Most optical storage devices record data ona thin film by burning microscopic holes(pits)in the film with a laser.A laser-reader device reads the dta by sensing the presence or absence of holes.Optical storage devices are used to record and to retrieve video,audio and image data as well as text and computer data.the extremely high-density recording capability of optical devices enable one 5-in. optical disk to store more than 5 gigabytes of data.Twelve-in. disks can store well over twice the amount. Most optical disks are read-only and are used for long-term 'archival' storage.Stored optical data are relatively unaffected by magnetic fields,so optical disks can be mailed and taken through X-ray machines and airport scanning devices. There are basically three types of optical disks:compact disk read-only memory(CD-ROM),write-once read-many(WORM) and erasable units and media. CD-ROM uses the same technique as audio disksand it is moreefficiently used for prerecorded optical storage.It can be used to distribute a database that doesn't have to be frequently updated,for example. CD-ROM is often used for archival purposes.A standard CD-ROM system stores around 700 mb of data on a 4.7-1n. disk and drive. WORM storage devices provide one-time writing but unlimited reading of data.Data cannot be overwritten or erased but can be updated by writing new information into a file at another location on the disk.The new file is then linked to the original file through software. WORM optical disks are usedto handle data that must be occasionally updated and changed.WORM's long life expectancy eliminates periodic updates. Erasable optical storage media is an emerging area where improvements are continually being made.Erasable optical media are used in applications where stored data requires frequent changes.