Optical media longevity

The X Lab™ regularly fields questions concerning the longevity of optical media, such as:

  • Which brand optical media should I buy?
  • How long will a CD or DVD last?
  • If I backup my pictures to CD or DVD, will that backup be good forever?

As a result, we investigated the available literature and assembled this FAQ to answer the following questions concerning optical media longevity:

Based on our research, we have drawn the following conclusions:

  • The true life expectancy optical media is unknown and perhaps unknowable. Many factors affect optical media longevity. Longevity estimates depend on expensive, time-consuming, narrowly-defined tests, the results of which can be subject to wide interpretation. Tests conducted by the National Institute of Standards and Technology (NIST) have found wide variations in the stability of optical media, leading to wide variations in estimated longevity. In general, the estimated longevity of CD-Rs is more consistent than that of recordable DVDs. However, the handling and storage of optical media are often greater determinants of longevity than the stresses examined in longevity tests.
  • Certified, archival-quality recordable optical media is unavailable and may never be available. While there is interest in long-lasting, "archival-quality" optical media in specific markets, no independent certification body exists. The costs of testing and certification, when compared to the potential market size and the rapid of evolution in optical media, has deterred manufacturers from marketing certified, archival-grade optical media.
  • Neither "CD rot" nor "DVD rot" are chronic problems with optical media. Known cases have been attributed to specific, isolated manufacturing defects. However, improper care and handling of optical discs can result in damage leading to "rot."
  • Attention to care and handling will promote the longevity of your optical media and the data saved thereon. By following the care and handling guidance provided by NIST, and by considering some important additional factors, you can reliably use optical media for backup purposes.

Ultimately, the longevity of current recordable optical media formats may soon be a moot point: the emergence of the Blu-ray Disc™ and HD DVD formats will bring new questions and concerns regarding longevity, backward compatibility, and migration.

As our personal documents, pictures, music, and movies become bit streams subject to the vagaries of technical obsolescence, digital preservation will increasingly concern us all.

Optical media longevity: questions and answers

Terminology

The following terminology is used in this FAQ:

  • Writable optical media refers to CD-R, DVD-R, and DVD+R.
  • Rewritable optical media refers to CD-RW, DVD-RW, and DVD+RW.
  • Recordable optical media is used to collectively refer to writable and rewritable optical media.
  • DVD±R refers to DVD-R and DVD+R media collectively.
  • DVD±RW refers to DVD-RW and DVD+RW media collectively.
  • Recordable DVDs refers to DVD±R and DVD±RW collectively.

How is optical media longevity defined?

The longevity, life expectancy, or service life of optical media is considered to be "the period of time in which the information recorded on the disc can be retrieved without loss." [1]

What factors affect the longevity of optical media?

The longevity of optical media depends on a plethora of factors, including: raw material quality, manufacturing quality, environmental conditions (exposure to temperature, humidity, sunlight, chemicals, and pollutants), storage, handling, recording quality, recording speed, rewrites, and the equipment in which the disc is used.

How is optical media longevity estimated?

The longevity of a product, like optical media, is estimated using Accelerated Life Tests (ALTs). ALTs are designed to estimate the life span of a given product in normal use based on its performance during rapid aging. Rapid aging simulates a lifetime's wear and tear by exposing the product to extreme levels of the stresses to which it would be subjected in regular use, resulting in product failure within a limited time period. The shortened product life span observed in the ALT is multiplied by an acceleration factor to estimate the expected life span of the product in normal use.

For optical media, stresses of temperature, relative humidity, or both are typically applied as elevated levels of these stresses have been shown to accelerate the natural degradation of the dyes used in writable optical media.

The International Standards Organization (ISO) currently defines two standards for testing optical media:

• ISO 18921:2000 for CD-ROM media.

• ISO 18927:2002 for CD-R media.

Research has demonstrated that ISO 18927:2002 is applicable to estimating the life expectancy of DVD-R media and it has been applied in ALTs involving all recordable DVD formats. [2]

In a typical optical-media ALT, multiple sample discs are exposed to stresses for a period of time ranging from tens to hundreds of hours. Discs are then tested for indicators of impending or actual failure, the end-of-life criteria for the test. The stress-and-test cycle is repeated until each disc meets the end-of-life criteria. Indicators of impending failure include unacceptable levels of jitter, Block Error Rate (BLER) for CDs, and Parity Inner Error (PIE) for DVDs. Uncorrectable errors — E32 errors on CDs and Parity Outer Errors (POE) on DVDs — indicate disc failure. [3]

Mathematical models are used to compute acceleration factors and hence estimated product life in normal use. The model employed depends on the stresses applied in the ALT:

  • When temperature is the only stress applied in the ALT, the Arrhenius model is employed.
  • When temperature is combined with other stresses, such as relative humidity or exposure to sunlight, the Eyring model is employed.

