Optical disc technology was invented in the 1960s and is the basis for the CD and DVD formats that are familiar to us today. In the 1990s, prerecorded compact discs (CDs) became the most popular medium for recorded sound. Various CD formats have since become popular for other applications as well, with CD-ROM (read only memory) and CD-R (recordable) both introduced in the 1990s for computer applications.
DVD (originally digital video disc) technology was introduced in 1995 as a medium for playing full-length movies and gradually became dominant over the videotape. There are various formats of DVD, which are likely to replace CD formats in the near future. These include DVD-ROM for computer data, DVD-RAM (a writeable version of DVD-ROM), and DVD-Audio.
Optical discs are multilayered objects that store digital data; they are used for audio, video, and computer data. Some are prerecorded, and others allow the user to record data. To record data onto an optical disc, a laser passes through a polycarbonate support and etches marks (known as pits and lands) into a data layer to represent the bits and bytes of digital data. To read data, the laser passes through the support and the data layer and is reflected off a metal reflective layer (sometimes these are separate layers and sometimes not).
Different types of discs have different structures and composition. A DVD is essentially two CDs glued together; that is, it has two layers of polycarbonate that are joined with adhesive in the center. Thus, CDs can be read and written on one side only, while DVDs can be read and written on one or both sides, depending on the format.
For illustrations of optical disc structure, see Chapter 3 (Disc Structure) of Care and Handling of CDs and DVDs: A Guide for Librarians and Archivists (PDF).
When audio CDs first became popular (in the late 1980s and early 1990s), it was thought that optical disc technology would prove to be very stable, but this is not the case. Like other media found in cultural collections, optical media are vulnerable to physical deterioration. In addition, they face the problem of media obsolescence—in many cases the technology to read the media will become obsolete and unavailable before the medium itself deteriorates.
The specific materials that make up the various CD and DVD formats (and their vulnerability to deterioration) will be discussed in this section. The problem of media obsolescence will be addressed in Session 6: Reformatting and Treatment.