Optical Media

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History

An explanation of the history of optical discs can be found on Wikipedia’s optical disc page.[1]


Technical Details

The term 'optical disk' describes a range of disk types where the stored information is read by a laser.

There are three main types of optical disk:

CD-ROM, (read-only memory) and DVD-ROM disks contain information that cannot be changed or added to by the user. On ROM disks, data is moulded as pits in the polycarbonate layer. A top metal layer usually aluminium, silver or gold reflects the laser.

CD-R (recordable) disks are blank when sold. While a region of the disk can only be recorded once, additional files can be recorded in different regions until the disc space is filled. Recordable disks use a photosensitive dye layer sandwiched between the polycarbonate and metal layers to capture and store data. Recording onto the disks requires dedicated hardware and software.

Rewritable optical disks, (CD-RW and DVD-RW) allow the user to record information on a disk, erase it, and replace it with new data. Rewritable disks have a phase-changing film between the polycarbonate and metal layers. To erase and rewrite data, the laser beam heats the film, changing its light transmission properties. They are used when information is being regularly revised, edited or updated. CD-RW disks also require dedicated hardware and software.

DVDs have a composite construction with the data stored between two layers of polycarbonate in the centre of the disk.[2]

Conservation Practices

Optical media suffers from the inherent vice of disc rot, which is the result of chemical and/or physical deterioration of the disc media. Compact discs can also be subject to "bronzing", which happens when the protective "outer coating of the CD erodes, leaving a silver layer exposed"[3]which then is subject to tarnishing.

References

This article uses information from The National Archives of Australia website which is released under the Creative Commons Attribution 3.0 Australia License.


  1. http://en.wikipedia.org/wiki/Optical_disc
  2. © Commonwealth of Australia (National Archives of Australia) 2013.
  3. Sydell, Laura. 2014. "How Long Do CDs Last? It Depends, But Definitely Not Forever". National Public Radio, August 18.