The subject of Accelerated Life Testing is rich and complex. Readers interested in a deeper understanding of this subject should consult the following references:

What estimates of recordable optical media longevity are available?

The Optical Storage Technology Association (OSTA) has published the following longevity estimates for recordable optical media:

CD-R CD-RW DVD±R DVD±RW
Unrecorded shelf life (years) 5-10 Unspecified
Manufacturers' estimated recorded life span (years) 50-200 20-100 30-100 30
Maximum rewrites N/A 1,000 N/A 1,000

Only a few independent laboratories have conducted accelerated life tests for such media. Those conducted by the National Institute of Standards (NIST) and the Library of Congress (LoC) may provide more realistic estimates of optical media longevity:

  • An unpublished report of the recently-terminated "NIST/LoC Optical Media Longevity Study" of recordable optical media stated that, after 11 months of testing: [4]
    • Virtually all CD-Rs tested indicated an estimated life expectancy beyond 15 years.
    • Only 47 percent of the recordable DVDs tested indicated an estimated life expectancy beyond 15 years. Some had a predicted life expectancy as short as 1.9 years.
  • A 2004 NIST study found wide variations in the stability of different bands and compositions of writable optical media. While some samples proved to be very stable and could be expected to "ensure data availability for several tens of years," others showed significant data loss after only 100 hours of rapid aging. [1] We interpret "several tens of years" to mean roughly 30 years.

How reliable are manufacturers' longevity estimates?

While some manufacturers have used accelerated life testing to support estimated life expectancy claims ranging from 20 to 200 years, how manufacturers interpret those tests to estimate longevity is often unclear. [5]

Tests involving only a single stress, such as temperature, may yield a greater estimated longevity than if the same media had been exposed to multiple stresses — temperature, relative humidity, and exposure to simulated sunlight — depending on the media's characteristics.

The measurements selected as end-of-life indicators can significantly affect the interpretation of life expectancy from test results. A number of published manufacturers' reports employ BLER as the sole end-of-life measurement [6, 7], but tests conducted by Media Sciences, Inc. have shown that BLER alone is an unreliable indicator of failure. [8, 9, 10]

Unless a manufacturer can provide their accelerated life testing results for a given recordable optical media product, any longevity claims for that product should be regarded as suspect.

Is certified, archival-quality recordable optical media available?

At present, no certified, archival-quality recordable optical media available in the market. This is despite interest in such from a number of users:

  • Digital archivists desire writable optical media with life expectancies of 50, 75, or 100 years.
  • Business and government entities are interested in how optical media might help them comply with regulatory requirements for record-retention, with retention periods ranging from six years to permanent. [11]

The following reasons help to explain the unavailability of certified, archival-quality recordable optical media:

  • No independent body is presently responsible for testing or certifying the longevity of optical media.
  • The complexity and cost of ISO 18927:2002 testing — up to two years — is impractical when compared to the pace of recordable optical media development. [12]
  • Innovation is being driven by the mass-market consumer side of the industry, rather than by professional archival requirements. [13]
  • The market for such specialized media may be too small for any manufacturer to profitably engage in the development, testing, and sale of such media. [14]

Of these reasons, the last may be the most significant hurdle to overcome.

It is technically possible to develop long-lasting writable optical media that meets stringent testing requirements. Imation scientists have developed CD-R and DVD+R media with estimated lifetimes in excess of 50 years, verifiable using the ISO 18927:2002 testing procedure, and within the caveats specified in an associated "Life Expectancy Statement." However, these products are yet to be commercially available. [12]

The complexity and cost of ISO 18927:2002 testing has led to proposals for a shorter-yet-reliable testing method that could lead to a certification procedure for "archival grade" optical media. [15] OSTA has two groups focused on archival storage via optical media, one of which is working on a faster testing standard:

  • The Optical Disc Archive Test Committee (ODAT) is a global working group of parties from industry, government, academia, and end users of optical media formed to establish and promote a standardized test methodology for archival-quality optical media.
  • The Commercial Optical Storage Applications Group (COSA) is concerned with optical data archival solutions to meet regulations for long-term, unalterable storage in government and industry markets.

OSTA has also established a Photo Archiving Roundtable to address the issue of digital photo backup. [16] While digital photo enthusiasts may form a substantial market for certified, archival-quality optical media, issues of price-sensitivity and technical obsolescence may still prevent such media from reaching the market.

In addition to OSTA, the Government Information Preservation Working Group (GIPWoG) is interested in optical media longevity and archival-grade media. GIPWoG facilitates dialog and information exchange between government agencies and the optical storage technology industry.

