Exploring

Inherent Vice: Optical Media

Data Layers

The type of data layer(s) used varies according to the format of the disc. Discs can be read-only (ROM), write-once (R), or rewritable many times (RW or RAM).

Read-Only (ROM) Discs

These include CD-ROM and DVD-ROM discs (both used for pre-ecorded audio, video, and computer applications). In ROM discs, the data layer is not actually separate; it is molded into the polycarbonate substrate and then coated with a metal reflective layer. In CD-ROMs and most DVD-ROMs, this reflective layer is aluminum. However, in order to put more data onto a disc, some DVD-ROMs are double-layered (on one or both sides), meaning that they have two metal reflective data layers, one of which is semi-reflective to allow the laser to pass partly through to the second fully reflective layer. In these types of discs, the semi-reflective layer is made from gold, silver alloy, or silicon.

If moisture or oxygen from the environment penetrates to the inside of the disc, the aluminum can oxidize, making it less reflective and eventually making the disc unreadable (this is known as "disc rot").

Write-Once Discs

These include CD-R, DVD-R, and DVD+R discs. In these discs, a separate data layer made up of organic dyes is sandwiched between the polycarbonate substrate and the metal reflective layer. These organic dyes are photosensitive, so marks are made in the data layer via chemical changes caused by the laser beam.

As with all organic dyes, these dyes will deteriorate slowly over time, especially if exposed to high temperatures and humidity. Extensive exposure to ultraviolet (UV) light and/or heat (e.g., sunlight) will also accelerate the deterioration of these dyes. However, if they are well cared for, these discs are much more stable than rewritable discs.

Rewritable Discs

These include CD-RW, DVD-RW, DVD+RW, and DVD-RAM discs. These discs also have a separate data layer sandwiched between the polycarbonate substrate and the metal reflective layer, but this data layer consists of a phase-changing metal alloy film. While this is a complicated process, in essence the laser beam makes marks in this film by heating it and reheating it during the rewriting process.

In rewritable discs, the metal alloy film deteriorates faster than the dyes used in write-once discs, thus making RW and RAM discs less stable. The metal alloy film deteriorates in response to heat, UV light, and high humidity. In addition, the rewriting process affects the life expectancy of rewritable discs; the more times a disc is rewritten, the shorter its life expectancy.