, the researchers have submitted a patent application for the method and apparatus used. Who knows? Perhaps laser-light experiments will soon be coming to a flea market near you.