Iowa State University

Care for CD & DVD Media

BASICS
CDs and DVDs of all types share certain features: they all have the same physical dimensions, are made at least partially from clear plastic, and have a metallic layer to reflect the laser beam that reads the data. The major identifying difference between currently manufactured discs and writable discs can be seen at first glance: manufactured discs have a silver metallic sheen on both sides, whereas writable discs typically are metallic or labeled on the top and colored on the bottom. The laser reads the disc from the bottom, so labels or inks on that side would get in the way. Because the bottom is always unobstructed, its overall color--whether silvery (CD-ROM/DVD-ROM) or colored (writable CD-R, CD-RW, DVD-R, DVD-RW, DVD+R, or DVD+RW)--provides quick identification.

The two types of discs, read-only and writable, differ in structure, materials, and manufacturing technology. When large numbers of copies are needed, CD-ROM is the natural choice because the discs are stamped from molds. After a CD-ROM leaves the mold, it's only a clear plastic disc with tiny pits; if you tried to read it on a CD player, the reading laser would pass right through it, never "seeing" a difference between a pit and a land (a smooth area). That's why CD-ROMs have their silvery metallic coating. The metallic layer forms a reflective surface from which the laser light bounces back into a light-sensing detector. When the tightly focused laser beam reflects off a land, the light-sensing detector "sees" the beam. When the beam encounters a pit, no laser light is reflected into the light-sensing detector.

The metallic reflective layer has been the source of problems in disc longevity. CD-ROM technology, which originated around 1980, is older than writable CD technology and much older than writable DVD technology. As with any new product, there were lessons to be learned and improvements to be made. The reflecting layer was a case in point. In CD-ROMs, the reflecting layer is actually aluminum or a chrome-aluminum alloy, not silver. The metal is applied to the pit side of the molded polycarbonate disc by means of "sputter" coating, wherein atoms of metal are deposited as a very thin film using a high-vacuum chamber. Thin metal films are semitransparent and easily damaged.

This Cross section of a CD-ROM, showing polycarbonate substrate, pits, lands, metallic reflective layer, and acrylic lacquer overcoat. Laser beam comes from below.  The protective lacquer layer is often times missing or much thinner on writable DVD and CD media!

STORING
A basic tenet of safe handling is to provide good containers or enclosures for discs. Cheap plastic sleeves (for example, those sometimes used to hold CDs in the backs of books or in some types of multiple CD holders). Upon exposure to extremes of temperature and humidity, the disc and sleeve may adhere to each other. The shear forces generated upon forced removal of a disc can lead to delamination. Acrylic "jewel cases" provided by many manufacturers and distributors are good protection against scratches, dust, light, and rapid humidity changes, but incorrect removal of the disc from the jewel case by flexing can easily cause the disc to crack or snap. Paper or cotton sleeves are the best protection for each CD or DVD. Spindling CDs or DVDs can also provide effective storage for large quantities of discs. Protect the correctly stored discs further by placing them in a closed box, drawer, or cabinet. This gives additional protection from light, dust, and climate fluctuations.

HANDLING
It is always a good idea to handle the disc themselves only when they are being used. Clean hands are always recommended when handling media. Never allow them to remain out of their enclosures for long periods of time. Always handle discs by their edges. Don't eat, drink, or smoke around CDs. Even moisture from the sweat of a pop can on your fingers will start the deterioration of the surface and significantly reduce the lifespan of writable media. Spilling water or soda on a disc can cause a disc to become immediately ruined.

The worst handling stresses for a disc are caused by application of a sharp point to the top surface. These actions deform the substrate, wiping out pits and causing areas of the disc to become unreadable. A ballpoint pen, for example, can cause compression of the polycarbonate substrate and the metallic reflecting layer in the area under the pen point. This can happen to CD-ROMs/DVDs and writable CDs/DVDs, but the danger is more acute for writable media. The adhesion between the dye and the other layers in a writable disc is weaker than the interlayer adhesion in a CD-ROM, where no dye is needed.

LABELING
User-applied labels of any kind may unbalance the disc and make it difficult for the player to read. Also, labels may peel in humid conditions. Labels will also protect the top of the disc to prevent it from scratches. Once a label is on the CD, however, it is especially important not to try and remove it. The act of peeling off a label creates a lever action that concentrates stress in a small area. Such stress can cause delamination, especially in a writable disc. If it is necessary to write on the top side of a disc, a soft felt-tip marker is preferable to other writing instruments, but with some solvent-based markers there may be a danger of the solvents migrating into the protective lacquer. So, if a disc must be marked, it should be done in a way that is approved by the disc manufacturer.

CLEANING DISCS
Discs may become dusty or dirty and may occasionally require a careful cleaning. Dirt, dust, and fingerprints are more significant when played when they are on the bottom (laser-incident) side of the disc. Light dust or dirt may be safely brushed off with a nonabrasive tissue. Always be gentle and wipe from the center hub toward the outside edge of the disc. The motion should be in a radial direction (like the spokes of a wagon wheel), not in a circumferential direction (around the circle). If a scratch is created by the cleaning process, it will do the least damage cutting across the track of pits and lands, rather than along it

LIFESPAN
Besides all the things mentioned above that can negate the inherent life expectancy, extensive tests have been done to suggest that most writable media will last 100 years or more when kept in dark moderate temperature storage conditions.  This is different for each type of media where some manufactures claim 200 to 1000 year life spans.  Accelerated aging is subject to uncertainties, but it does rest on firm scientific footing. Behind the data is the simple assumption that raising the temperature causes the reactions of decay to happen faster--so fast, in fact, that they occur within a few months, rather than decades. The science of reaction rates is called kinetics, and the lifetime predictions are based on well-established principles of that branch of chemical science. These same principles are used every day to design the chemical plants and processes of the modern world. Because there is so much practical experience with the laws of kinetics, lifetime predictions based on them are approximately correct. Such test methods soon will be part of a forthcoming ANSI (American National Standards Institute) standard dealing with tests for CD permanence.

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