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7.4 Permanence, care, and handling of CDs including CD-ROM, Writable CD, and Kodak Photo CD
Kodak Imaging in Action: Eastman Kodak Company, Rochester 1995
Collections of audio CDs, CD-ROM multimedia titles represent significant investments of money and efforr. Users of writable CDs often are especially concerned for their unique data and pictures.
Long Life for any information storage medium is always a combination of manufacturers' and users' responsibilities. As a manufacturer of recordable CD products, Kodak believes strongly in sharing information that helps users of CD technology understand all aspects of CD longevity.
Life Expectancy of CDs
Compared with other digital storage media (eg., magnetic tape), CDs have much longer life expectancies. Accelerated-aging, tests, which speed up the reactions of decay, can determine the rate at which slow chemical changes can make discs unreadable. When tested this way, Kodak writable media stored under archival conditions showed a life expectancy of around 200 years. By contrast, magnetic tape storage media (according to a recent report by the Commission on Preservation and Access) 'will only last a few decades'. While this is a powerful advantage for CD data storage, there are other important aspects to consider.
Data stored on CDs is encoded in digital form. First the data must be read then it is converted into music, text, pictures, and so on by software programs. Because the real goal is access to the discs' contents, not merely the preservation of the discs - the survival of playback hardware devices and software formats is key. Migration of digital data from one storage medium or software standard to another is essential.
Because migrating data requires effort and expense, there is a risk that it will be neglected. History teaches that many things are preserved only by accident, merely because they were tough enough to endure until they could be valued once more by new owners, who saw them through new eyes. It is comforting to know that CDs, because of their century or more life expectancy, will physically survive long enough to be rediscovered by new generations. At the same time, it is prudent to create redundant copies of stored digital data. If images or data, for example, are recorded on film as input or output from digitization, the film serves well as a collection back-up. Having, two copies in different forms and in different physical locations increases the chances that the information will survive.
In addition to redundancy, a key aspect of archiving information on writable CD is the principle of master copies and derivatives. The power of writable CD technology is the ability to store so much information on durable discs, together with the easy generation of additional copies. This means, for example, that multiple departments of an institution can have and use their own CDs, all for very low cost. These copies, used for different purposes, are the derivatives. Someone should have responsibility for the creation and physical care of a master. This master copy should be stored properly, be physically secure, and be handled only to make derivative copies. Such a strategy will help to ensure both long life and economical access to stored data for many years.
CD TAPES. CD-ROM and Writable CD (CD- Recordable)
The CD is a new medium, but several different types already exist. The References in technology between read-only CDs (for example, those used for audio discs and multimedia publishing) and writable CDs (for example, those used as data storage and for image storage on Kodak Photo CD) need to be understood. Among CD types, not all discs are created equal, from a permanence point of view. It is important for those concerned about CD longevity to know certain things, so that they can make the best selection among available product types and brands. It is also important that they know about the special handling and storage requirements of each CD type.
CDs of all types share certain features: they all have the same physical dimensions, are made at least partially from clear polycarbonate plastic, and have a metallic layer to reflect the laser beam that reads the data. All CDs are 'groovy' they have a microscopic spiral - groove pattern molded into the polycarbonate plastic. The lines of this spiral are so small and close together that they act like a diffraction - rating. This creates the shimmering lines of 'rainbow' colours that emanate from the centre hub toward the outer edges on the bottom side of every CD. The spiral groove is continuous on writable CDs because it's needed to guide the laser during writing.
The major identifying difference between currently manufactured read only discs (CD-ROM) and writable CD discs can be seen at first glance: CD-ROMs have a silver metallic sheen on both sides, whereas writable CDs are typically metallic gold on the top and green or green-golden on the bottom. The top of a CD bears a label, logo, or other printed information. The bottom has no markings except in the area near the centre hub. 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 colour - whether silvery (CD- ROM) or green/green-golden (writable CD or CD-Recordable) provides quick identification.
