Optical disc

An optical disc is a flat, usually round storage medium that encodes information through tiny physical marks on its surface, which are read using a beam of light. These discs can work in one of two ways: reflective, where the light source and detector sit on the same side, or transmissive, where light passes through the disc to a detector on the opposite side. They can hold analog data, digital data, or a mix of both, and are primarily used for distributing electronic media and data, as well as for long-term archival storage.

The disc’s data layer sits on a thicker polycarbonate substrate that forms the bulk of the disc and helps keep dust out of focus. The encoded pattern runs in a continuous spiral from the innermost track to the outermost edge. Data is written using a laser or a stamping machine and is read back by a laser diode in an optical disc drive, which spins the disc at speeds ranging from about 200 to over 4,000 RPM, depending on the drive, format, and how far the read head is from the center (outer tracks yield faster data rates at the same spin speed). Most optical discs have a characteristic rainbow sheen caused by the diffraction grating of their grooves; this data side is usually coated with a clear lacquer. The opposite side typically bears a printed label, which may be paper or stamped directly onto the disc. Unlike floppy disks, most optical discs lack a protective casing, making them vulnerable to scratches, fingerprints, and other environmental damage—though Blu-ray discs feature a special coating called Durabis to reduce these issues. Disc diameters range from 7.6 to 30 cm, with 12 cm becoming the standard after 1997. The program area usually starts 25 mm from the center, the disc is about 1.2 mm thick, and track pitch varies from 1.6 μm for CDs down to 320 nm for Blu-ray discs.

Optical discs come in three recording types: read-only (e.g., CD, CD-ROM), recordable or write-once (e.g., CD-R), and rewritable (e.g., CD-RW). Write-once discs often use an organic dye layer—such as phthalocyanine (widely used by Verbatim), azo dye (introduced in 1996 and mainly used by Verbatim), or oxonol dye (used by Fujifilm)—sandwiched between the substrate and a reflective layer. Rewritable discs typically use a phase-change alloy, most commonly AgInSbTe (silver, indium, antimony, and tellurium). The dye and reflective material can be identified by shining light through the disc, as different combinations produce distinct colors. Blu-ray recordable discs generally avoid organic dyes in favor of an inorganic recording layer, except for so-called low-to-high (LTH) discs, which use organic dyes and can be made on existing CD/DVD production lines, though they are lower quality than standard Blu-ray recordable media.

File systems designed specifically for optical discs include ISO 9660 and the Universal Disk Format (UDF). ISO 9660 can be extended with the Joliet extension for longer filenames, or with the Rock Ridge extension for even longer names and Unix/Linux-style permissions, though Rock Ridge is not recognized by Windows or most DVD players. For cross-platform use, multiple file systems can coexist on a single disc, all referencing the same files.

Optical discs are most commonly used for digital preservation, storing music (especially for CD players), video (for Blu-ray players), and data or programs for personal computers, as well as for offline distribution of large files due to their low per-unit cost compared to other media. The Optical Storage Technology Association (OSTA) helped standardize these formats. Libraries and archives follow specific preservation procedures to keep discs readable in drives or players. File operations typical of flash drives, memory cards, or hard drives can be simulated using a UDF live file system. However, for computer backup and physical data transfer, CDs and DVDs are gradually being replaced by faster, smaller USB flash drives, a trend expected to continue as those drives grow in capacity and drop in price. Additionally, the rise of streaming and digital downloads for music, movies, games, software, and TV shows has significantly reduced annual sales of audio CDs, video DVDs, and Blu-ray discs. Still, some people prefer audio CDs and Blu-rays because they offer tangible support for favorite works, and because audio CDs (like vinyl records and cassette tapes) provide uncompressed audio free of lossy compression artifacts, while Blu-rays deliver better image and sound quality than streaming—thanks to higher bitrates and more storage space—without visible compression artifacts. Although Blu-ray content can sometimes be torrented, this option may not be available to everyone due to internet restrictions.