Optical disc

An optical disc is a flat data-storage medium that uses laser light to read encoded microscopic features. Some formats also use lasers to write data.

Laser diode: A semiconductor device that emits coherent light when electric current passes through it. Its focused beam illuminates the disc’s data track during reading and writing.
Optical storage: A data-storage method that uses light to record or retrieve information from a medium. Optical discs are removable implementations of this broader storage method.
Compact Disc: An optical disc format introduced for digital audio and later used for computer data. Its infrared laser and relatively wide track spacing define the earliest widely adopted digital disc family.
CD-ROM: A read-only Compact Disc format used to distribute computer data and software. It adapted CD manufacturing for mass-produced computer files and programs.
Pit (optical disc): A microscopic depression in a pressed optical disc’s reflective data layer. Pits and the lands between them alter reflected light to encode information.
Track pitch: The distance between adjacent tracks on a storage medium. Track spacing helps determine how densely an optical disc can store data.
DVD: An optical disc format designed for higher capacity than a Compact Disc. A shorter-wavelength red laser and tighter data geometry increase capacity over CDs.
CD-R: A Compact Disc format that can be recorded once using a compatible drive. Its write-once layer made personal disc creation possible without pressing equipment.
Optical disc drive: A device that uses a laser and photodetector to read or write optical discs. Its optics track the data spiral and convert reflected light into signals.
Spiral track: A continuous spiral path along which data is arranged on a storage medium. Most optical discs organize their data along a spiral read from the center outward.

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