Laser rot

Disc rot is a phrase describing the tendency of CD or DVD or other optical discs to become unreadable due to physical or chemical deterioration. The causes of this effect vary from oxidation of the reflective layer, to physical scuffing and abrasion of disc surfaces or edges, including visible scratches, to other kinds of reactions with contaminants, to ultra-violet light damage and de-bonding of the adhesive used to adhere the layers of the disc together.

In CDs, the reflective layer is immediately beneath a thin protective layer of lacquer, and is also exposed at the edge of the disc. The lacquer protecting the edge of an optical disc can usually be seen without magnification. It is rarely uniformly thick; thickness variations are usually visible. The reflective layer is typically aluminium, which reacts easily with several commonly encountered chemicals such as oxygen, sulphur, and certain ions carried by condensed water. In ordinary use, a surface layer of aluminium oxide is formed very quickly when an aluminium surface is exposed to the atmosphere; it serves as passivation for the bulk aluminium with regard to many, but not all, contaminants. CD reflective layers are so thin that this passivation is less effective. In the case of CD-R and CD-RW media, the materials used in the reflecting layer are more complex than a simple aluminium layer, but also can present problems if contaminated. The thin 0.25-0.5mm layer of protective lacquer is equivalent.

DVDs have a different structure from CDs, using a plastic disc over the reflecting layer. This means that a scratch on either surface of a DVD is not as likely to reach the reflective layer and expose it to environmental contamination and perhaps to cause corrosion, perhaps progressive corrosion. Each type of optical disc thus has different susceptibility to contamination and corrosion of its reflecting layer; furthermore, the writable and re-writable versions of each optical disc type are somewhat different as well. Finally, discs made with gold as the reflecting layer are considerably less vulnerable to corrosion problems, though no less susceptible to physical damage to that layer. Because they are less expensive, the industry has adopted aluminium reflecting layers as the standard for factory pressed optical discs.

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