The Nature of CD-ROM Discs
The two types of CDs, 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. The molding process (which is not unlike how phonograph records are made) forms a polycarbonate disc that makes up the bulk of the finished CD. This molded disc has a spiral track of pits impressed on one side; these pits embody the encoded data. As the laser beam in a CD is guided alone the length of the track, it is interrupted by the pits and these interruptions are decoded into music, text, or pictures.
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 through it, never "seeing" a difference between a pit and a land pass (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. 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 aluminium or a chrome-aluminium 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. Familiar examples are 'mirror' sunglasses and the golden faceplates of space suits.
Oxidation of Metallic Reflecting Layers
If the metallic layer in a CD ceases to be reflective, then there can be problems in reading its data. In some early CD-ROMs (and possibly some still being manufactured by lower-quality producers), the metallic layer, gradually lost its ability to reflect the laser beam, especially when discs were stored in warm or humid conditions. The problem was oxidation of aluminium metal, a slow chemical change that robbed the layer of its reflecting properties. Oxidation made the discs more transparent in the affected areas, sometimes almost clear. The use of aluminum alloys improved oxidation resistance.
The tendency of CD-ROM metallic layers to oxidize over time can be measured with accelerated-aging tests. Today's CD-ROMs from quality manufacturers have a predicted life of over one hundred years (in normal storage conditions) without danger of data loss because, of oxidation. How can a consumer judge when a CD is manufactured to the highest standard of quality. Unfortunately, it is as not possible to tell definitively by just looking. Nevertheless, it is possible to obtain some clues.
The metallic layers of CD-ROMs have varying degrees of transparency. By holding a CD-ROM up close to the eye in front of a strong, light source, one can determine how transparent the metallic layer is (unless a label or heavy printing prevents this). Some CD-ROMs are opaque, some almost opaque, and others almost transparent. Those that are quite transparent may be more susceptible to oxidation damage. (This does not apply to writable CDs because they have gold metallic layers. Gold is well known for superior oxidation resistance.) The presence of obvious pinholes in the metallic layer is another undesirable feature.
Scratches. Top and Bottom
The metallic layer in all types of CDs needs protection from physical wear and tear as well from as oxidising or airborne contaminants. The sputtered metal film is physically delicate and would be damaged quickly if touched or rubbed. For this reason CDs have a clear layer of lacquer covering the metallic reflecting layer. It also forms a surface upon which the label informations can be printed.
The molded polycarbonate substrate layer is on the bottom of a CD (the side from which the laser reads the data). The pits must be read through it. This part of a CD's design helps make it tolerant of scratches. The lens of the reading laser focuses the beam directly on the pits, through the 1.2-millimeter thickness of the polycarbonate. The beam 'looks past' scratches on the bottom of the disc, unless they are really deep. This first line of defense against scratches is not the only one. There is a second in the software that decodes the data stored on the disc. Known as 'error detection and correction,' this technology is able to deal with errors in the data because some degree of redundancy and cross-checking is built into the encoded data in the first place.
Together, the acrylic layer, the out-of-focus scratches, and software error correction add up to formidable physical robustness for all types of CDs, especially when compared with phonograph records or magnetic media. (Writable CDs are somewhat different in structure from CD-ROMs and have some special handling considerations - see below.) However, physical damage from scratches or other kinds of mishandling is still a serious threat. Proper handling, including appropriate storage containers for CDs, is a vital contributor to CD longevity.
Readability. or When Is a CD's Life Really Over?
Still, nothing lasts forever, and it is important to know what the inherent life expectancy of a CD disc actually is. As mentioned, life expectancy at moderate storage conditions can be determined by means of accelerated-aging, tests, and most manufacturers provide such data to consumers upon request. To understand the test results, one has to know what was considered the 'end of life' for test purposes and, of course, what the predicted life expectancy turned out to be. End of life for a CD disc seems a simple concept - when the disc can no longer be read, its useful life is over.
