optical storage The storage or retrieval, or both, of data or images by optical means. Numerous methods have been explored, including holography, but current techniques depend on the use of a semiconductor laser and optical system to generate a very small spot of light (typically one micrometer in diameter) focused on a thin layer of a suitable medium to access each information element in turn. The principal configurations used are optical disk, optical card, and optical tape.

When writing data or images, the beam power is sufficient (typically 10 milliwatts) to heat the illuminated area of the medium so as to change its optical characteristics, reversibly or irreversibly. (In the case of magneto-optic recording, a magnetic field is also applied to control the state taken by the element of the medium as it cools.) When reading, the beam power is reduced to the point where it does not produce any change in the state of the medium, and the light reflected (or in some cases transmitted) by each element is detected and its intensity or polarization is observed to decide whether it represents a 1 or 0 bit.

Optical storage is capable of higher areal torage densities than have been achieved by magnetic media, and does not require the close medium-to-head spacing of magnetic storage. The medium is rugged, since the sensitive layer is beneath a clear protective layer; the light beam is out of focus at the surface of this and thus reasonably insensitive to dust or scratches. There is thus the potential of low-cost storage on rugged readily interchangeable volumes. On the other hand the optical components are relatively expensive and bulky, and the flaw density in the medium (relative to the element size) is higher than in magnetic storage thus requiring elaborate error correction techniques for most applications.

Three classes of media can conveniently be distinguished: rewritable, where recorded data can be erased and rewritten as in magnetic storage; write-once (or WORM), where information once written cannot be erased; read-only, where the information is impressed on the medium during manufacture and cannot subsequently be changed. Some technologies allow two of these classes to be combined on the same media volume.

Write-once media offer permanent storage once recorded and so are an attractive alternative to magnetic tape for archival storage. Read-only media are a very cheap means of istributing large amounts of data, such as images, or software in machine-readable form (see CD-ROM, DVD). Rewritable media compete more directly with magnetic storage, but are becoming popular for backup and for the storage of bitmapped images, which tend to need very large files.