Exploring

Inherent Vice: Optical Media

Reflective Layers

The reflective layer in an optical disc reflects the laser beam to transmit the data back to the laser head. It is made from different types of metal, depending on the disc format.

Read-Only Discs

These include CD-ROM and DVD-ROM discs (both used for prerecorded audio, video, and computer applications).The data/metal reflective layer is made from aluminum in most of these discs. Aluminum can be subject to oxidation if kept in a humid and hot environment (the polycarbonate substrate will absorb moisture in such a situation, and cracks or scratches will make this worse). In double-layer DVD-ROM discs, there are two data/reflective layers; the first is semi-reflective and made from silicon, gold, or silver alloy, which are much less subject to corrosion.

Write-Once Discs

Write-once discs (such as CD-R) have a significantly longer life expectancy than rewritable discs.

These include CD-R, DVD-R, and DVD+R discs. Gold, silver, or silver alloy is used for the metal reflective layer in these discs as well. Silver can corrode by reacting with sulphur dioxide, a pollutant that can migrate through the substrate when it absorbs moisture. Silver alloys are somewhat less vulnerable to this corrosion. Gold is very stable, but it is expensive. All of these metals do not deteriorate as quickly as the dyes used in the data layer, however.

Rewritable Discs

These include CD-RW, DVD-RW, DVD+RW, and DVD-RAM discs. Like read-only discs, these discs use aluminum for the reflective layer, which can be subject to oxidation in poor environmental conditions. However, the metal alloy film used for the data layer will likely deteriorate much more quickly.

Little independent testing has been done to determine the lifespan of optical discs. However, from general knowledge about the composition of these discs, it is assumed that write-once discs should have a significantly longer life expectancy than rewritable discs. Rewritable discs are not intended, and should not be used, for long-term or archival use.