Optical media preservation
The preservation of optical media is essential because it is a resource in libraries, and stores audio, video, and computer data.
While optical discs are generally more reliable and durable than older media types, (magnetic tape, LPs and other records) environmental conditions and/or poor handling can result in lost information.
Instead, one must maintain data transferred to storage and provide access and ensure integrity of the information for the long-term.” As technology changes data can be migrated from an older to a newer type of media to avoid media failure or “format obsolescence”—a real threat for technology when it is no longer supported.
If the machine required to play and read the discs is not kept in working order and maintained, data loss may result.
Issues which affect data longevity of nominally archival-grade discs include the following: dye failure (discs with premium organic and pthalocyanine-based long-life dyes are more suitable); bonding failure (premium bonding agents and edge-to-edge coverage improve longevity); scratches, minimised by careful handling and a scratch-resistant coating; production quality (some factories have better quality control standards, and discs from a batch known to be good may be more reliable than another batch).
CD-ROMs/DVD-ROMs (read-only-memory) are commercial grade discs and use a metallic data layer created using a molding machine that stamps pits (depressions) and lands (flat surfaces) into a polycarbonate substrate base.
While aluminium is most frequently used, it can potentially oxidize and lose data, a process sometimes called "disk rot".
For archival discs, silver or gold layers are preferred because of longer life expectancies and better reflectivity.
CD-RW/DVD-RWs (rewritable) are recordable, erasable and re-recordable discs that use a phase changing film data layer that reacts to heat.
Both disc types use unique blue-violet laser beams to read data, and are not compatible with equipment for other formats.
The structure of Blu-ray discs is more similar to CDs, with the information stored directly beneath the surface.
Early Blu-ray discs were easily damaged, but a protective layer that made them less delicate was developed.
Optical discs are not subject to wear as they are read without mechanical contact with the surface, but are susceptible to scratches from handling.
Data recorded on discs has redundancy, so that error detection and correction can compensate for some degree of damage.
UV rays in sunlight are energetic enough to produce a photochemical reaction that changes the optical properties of the dye.
It has been recommended that discs be stored vertically, if possible in a cabinet or drawer less susceptible to changes in temperature or humidity.
[15] If scratches on the laser-reading side of an optical disc prevent it from being read, it may be possible to recover all or most of the content once, and transfer it to another storage medium.
There are software packages that analyse data on a damaged storage medium and can recover some or all otherwise inaccessible information.
However, since this is an ablative process in which a layer of the substrate is removed, it is only safe to do a few times before risking the loss of the whole disc.