Readability, however, is not a simple thing, to measure. The readability of a CD is determined partly by the disc and partly by the player device. It can be difficult to know whether a disc itself has deteriorated or whether hardware or software problems in the playback system are involved. Audio CDs are perhaps the most familiar example of the problem; most people who own several CD players have had the experience that some discs play well on one player but not on another. All types of CD players are electromechanical devices with complex moving, parts that must operate within very close tolerances, and it is easy for misalignments or other troubles to occur. For data discs that are read on computers, software problems could also make a particular disc unreadable. As a practical matter, an unreadable CD more often indicates player problems than physical or chemical deterioration of the disc.
In life testing, manufacturers take great pains to make sure that their playback hardware gives a reliable indication of a disc's readability. Specially calibrated players read discs periodically during the aging tests and report on how many reading errors the error detection and correction system has had to cope with. Although readability to the end user seems like an all-or-nothing proposition, in actuality some reading errors occur in nearly all discs and are handled routinely by the error detection and correction system. As a disc ages, more and more reading errors can be measured. The life-expectancy data published by Eastman Kodak and other manufacturers is really a prediction of how long it would take for a well-written disc kept at moderate storage conditions to develop a certain number of playback errors spontaneously. Specifically, life expectancy data is based on a measurement called 'BLERmax 50.' The 'BLER' part stands for 'block error rate,' a measure of how many errors are discovered by the error detection and correction system in a ten-second period. The 'max' part stands for the maximum BLER encountered in reading a given disc. For test purposes, end of life occurs when the BLERmax reaches 50 errors. This is a very conservative definition of end of life, however, because most discs that reach BLERmax 50 are still readable.
So, How Long Can CDs Last?
Leaving aside scratches, fires, floods, and peanut butter sandwiches and concentrating on the slow chemical changes that determine the inherent life expectancy of a CD, extensive accelerated-aging tests suggest that Kodak writable CD products, including Photo CD discs, will not reach a BLERmax of 50 for a period of around 200 years when kept in the dark at moderate storage conditions. This long potential life expectancy is mainly a function of the greater dark stability of the dye used in Kodak writable CD products. Considering that BLERmax 50 is still not an unreadable level of error, Kodak writable CDs have a very long life expectancy indeed. Similar research by the 3M Company shows that CD-ROM products made by them will not attain a block error rate of 50 per second for more than 100 years in moderate storage conditions.
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.
The Nature of Writable CDs
A writable CD starts out blank. A laser 'writes' data on it by creating discoloured areas in a layer of greenish dye. The areas of discoloured dye on a writable CD act like the pits in a CD-ROM: they interrupt the beam of the reading laser. Writable CD technology makes it possible for people to create their own CDs, which then can be read by computers equipped with CD-ROM drives or (providing the data is written as music) on home audio CD players. Kodak Photo CD is an example of a writable CD that can be read on computers and special TV players.
The physical structure of a writable CD differs from that of a CD-ROM. A writable CD is molded from polycarbonate too, but without pits. Like a long-playing phonograph record, a writable CD has a smooth spiral groove running over most of it's surface. This groove, molded into the top side of the polycarbonate substrata, serves to guide the powerful laser beam that does the writing.
Both the reading and writing are done from the bottom side of the disc, through the full thickness of the polycarbonate substrata. This provides scratch resistance as it does for the CD-ROM, but there is a catch. It takes a fair bit of energy for the laser beam to make a "mark" on a writable CD. That is why scratches and especially fingerprints or dirt are much more dangerous to a writable CD before writing than afterwards. The fingerprint, dirt, or smudge can scatter the beam of the writing laser, perhaps weakening it to the point where the mark is too small or too light to be read. For this reason, extra care must be taken in the handling of writable CDs before the data is written.
The Gold and the Greens: Structure of a Writable CD
Just as in a CD-ROM, the bulk of a writable CD is a clear polycarbonate substrata about one millimetre thick. The layer above this one is something writable CDs have but CD-ROMs don't have: a layer of light-sensitive green organic dye. Above this dye layer is a thin film of metallic gold. The gold layer has the same purpose as the aluminum-chromium layer in a CD-ROM, namely, to reflect the reading laser beam back into the photodetector. In its original state, the dye isn't dark enough to block light reflection from the gold layer above. But when the dye has been 'zapped" by the writing laser (which has about ten times the power of the reading laser), a dark discolouration is created that blocks reflections and forms a permanent, readable mark. The dye molecules absorb enough energy during the writing process to break down physically and chemically, leaving an area that no longer reflects light. The marks made by the writing, laser follow the same encoding scheme as the pits in a CD-ROM, so writable CDs can be read by CD-ROM drives.
