In: Root » Computers and technology » Storage Devices
Two aspects of the design of the Compact Disc make the media resistant to abuse. Because the laser that reads the disc shines through the polycarbonate plastic base before reaching the data, it is focused below the surface of the disc. At the reading surface of the disc, the laser beam is still rather diffuse-spread out over a wide area-so smudges and scratches have a smaller effect on the readability of a given disc. Moreover, all CD formats use heavy duty error correction that compensates for any errors that do occur, at least if they are not severe. Despite the relative ruggedness of CDs, they are not invulnerable. Major scratches, heavy smudges, and contamination can all affect the disc, causing temporary or permanent errors. You can manually correct some of these errors; others may render a disc unusable. Prevention-which means proper disc care-is the best way to avoid disc problems. Because the surfaces of any Compact Disc are vulnerable to damage, you should always handle your CDs by the edges. If your hands are too small to reach across the expanse of a disc, you can always slide a finger partly into the center hole. If you must hold the surfaces of a disc, pinch them so that only your fingertips touch the disc as near as possible to the center hole. Never stack CDs atop one another as they may scratch each other. And avoid leaving CDs in direct sunlight. An excess of heat or light-and direct sun can overwhelm your discs with both-can damage them. The label side of a CD is actually more vulnerable to damage than the clear side through which the laser shines. It is protected only by a thin layer of lacquer that is more easily scratched than the polycarbonate disc base. A scratch on the label side can dig deep into the optical medium of the disc and damage your data. Chemicals can affect either surface of a CD. Solvent-based markers can penetrate the label-side lacquer and affect the optical medium. Labels, unless they are particularly designed for CDs, can leach chemicals that can damage discs. If you accidentally get fingerprints or other smudges on the reading surface of a Compact Disc, you can often clean them off. To avoid scratching the disc, use lens tissue or soft cotton and wipe the disc radially-that is, from the center to the edge in a straight line. Use no solvents other than clear water or lens cleaning solution. Wipe gently. Do not scrub your discs. CD makers report that filling in surface scratches is not practical, although in theory it is possible. The chemicals required are exotic-an optical adhesive-and they must be cured with ultraviolet light. According to the CD makers, it is far easier and less expensive to replace a scratched disc than even to attempt such repairs. In the early 1990s, a great deal of concern arose about the deterioration of CDs in storage, a process termed at the time CD rot. In fact, many CDs did actually decay to unreadability. The causes were twofold. The lacquer protective coating on some discs did not completely encapsulate the aluminized reflective layer of the disc. The aluminum oxidized, which changes its reflectivity and response to the reading laser of the drive. With sufficient oxidation, the disc became unreadable. In addition, the inks used for silk screened labels on some discs still were chemically active even after they had been cured with ultraviolet light. The chemicals in the inks reacted with the optical layer of the disc, causing deterioration and reading errors. Once these problems were discovered and understood, the CD industry quickly rectified them. CD rot is no longer a problem.
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