Section 5: Optical media
Audio compact discs (CDs) date from 1982 to the present. They represent the first successful consumer digital audio format. Composition varies between pressed CDs and DVDs (digital versatile discs) and those that are writeable and rewriteable (CD-R and CD-RW).
Pressed CDs are comprised of a protective lacquer layer, a metal data layer, and a polycarbonate plastic layer. Digital information is expressed in binary, and the metal data layer is molded to create pits; these pits reflect light differently than unaffected areas called lands. Playback is achieved through optoelectronic means: an optical stylus laser reads the pits and lands to reproduce the encoded information.
CD-Rs have a protective lacquer layer, an organic dye layer, a gold or silver metal reflective layer, and a polycarbonate plastic layer. The organic dye layer functions in much the same way as the metal data layer in a pressed CD; however, instead of being molded, the dye layer allows or blocks light transfer through the data layer, functioning in the same manner as the pits and lands.
CD-RWs have a protective lacquer layer, metal alloy recording layer, aluminum reflective layer, and a polycarbonate plastic layer. The metal alloy recording layer functions similarly to the dye layer in CD-Rs. The phase-changing metal alloy film can be altered in the same way the organic dyes can to reflect light.
DVDs were introduced to the market in 1995 and continue to be used today. They may contain any type of digital file, but commercially produced DVDs containing videos are common. Pressed DVDs are essentially two CDs glued together: a polycarbonate plastic layer, adhesive, a metal data layer, and a polycarbonate plastic layer. (The protective lacquer present on a CD is not needed on a DVD because the second polycarbonate layer acts as a protective layer.) DVDs can be two sided, meaning there may or may not be two recording layers present. Digital information is recorded in the same way as on a pressed CD: binary information is pressed into the metal data layer. Again, playback is through optoelectronic means using an optical stylus laser.
DVD-Rs have a polycarbonate plastic layer, an organic dye layer, a gold or silver metal reflective layer, and a polycarbonate plastic layer. The organic dye layer works the same as in a CD; the dye layer allows or blocks light transfer through the data layer. These, too, can be two-sided, with two dye layers and two metal reflective layers.
DVD-RWs have a polycarbonate plastic layer, metal alloy recording layer, aluminum reflective layer, and a polycarbonate plastic layer. The metal alloy recording layer functions the same as in CD-RWs; the phase-changing metal alloy film can be altered to reflect light. CD-RWs can be two-sided with two metal alloy recording layers and two reflective layers.
The polycarbonate layers in all optical disc types are easy to scratch or smudge, which may cause read errors or, if the damage is great enough, prevent successful playback. In addition, inks and adhesives used to label optical discs can harm the data stored in the top layer of the disc. The organic dyes used in optical discs will degrade over time, eventually making the disc unreadable. Delamination is possible through large swings in temperature and humidity; oxygen is introduced through the delamination process and can damage the data layer in CD- and DVD-RWs.
Improper storage, care, and handling practices may result in the following issues, impairing reproduction:
- Scratches or other means of damaging the data layer, therefore removing the data itself.
- Scratches on the polycarbonate layer, resulting in the inability of light to reach the data layer without distortion.
- Warping of the disc.
- Breakage of the disc.
- Discoloration of the layers resulting in the distortion of light values and the inability to retrieve the data.
- Optical discs should be stored at temperatures between 41 and 68 degrees Fahrenheit, with a relative humidity (RH) between 30%-50%. Temperatures should not fluctuate more than ±2 degrees within a 24 hour period. RH should not fluctuate more than ±5% within a 24 hour period.
- Optical discs should be kept away from sources of heat, including sunlight or room lighting. Exposure to light greatly accelerates the fading of dyes.
- Optical discs should be stored on their edge so that they stand vertically.
- Optical discs should be in individual cases or Tyvek envelopes.
- Paper inserts should be removed and stored alongside the case or envelope. Paper can attract and hold moisture in humid environments.
- Handle the optical disc from the interior and exterior edges only. Do not touch the surface of the optical disc without using non-abrasive, non-shedding gloves. Fingerprints, smudges, scratches, particles of dirt or dust, solvents, or excessive moisture all may interfere with the ability of the laser to read the data layer.