Most writable CDs have a gold reflective layer, and, therefore appear golden when viewed from the top or label side. However, the bottom side where we get a good look at the dye through the clear substrata reveals some key differences among subtypes of writable CDs. There are two principal kinds: those with a dark green appearance and those with a light green-golden appearance. Kodak writable CDs and Photo CDs are always clearly marked as such. Another distinguishing feature of Kodak writable CD products is the presence of a unique serial number, both in numerals for a person to read and as a bar code that can be read by certain player devices. The serial number and bar code are put on at the factory. They allow each writable disc to be identified and tracked, making them valuable tools for an archive to use in managing a collection of digital information.
Lightfastness of Writable CDs
Tests show that the writable CDs of some manufacturers can be affected by casual exposure to light (for example, by leaving, a disc bottom-side up on a desk for days). This can cause the dye to fade and the contrast between marks and lands to disappear. The dye formation in Kodak writable CD products, however, provides much superior lightfastness, and the risk of accidental damage is much lower. Nevertheless, as a general rule it is prudent to keep all writable CD products in suitable enclosures (acrylic cases, caddies, or other opaque enclosures) when not in use and never to leave them exposed to direct sunlight.
Do Not Fold, Spindle, or Mutilate: Safe Handling of CDs
Back when cardboard punch cards were the only form of digital data storage they were often marked, 'Do not fold, spindle, or mutilate.' Apart from the fact that most people were a little unsure about the meaning of 'spindle,' the advice was straightforward. CDs also come with handling advice, now in the form of graphical symbols that are intended to be easily understood by speakers of all languages. Some of these symbols are obvious, others about as meaningful as 'Do not spindle.' What are the key points in handling CDs - the do's and dont's?
A basic tenet of safe handling is to provide, good containers or enclosures for CDs. Cheap plastic sleeves (for example, those sometimes used to hold CDs in the backs of books or in some types of multiple CD holders) are not suitable for long - term storage. 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 in such circumstances can lead to delamination. The acrylic 'jewel cases' provided by many manufacturers and distributors are good protection against scratches, dust, light, and rapid humidity changes. If the manufacturer provide a spacer card or other material as part of the jewel case package it should be retained. Protect the individually cased CDs further by placing them in a closed box, drawer, or cabinet. This gives additional protection from light, dust, and climate fluctuations. It is always a good idea to handle the CDs themselves only when they are being used. 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.
The worst handling stresses for a CD are caused by severe flexing or application of a sharp point to the top surface. These actions deform the substrata, wiping out pits and causing areas of the disc to become unreadable. A sharp stylus - a ballpoint pen, for example - can cause compression of the polycarbonate substrata and the metallic reflecting layer in the area under the pen point. This can happen to both CD-ROMs and writable CDs, but the danger is more acute for writable CDs. The adhesion between the dye and the other layers in a writable CD is weaker than the interlayer adhesion in a CD-ROM, where no dye is needed. In response to this danger, Kodak writable CD products have a unique feature - the 'durability' coating. This is a specially formulated coating applied over the protective acrylic lacquer layer. The coating, part of the Infoguardtm feature set, has physical properties that keep scratching objects from getting a grip on the disc surface, thus reducing the likelihood of a scratch or delamination. Besides providing scratch tolerance, it minimises danger of dye delamination when a sharp pressure point is applied to the label side of the disc and helps protect against solvents and acids.
Be Careful Marking and Labelling CDs
User-applied labels of any kind may unbalance the CD and make it difficult for the player to read. Also, labels may peel in humid conditions. 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 CD. If it is necessary to write on the top side of a CD, 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 CD must be marked, it should be done in a way that is approved by the disc manufacturer.
