Optical storage, electronic storage medium that uses low-power laser beams to record and retrieve digital (binary) data. In optical-storage technology, a laser beam encodes digital data onto an optical, or laser, disk in the form of tiny pits arranged in concentric tracks on the disk’s surface. A low-power laser scanner is used to “read” these pits, with variations in the intensity of reflected light from the pits being converted into electric signals. This technology is used in the compact disc, which records sound; in the CD-ROM (compact disc read-only memory), which can store text and images as well as sound; in WORM (write-once read-many), a type of disk that can be written on once and read any number of times; and in newer disks that are totally re writable.
Optical storage provides greater memory capacity than magnetic storage because laser beams can be controlled and focused much more precisely than can tiny magnetic heads, thereby enabling the condensation of data into a much smaller space. An entire set of encyclopedias, for example, can be stored on a standard 12-centimeter (4.72-inch) optical disk. Besides higher capacity, optical-storage technology also delivers more authentic duplication of sounds and images. Optical disks are also inexpensive to make: the plastic disks are simply molds pressed from a master, as phonograph records are. The data on them cannot be destroyed by power outages or magnetic disturbances, the disks themselves are relatively impervious to physical damage, and unlike magnetic disks and tapes, they need not be kept in tightly sealed containers to protect them from contaminants. Optical-scanning equipment is similarly durable because it has relatively few moving parts.
Early optical disks were not erasable—i.e., data encoded onto their surfaces could be read but not erased or rewritten. This problem was solved in the 1990s with the development of WORM and of writable/rewritable disks. The chief remaining drawback to optical equipment is a slower rate of information retrieval compared to conventional magnetic-storage media. Despite its slowness, its superior capacity and recording characteristics make optical storage ideally suited to memory-intensive applications, especially those that incorporate still or animated graphics, sound, and large quantities of text. Multimedia encyclopedias, video games, training programs, and directories are commonly stored on optical media.
CD, DVD and Blu-ray discs are forms of optical storage. They are a useful lightweight media for transporting data and programs. Data is written onto the disc by a laser and is read from the disc by another laser. The discs are cheap, but they are vulnerable to scratches. They cannot store as much data as a hard drive. A CD stores up to 800 MB, a DVD holds up to 4.7 GB and a Blu-ray holds up to 50 GB.
You need to have a CD, DVD or Blu-ray writer on your computer to write information to the discs. CD, DVD and Blu-ray discs are useful for creating film or music discs which are compatible with hardware such as DVD players.
Optical disk A type of optical storage in which the medium is in the form of a disk that is rotated to give one dimension of access while the light beam is scanned radially to give a second dimension. In nearly all cases the disk is exchangeable. This is easily arranged because there is a substantial clearance, typically 1 mm, between the surface of the disk and the nearest component of the optical system. The optical system is heavy and expensive compared to the corresponding components of a magnetic disk drive, so most optical drives are designed to access a single recording surface: if the disk has recording surfaces on both sides it is removed from the drive and reversed to give access to the second surface. (This is done automatically when the drive forms part of an optical disk library.) A few drives can access both sides of the disk. Multiple disk packs are not used.
Rewritable, write-once, and read-only media have been developed for optical disk drives: multifunction drives can read two or all three of these media types. Disk sizes range from 350 mm downward with 300 mm and 130 or 120 mm the most widely accepted, although smaller sizes are becoming popular. The only widely used format for read-only disks is CD-ROM.
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