Definition

optical disc

Contributor(s): Rodney Brown

An optical disc is an electronic data storage medium that can be written to and read from using a low-powered laser beam.

Originally developed in the late 1960s, the first optical disc, created by James T. Russell, stored data as micron-wide dots of light and dark. Russell's optical storage system used a powerful backlight to read the dots through a transparent sheet of material on which the dots were encoded.

In later optical disc systems, a laser read the dots and the data was converted to an electrical signal. Then the data was converted to audio or visual output. However, the technology didn't appear in the marketplace until Philips and Sony came out with the compact disc (CD) in 1982. Since then, there has been a constant succession of optical disc formats, first in CD formats and followed by a number of digital versatile disc (DVD) formats.

Initially, an optical disc could hold much more data than similarly sized magnetic storage media. Increases in hard disk drive (HDD) technology, and particularly the introduction of solid-state memory, helped magnetic media capacity outgrow optical discs. However, optical disc storage is less likely to degrade over time than magnetic storage, such as HDD or magnetic tape, so it is often used for archival or cold storage.

Storage capacity has increased with each new generation of optical media. The newest standards, such as Blu-ray, offer up to 27 gigabytes (GB) of storage on a single-sided, 12-centimeter disc. Optical discs are inexpensive to manufacture and data stored on them is relatively impervious to most environmental threats, such as power surges or magnetic disturbances. That makes optical disc storage well-suited for archival storage. In 2016, Sony announced a disc based on Blu-ray technology that can hold 3.3 terabytes (TB) of data.

optical storage media
Optical media formats

Development/history

While Russell is credited with developing the first method of storing digital information on an optical medium, his creation bears little resemblance to later CDs or DVDs. Russell used transparent foil as the medium, and the data was read by shining a light through it instead of reflecting a laser off of it. In addition, Russell's system didn't spin as the data was read, so it could be any shape, not just a disc.

The modern CD and DVD are based on technology developed in 1969 in the Netherlands by physicist Peter Kramer while working for Philips Research. Kramer developed the method of encoding data on a reflective metallic foil that could be read via a small, low-powered laser. His work went on to become the basis of all digital optical storage media, but it was initially used to store analog video on the first laserdisc.

Philips teamed up with Sony in a joint consortium and, in 1979, developed the first audio CD, the first commercial use of digital optical storage. Just five years later, Sony, working this time with Denon, produced the first CD-ROM for the storage of any digital data, not just audio. The CD-ROM could hold approximately 680 MB of data. Almost 10 years after that, Sony again teamed with Philips, as well as Toshiba and Panasonic, to create the DVD, which increased data capacity to 4.7 GB.

It took another 10 years before the next generation of optical storage, the Blu-ray disc, hit the market. Boasting storage of up to 27 GB, the Blu-ray was developed by a consortium that was again led by Sony. Toshiba was not in the consortium this time, as it had developed and tried to market its own format, the HD-DVD. After a short format war, Sony and Blu-ray emerged as the industry standard.

How optical storage discs are made

All modern formats of optical discs use the same basic sandwich of materials structure. A hard plastic substrate forms the base, and then a reflective layer of a metallic foil -- typically aluminum for mass-produced discs -- is used to encode the digital data. Next, a layer of clear polycarbonate protects the foil and allows the laser beam to pass through to the reflective foil layer.

With an audio CD, or software or computer game distribution disc, the digital dots in the foil are physically stamped in from a negative disc image made from nickel, which is itself made from a glass master. This enables mass production at a level not possible by individually encoding CDs with a laser, as happens when a CD-ROM is written, or burned, in a computer.

Optical discs that are intended for storage written by a user include different materials in the sandwich depending on whether the disc is write-once or rewritable. A write-once CD-ROM has an organic dye layer between the unwritten reflective foil and the polycarbonate. Rewritable optical discs swap the aluminum foil for an alloy that is a phase-change material so it can be erased and rewritten multiple times.

This was last updated in September 2018

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