opticaldisc

An optical disc is a storage medium that records and reads data with a laser. Made of a polycarbonate substrate with reflective layers and a protective coating, data are stored as microscopic pits and lands arranged along a spiral track. An optical drive directs a focused laser beam at the disc, and the spacing of pits and lands alters the reflected light, producing digital signals detected by a photodiode. The disc is spun at varying speeds, and precise focusing and error correction enable data retrieval.

Common formats include CD, DVD and Blu-ray. Compact discs were originally designed for audio but later standardized as CD-ROM for data and CD-R/CD-RW for recordable media. DVDs added higher density with smaller pits and multiple layers, yielding about 4.7 GB per side, with dual-layer versions. Blu-ray discs use a shorter-wavelength laser and tighter tolerances to achieve much higher capacity, typically 25 GB per layer, with multi-layer variants.

Data formats and compatibility include CD-ROM, CD-ROM XA, CD-DA; DVD-ROM, DVD-Video; and Blu-ray Disc formats such as BD-ROM, BD-R and BD-RE. Recordable and rewritable versions exist for each family. Handling concerns include susceptibility to scratches, warping, and dye or binder degradation in some media. Long-term storage can be affected by environmental conditions; archiving uses hard media or M-DISC technology.

History and usage: Optical discs were introduced in the 1980s with the CD for audio, followed by the DVD in the 1990s and Blu-ray in the mid-2000s. Although streaming and solid-state storage have reduced their dominance, optical discs remain widely used for media distribution, software distribution, and archival backups, benefiting from standardized formats and relatively low cost per gigabyte.