CDs may become dusty or dirty and may occasionally require a careful cleaning. Dirt, dust, and fingerprints are more significant 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 such as Kodak lens tissue. Always be gentle and wipe from the centre 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. If more than dust removal is require do not use solvents. Use Kodak lens cleaner and Kodak lens tissue, again working in a radial direction. Use only a small amount of lens cleaner and work carefully. Spilled liquids may be removed from the disc by this method.
Storage Conditions for CDs
Although CDs can tolerate a fairly wide range of conditions without immediate damage, it is best to store CDs in cool and moderately dry environments. Slow chemical changes such as oxidation of metallic reflecting layers, dark fading of dyes, and deterioration in polymer substrates and coatings are temperature - and humidity-dependent processes. They will always proceed faster under warmer and more humid conditions. For Kodak writable CD products, for example, the predicted time of more than 200 years to reach BLER of 50 is based on storage at 25EC (77F), 40% RH. This means that cooler and/or drier conditions would lead to even greater life expectancies. Of course, warmer and damper conditions mean a lowered life expectancy. It is not necessary or desirable to freeze CDs to get a long life expectancy.
Cycling conditions especially between extremes of temperature and humidity, can be dangerous to CDs. Although moderate temperature and humidity changes ordinarily do no harm, fast changes between very warm and wet conditions to cooler and very dry conditions might produce warping, and distortion. The recommended maximum limit for temperature change is 15EC (27F) per hour. For humidity, the recommended maximum RH change per hour is 10%. In practice, such steep gradients of temperature and RH are rare. The enclosures in which CDs are kept provide the first line of defense against such cycling, damping the rate of change in RH so that the CD inside its case or storage cabinet does not "feel' fast changes occurring in the outside atmosphere.
At present, no national or international standards for long-term storage conditions for CDs have been published by ANSI or the International Standards Organisation (ISO). Until such standards are available, the recommendations given in this publication must be considered provisional.
CD Permanence in Perspective
Ensuring the permanence of CD-ROMs and writable CDs (and indeed of any modern information medium) is a combination of manufacturers' and users' responsibilities. CDs in general last longer than other digital storage media. Under accelerated test conditions, Kodak writable CD products have a projected life expectancy of more than 200 years when stored appropriately. Many CD-ROM discs can be expected to have 100 years of life, or more, under similar circumstances. On the basis of long inherent life expectancy and worldwide standardization, CDs are a sensible choice for archiving data, text, and pictures in digital form. In light of the large investments represented by collections of CD-ROM and writable CD media, this expected long life is encouraging, but it is not the whole story.
Satisfactory permanence for CD-ROM and writable CD media starts with knowing, about the physical nature of the discs themselves and with choosing, products carefully in the first place. This publication has surveyed the currently available technology choices in CD media. It has explained the special handling and storage requirements of CD discs which arise from their physical nature as objects of plastic and metal. Beyond this, however, permanence of the information stored on CDs depends as much or more on sound archival practices than on the inherent long life of the discs. Users of CD technology, whether in a personal or professional context, are the critical links in assuring data survival in the long term. Master copies kept in appropriate storage and handled only when necessary may last centuries. Copies for use are likely to have considerably shorter lives. Therefore, a strategy of master discs from which derivatives are made for use is fundamental to success.
Another useful approach is to maintain data in more than one form or in more- than one type of storage media, perhaps in analog form, if possible. In addition, keeping copies in different physical locations helps guarantee survival.
The principal fact of life for all digital storage media is the rapid obsolescence of hardware and software. Users of CD technology should be reassured by the long physical life of CD discs, but they must not lose sight of the need to maintain a viable path for migration of data to new hardware and software platforms. Digital storage media impose a strict discipline that human-readable records do not: their rapid evolution creates a continual progression of technology that cannot safely be ignored for too long. However, armed with knowledge of the physical nature of the media and of appropriate archival approaches, users of CD technology can have the best of both worlds - the functionality of digital media and a long life for stored data and pictures.
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