encoded in microscopic pits (or bumps) on a special material (often aluminium) on one of its flat surfaces. The encoding material sits atop a thicker substrate (usually polycarbonate) which makes up the bulk of the disc and forms a dust defocusing layer. The encoding pattern follows a continuous, spiral path covering the entire disc surface and extending from the innermost track to the outermost track. The data is stored on the disc with a laser or stamping machine, and can be accessed when the data path is illuminated with a laser diode in an optical disc drive which spins the disc at speeds of about 200 RPM up to 4000 RPM or more depending on the drive type, disc format, and the distance of the read head from the center of the disc (inner tracks are read at a faster disc speed). The pits or bumps distort the reflected laser light, hence most optical discs (except the black discs of the original PlayStation video game console) characteristically have an iridescent appearance created by the grooves of the reflective layer. The reverse side of an optical disc usually has a printed label, generally made of paper but sometimes printed or stamped onto the disc itself. This side of the disc contains the actual data and is typically coated with a transparent material, usually lacquer. Unlike the 3½-inch floppy disk, most optical discs do not have an integrated protective casing and are therefore susceptible to data transfer problems due to scratches, fingerprints, and other environmental problems.
Optical discs are usually between 7.6 and 30 cm (3 to 12 inches) in diameter, with 12 cm (4.75 inches) being the most common size. A typical disc is about 1.2 mm (0.05 inches) thick, while the track pitch (distance from the center of one track to the center of the next) is typically 1.6 µm (microns).
An optical disc is designed to support one of three recording types: read-only (eg: CD and CD-ROM), recordable (write-once, e.g. CD-R), or re-recordable (rewritable, e.g. CD-RW). Write-once optical discs commonly have an organic dye recording layer between the substrate and the reflective layer. Rewritable discs typically contain an alloy recording layer composed of a phase change material, most often AgInSbTe, an alloy of silver, indium, antimony and tellurium
Optical discs are most commonly used for storing music (e.g. for use in a CD player), video (e.g. for use in a DVD player), or data and programs for personal computers. The Optical Storage Technology Association (OSTA) promotes standardized optical storage formats. Although optical discs are more durable than earlier audio-visual and data storage formats, they are susceptible to environmental and daily-use damage. Libraries and archives enact optical media preservation procedures to ensure continued usability in the computer’s optical disc drive or corresponding disc player.
For computer data backup and physical data transfer, optical discs such as CDs and DVDs are gradually being replaced with faster, smaller, and more reliable solid state devices, especially the USB flash drive. This trend is expected to continue as USB flash drives continue to increase in capacity and drop in price. Similarly, personal portable CD players have been supplanted by portable solid state MP3 players, and MP3 music purchased or shared over the internet has significantly reduced the number of audio CDs sold annually.
The optical disc was invented in 1958. In 1961 and 1969, David Paul Gregg registered a patent for the analog optical disc for video recording, U.S. Patent 3,430,966. It is of special interest that U.S. Patent 4,893,297, filed 1968, issued 1990, generated royalty income for Pioneer Corporation’s DVA until 2007 — encompassing the CD, DVD, and Blu-ray Disc systems. In the early 1960s, the Music Corporation of America bought Gregg’s patents and his company, Gauss Electrophysics.
Later, in The Netherlands in 1969, Philips Research physicists began their first optical videodisc experiments at Eindhoven. In 1975, Philips and MCA join efforts, and in 1978, commercially much too late, they presented their long-awaited laserdisc in Atlanta. MCA delivered the discs and Philips the players. However, the presentation was a technical and commercial failure and the Philips/MCA cooperation ended.
In the mid-1990s, a consortium of manufacturers developed the second generation of the optical disc, the DVD.
The third generation optical disc was developed in 2000-2006, and the first movies on Blu-ray discs were released in June 2006. Blu-ray eventually prevailed in a high definition optical disc format war over a competing format, the HD DVD. A standard Blu-ray disc can hold about 25 GB of data, a DVD about 4.7 GB, and a CD about 700 MB.
|CD–ROM 74 min||SS SL||1||1||12||0.682||0.635|
|CD–ROM 80 min||SS SL||1||1||12||0.737||0.687|
|DVD–R 1.0||SS SL||1||1||12||3.95||3.68|
|DVD–R (2.0), +R, –RW, +RW||SS SL||1||1||12||4.70||4.37|
|DVD-R, +R, –RW, +RW||DS SL||2||2||12||9.40||8.75|
|DVD–RAM 1.0||SS SL||1||1||12||2.58||2.40|
|DVD–RAM 2.0||SS SL||1||1||12||4.70||4.37|
|DVD–RAM 1.0||DS SL||2||2||12||5.16||4.80|
|DVD–RAM 2.0||DS SL||2||2||12||9.40||8.75|
|HD DVD||SS SL||1||1||8||4.70|
|HD DVD||SS DL||1||2||8||9.40|
|HD DVD||DS SL||2||2||8||9.40|
|HD DVD||DS DL||2||4||8||18.80|
|HD DVD||SS SL||1||1||12||15.00|
|HD DVD||SS DL||1||2||12||30.00|
|HD DVD||DS SL||2||2||12||30.00|
|HD DVD||DS DL||2||4||12||60.00|
|HD DVD–RAM||SS SL||1||1||12||20.00|