managing electronic records in governmental bodies - National ...

73
in optical disks.
WORM and rewritable disks share the same basic structure as a CD-ROM, but with
extra layers to allow for the recording process. In the case of rewritable disks one of
the additional layers is magnetic.
DVD's are formed by back to back bonding of two 0.6 mm thick 12 cm optical disks.
They have four to five times the capacity of normal CD's.
3. Deterioration of optical disks
We cannot control the inevitable deterioration of materials, but we can control how
fast it happens.
Certain materials are susceptible to deterioration in particular ways due to their
properties, and other materials deteriorate as a result of particular environmental
conditions. Optical disks are a very dense form of information storage, so even
small amounts of degradation can lead to significant information loss.
Optical disks can be particularly prone to deterioration due to flaws arising in their
manufacture, for example:
• Water or air trapped under the coating during moulding can lead to corrosion of
the aluminum reflective layer.
• Rapid cooling of the plastic base or coating can result in cracks that also lead to
dropout of information.
• Bonding between the different layers may be weak, leading to delamination.
• Inks used to print information on the outer surface may corrode the plastic.
• The polycarbonate plastic layer has a tendency to ‘flow’ over time. This means
that the plastic layers may slowly lose their shape, eventually making it difficult
for them to be processed by the machinery used to read them.
• Because they are read optically any marking that interferes with the light path,
e.g. scratches or surface deposits, can cause reading problems such as skipping
or repetition of tracks. Some deposits, such as fingerprints, may cause etching of
the plastic surface and can lead to irreversible damage.
Improvements have been made in optical disk technology to address some of these
problems. For example, some optical disks are now being produced with gold
metallic layer, which cannot corrode, rather than aluminum.
4. Handling and care of optical disks
o Handle with care.
o Lint-free cotton gloves should be worn to avoid scratching or other marking of the
surface. If disks must be handled with bare hands then fingers should never be
allowed to touch the reflective side of the disk.
o Disks should only be removed from their protective packaging for use and
returned immediately after use.
o Food and drink should never be consumed where optical disks are in use.