External backing stores - optical discs There are several different types of optical disc, although they all look pretty much the same. There are several formats on the market, such as: CD-ROM - read only, the data is written to them before they are sold.
CD-R - meaning CD-Recordable, the user can write data to the CD once or fill it over time using multi-session writing to the same disc on separate occasions.
Unlike multi-session discs, existing data can be overwritten. Aluminum is used behind the inner layers, but a semi-reflective gold layer is used for the outer layers, allowing the laser to focus through the outer and onto the inner layers. After all of the layers are made, each one is coated with lacquer, squeezed together and cured under infrared light. For single-sided discs, the label is silk-screened onto the nonreadable side. Double-sided discs are printed only on the nonreadable area near the hole in the middle.
Cross sections of the various types of completed DVDs not to scale look like this:. Each writable layer of a DVD has a spiral track of data. On single-layer DVDs, the track always circles from the inside of the disc to the outside.
That the spiral track starts at the center means that a single-layer DVD can be smaller than 12 centimeters if desired. What the image to the left cannot impress upon you is how incredibly tiny the data track is -- just nanometers separate one track from the next a nanometer is a billionth of a meter.
And the elongated bumps that make up the track are each nanometers wide, a minimum of nanometers long and nanometers high. The following figure illustrates looking through the polycarbonate layer at the bumps. Both parts are thus reflected to, and detected by, the photosensor in the laser head, which focuses on one layer at a time. Figure 8 shows the most common construction for single-sided, double-layered DVDs, while Figure 9 shows an alternative construction.
The difference between these two types is that one Figure 8 has the metal data layers on separate sides halves of the disc. In addition to passing through a semireflective metal layer, the laser beam must also pass through a special adhesive that binds the two disc halves together and does not hinder the laser beam. In the example shown as Figure 9, the two metal data layers are on the same half of the disc. Figure 10 shows a double-sided DVD that may typically be a DVD-Video providing the video in a full-screen TV version on one side of the disc and a wide-screen version on the other side.
A very thin lacquer layer is applied to the label side of CDs to protect the metal from exposure to the environment. DVDs have no such protective lacquer coating. That layer also gives some limited protection from writing on or labeling the disc. However, the CD is more sensitive to damage on this side than on the polycarbonate side. Since the metal layer is so close to the surface of the label side, pointed objects can easily damage the CD by deforming the metal or exposing it to the environment.
Some solvents can also affect lacquer coatings and expose or react with the metal. Once the metal is damaged, the laser cannot read data in the damaged areas. Sometimes a manufacturer will add an additional layer designed specifically to provide more resistance to fingerprints and scratches on the label side of CDs. One particularly effective modification has been the application of lacquer completely around the edges of the disc. In earlier CDs, moisture had been allowed to penetrate to the metal through unprotected areas of the disc edge.
Clearly, it is as important to protect the edges of CDs as it is their surfaces. Such surfaces are of four types:. These layers are applied over the lacquer layer on CDs or over the polycarbonate substrate on a single-sided DVD. Some discs have an extra coating on which text or logos are printed. In many cases, the lettering appears to be stenciled, but it is not part of the coating; what one sees is the reflected surface of the metal rather than imprinted text or logos.
Typically, one can see through this lettered area-and even through the metal-by holding the disc up to light. Because these lettered areas are particularly susceptible to damage, it is most important to avoid writing on or scratching in these areas.
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