Disc Rot or Laser Rot

Disc rot, laser rot, CD rot, and DVD rot are terms used to describe the degradation of optical discs. However, this type of deterioration is not an actual rotting of the disc and it is not the laser light causing the deterioration.

Optical discs contain a metal layer within the disc structure. The purpose of the metal layer is to reflect the laser light from the player or reader back to the equipment’s signal detector after the data or information layer is read. This allows a signal to be registered by the equipment. Without the metal layer, the laser light would read the data layer and then shine right through the disc and the detector would never receive any information from the disc. Therefore, the metal layer and the integrity of this metal layer is a vital component in the proper functioning and readability of optical disc formats.

The composition of the metal layer differs depending on the disc format. A summary of the types of metals used is provided below.

  • Read-only CDs such as purchased audio CDs – aluminum or aluminum alloy
  • CD-Rs and DVD-Rs and DVD+Rs – silver, silver alloy, or gold
  • CD-RWs and DVD-RWs and DVD+RWs – aluminum, aluminum alloy, silver, or silver alloy
  • Read-only DVDs such as purchased movie DVDs (single layer) – aluminum, aluminum alloy, silver, or silver alloy
  • Read-only DVDs such as purchased movie DVDs (dual layer) – first layer or semi-reflective layer can be silver alloy, silicon, or gold; second layer can be aluminum, aluminum alloy, silver, or silver alloy
  • Blu-ray discs use similar materials although silver alloy seems to be in more widespread use.
disc rot or laser rot as seen from the back of an audio CD

Disc rot and laser rot are basically the chemical degradation of the metal reflective layer. With the exception of gold, metals used in optical discs will oxidize or corrode when exposed to moisture and oxidizing compounds. These oxidizing compounds can be from the oxygen in the air we breathe, atmospheric pollutants, volatile agents coming from poor quality storage enclosure materials, inks and labels on the discs, paper or other materials stored in the case with the disc, etc. When CD disc rot or DVD disc rot occurs the metal layer may discolor, may form holes (from very small pinholes to large holes to complete disappearance of the metal layer), or the metal layer may get thinner. If any of these are noticed, then laser rot is the likely problem.

This type of degradation is not repairable. The error correction system associated with optical disc technology may or may not be able to compensate for the damage. However, the capacity of the system is limited and eventually extensive laser rot will cause playability problems or disc failure.

Disc Rot and Disc Protective Layer

Disc rot generally occurs in discs that have a poor protective layer. This layer is supposed to provide physical and chemical protection for the metal layer. If the protective layer is poor quality and not completely covering the metal layer then oxidation of the metal layer can occur. Early audio CDs did not have a good quality protective layer and many discs failed because of this type of degradation. Laserdiscs also suffered from this problem. The formulation for the protective layer was adjusted once it was noticed that this deterioration problem was common. Nowadays, laser rot can still be a problem, especially in poorly manufactured discs. If gold is used as the metal layer, as in the best CD-Rs and DVD-Rs, then laser rot is not a problem since gold metal is very stable or inert and does not oxidize.

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