Optical Disc

An optical disc is a data storage media, usually produced from combined layers of polycarbonate substrate(a type of plastic); organic polymer dye; a reflective layer often made from a gold, silver, or aluminium alloy; and an ultraviolet-cured lacquer.

Formats of optical discs include CD(R, -RW), (HD)DVD(-R, +RW, -RW), and Blu-Ray. Each format has different maximum size capabilities and are made from slightly varying materials. More importantly, not all formats will read in every drive. A CD-ROM drive can only read CDs, but not DVDs; a DVD-ROM drive can read all kinds of CDs and DVDs; a BD-ROM drive can read all kinds of CDs, DVDs and BDs.

A compact disc (CD) is fabricated from a polycarbonate disc measuring 12 cm in diameter and 1.2 mm in thickness which is bonded to an aluminium layer and covered with a lacquer finish. CDs can hold up to 800 MBs of data. The data is stored in bumps (pits if looked at from above; however, it is the reflection from the bumps which is interpreted as binary data). These bumps (pits) have a minimum length of 0.8 microns and are arranged in circular tracks which are 1.6 microns apart (known as the track pitch). To read the data, an infrared 780 nm laser dioded is reflected against the disc surface.