optical disc

An optical disc is a flat, circular substrate, typically 120 mm or 80 mm in diameter, onto which digital data is encoded as a spiral track of microscopic pits and lands. A laser beam, usually in the near-infrared range (780 nm for CD, 650 nm for DVD, 405 nm for Blu-ray), reflects off this surface with different intensity depending on pit geometry. A photodiode detects these reflections and converts them back into binary data. The disc itself is polycarbonate plastic, 1.2 mm thick, with a reflective aluminum or gold layer underneath the data layer and a protective polycarbonate top coat.

The three dominant formats differ in pit dimensions and wavelength. Compact discs (CD) hold 650 to 700 MB and spin at 200 to 500 rpm depending on read speed. DVDs increased capacity to 4.7 GB (single layer) or 8.5 GB (dual layer) by using shorter wavelengths and tighter spiral pitch. Blu-ray discs push this further with 25 GB single-layer and 50 GB dual-layer capacity using a 405 nm violet laser and pits as small as 0.32 micrometers along the track. LaserdiscĒMediaa predated the CD but saw limited adoption outside video playback.

Data on optical media degrades through oxidation of the reflective layer, polycarbonate embrittlement, and dye breakdown in writable variants (CD-R, DVD-R, BD-R use organic dyes instead of embossed pits). Humidity accelerates this; claimed archival life ranges from 20 years (consumer media) to 50 to 100 years for gold-layer pressed discs in controlled storage. Surface scratches that penetrate the protective layer cause read errors or complete data loss in that sector.

Manufacturing and variants

Pressed discs (read-only) are stamped from a master, while writable discs use a polycarbonate blank with a dye layer. When a write laser heats the dye, it changes reflectivity to mimic pits. Rewritable discs (CD-RW, DVD-RW) use phase-change alloys that can switch between crystalline (reflective) and amorphous (non-reflective) states thousands of times. ROM production requires injection molding and metallization; manufacturing tolerances are tight, with track pitch as narrow as 0.74 micrometers on Blu-ray.

Optical media dominated consumer and enterprise backup from the 1990s through 2010s but has largely ceded ground to solid-state storage and cloud backup. However, pressed DVDs and Blu-rays remain standard for film distribution, and archival organizations still press optical media for long-term preservation because the format is less susceptible to electromagnetic interference than magnetic tape and has no moving parts once manufactured. Reading equipment is now scarce; drives are being discontinued, making long-term access a growing concern.