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.

The first optical disk drives suitable for data storage (with a read error rate after correction better than 1 in 10¹² bits) appeared on the market at the end of 1984: these all used write-once media on large-diameter disks (350 mm to 200 mm). Read-only disks, particularly CD-ROM, appeared about the same time. Rewritable disks became practicable several years later because of difficulty in developing reliable media, but have now replaced write-once disks for many purposes. Optical disks are very robust, need no special environmental control, and have an almost unlimited life.

Optical disks have higher capacity than magnetic disks of similar cost, but their performance is lower than that of hard magnetic disks although higher than that of floppy disks. They are therefore rarely used as the working store of a computer, but are suitable for archival storage, backup, and data distribution and exchange. They are widely used for storage of bitmapped images, such as scanned documents, because of their low cost per bit. Such images have natural redundancy so a poorer error rate is acceptable; in the early days of optical storage, optical disks without error correction (giving a read error rate of about 1 in 10⁵) were used for storing scanned documents written in Japanese Kanji characters, but nowadays a high level of error correction is used in nearly all cases so that the disks and drives are adaptable to all purposes. An exception is the format used for sound and video (but not text or digital data) on CD-ROM, where a higher error rate is acceptable as this material is naturally redundant.

See also DVD, optical disk library.

optical disk In computing, a high-density storage device consisting of a disk on which data is recorded and read by a laser. The most common type is a CD-ROM, although an audio compact disc (CD) is also a read-only device of this kind.