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, any of a variety of information storage disks that are played or read using a laser. Optical disks include compact discs (CDs and CD-ROMs), laser discs (see videodisc), and digital versatile discs (or digital video discs; DVDs and DVD-ROMs). WORM [Write Once/Read Many] disks can be used to record data, but once data is recorded it cannot be altered except by obliterating the old version and storing the new version on a previously unused portion of the disk. Magneto-optical disks, such as the rewritable optical disk and the recordable disk used with the Mini Disc player, have a special layer, as of barium ferrite, that can be magnetically polarized by a recording head when heated with a laser. Data or sound may be recorded to and erased from any portion of a magneto-optical disk multiple times.