managing electronic records in governmental bodies - National ...

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ANNEXURE G: Handling optical storage media
1. Types of optical disk
The term ‘optical disk’ is used to describe a range of disk types where the
information is held in a form that is read optically, i.e. by a light source (usually laser)
and photoelectric cell.
There are two main types of optical disk:
- Compact Disk (CD) that consist of the following types:
• ROM disks contain information that cannot be changed or added to by
the user (ROM stands for ‘read-only memory’). The best-known type
of CD-ROM is the music compact disk, but they are also becoming
popular as a replacement for print copies of large publications, such as
encyclopaedias. CD-ROM disks are usually 12 cm in diameter,
although other sizes have been used such as the 30 cm laser disks
used for motion pictures.
• WORM disks are also known as read-write optical disks (WORM
stands for ‘write once read many’). They are blank when sold, and
allow the user to record information on them, which cannot be removed
or changed. Recording onto the disks requires dedicated hardware.
They can be read in CD-ROM disk drives. Executable CD-WORM
disks are disks where the programme needed to access the content of
the disk is recorded on the disk itself.
• Rewritable disks – are a form of optical disk technology that allow the
user to record information on a disk, erase it, and replace it with new
data. They are used when information is being regularly revised,
edited or updated. They are also used for short-term information as
they can be wiped and reused when the information is no longer
needed. As with WORM disks, recording on to rewritable disks
requires dedicated hardware but once this is done they can be read in
CD-ROM disk drives.
- Digital Versatile Disks (DVD) which is basically a 2 nd generation high density
compact disk that also consist of ROM, WORM and Rewritable versions.
DVD drivers are backward compatible to enable them to read CD's
2. Composition of optical disks
An optical disk has a number of layers. CD-ROMs have a stable polycarbonate
plastic base. The polycarbonate base is pressed out from a master mould and holds
its information as a series of tiny depressions. The base layer is then covered with a
thin layer of metal (usually aluminium) to make it reflective. To protect the metallic
layer the whole disk is entirely sealed with a thin layer of clear polycarbonate. This
laminar structure is the main source of many of the preservation problems that arise