Any manufacturer's claim that their recordable optical media is suitable for "archival" use should be challenged by requesting the results of the manufacturer's accelerated life tests of that media, with full disclosure of all factors involved in interpreting those tests.

Should I worry about CD rot and DVD rot?

CD rot, also known as CD bronzing, has been an issue with certain discs manufactured from the late 1980s to the early 1990s. [17] A thin layer of lacquer on the label side of the disc protects the reflective data layer. In CD-ROMs and Audio CDs — Compact Disc Digital Audio (CDDA) — the reflective data layer is normally aluminum. If the lacquer is damaged, applied improperly, or formulated incorrectly then the reflective data layer may become exposed, leading to aluminum corrosion. [18] The corrosion of the reflective data layer is the phenomenon described as "CD rot" and results in data loss and playback issues.

While the lacquer-related manufacturing problems have been resolved, one should protect the label side of their CDs from damage: the label side, rather than the clear side, is the most vulnerable.

A testament to the durability of Audio CDs is a natural aging study conducted by the Library of Congress. The study found discs that, despite exhibiting both unacceptable levels of BLER and uncorrectable errors, remained playable and failed to exhibit noticeable audio defects. [19]

DVD rot has been debunked as a chronic problem, yet it remains a persistent urban legend. [20] While there have been documented cases of deterioration in specific discs, they appear to be the result of poor manufacturing. [21] Improper storage and handling can damage DVDs. In particular, excessive bending, such as when removing a DVD from its case, can result in delamination, the physical separation of the DVD's layers where they are glued together. [22]

How can I promote the longevity of my optical media?

To promote the longevity of one's optical media, adhere to the advice in the following NIST publications:

What other things should I consider in using optical media for backup?

  • Do not use rewritable optical media for long-term or archival backups. Rewritable discs use a phase-changing metal alloy film for recording data that is less stable than the dye used in writable discs. Burn your archival backups to writable optical media, using CD-R media when feasible given the variability in recordable DVDs reported by NIST. [1, 5]
  • You will probably get what you pay for: cheap optical media is cheap for a reason. Likewise, if it seems too cheap to be true, then it may be counterfeit.
  • If you buy a new computer, assure it can read the optical media you burned with your old computer before disposing of the latter. While the optical drive that burned a disc should be able to read that disc, manufacturing and quality variations between optical drives could result in cases where a disc burned in one optical drive cannot be read by another, despite the fact that both drives support the disc's format.
  • Have multiple concurrent backups of important data:
    • Store at least one backup off-site, such as in a bank safe-deposit box.
    • Periodically check each backup to assure that the data remains readable and intact.

Related links

Notes

[1] Slattery, et. al. "Stability Comparison of Recordable Optical Discs — A Study of Error Rates in Harsh Conditions." Journal of Research of the National Institute of Standards and Technology. Volume 109, Number 5. September-October 2004.

[2] Irie, M. and Okino, Y. "Statistical Analysis of Lifetime Distribution for Optical Recordable Disks." Japanese Journal of Applied Physics. Vol. 45, No. 2B, 2006, pp. 1460-1462. 24 February 2006.

[3] Jitter is a measure of the degree to which the pits and lands on an optical disc are well-defined, with distinct, acceptable levels specified for CDs and DVDs. For definitions of BLER, E32, PIE, and POE see the "Definitions" section of the "NIST/Library of Congress Optical Media Longevity Study."

[4] Olson, Nels and Zheng, Jian. "NIST/LoC Final Report to ODAT." Presentation to the 30 January 2007 meeting of the Government Information Preservation Working Group (GIPWoG).

[5] Byers, Fred R. Care and Handling of CDs and DVDs — A Guide for Librarians and Archivists. NIST Special Publication 500-252. National Institute of Standards and Technology. Gaithersburg, MD. October 2003.

[6] Stinson, et. al. "Lifetime of KODAK Writable CD and Photo CD Media." Eastman Kodak Company. 1995. Viewed February 2007.

[7] "TDK CD-R Technology." Viewed February 2007.

[8] Hartke, J. "Recordables — CD-R Longevity Claims: Fact or Fiction?" MediaLine. August 2001. [Dead link as of 2011.10.22]

[9] Hartke, J. "Measures of CD-R Longevity." Media Sciences, Inc. Posted 17 July 2001.

[10] Hartke, J. "Why CD-Rs Fail." Media Sciences, Inc. October 2002.

[11] Trustworthy Storage and Management of Electronic Records — The Role of Optical Storage Technology. Cohasset Associates, Inc. Chicago, Il. April 2003.

[12] Edwards, J. "Distinguishing an Archival Grade Optical Medium." Imation, Inc. Presentation to the 2 February 2006 meeting of the Government Information Preservation Working Group (GIPWoG).

[13] English, D. "Live Long and Prosper: Defining Disc Longevity." Mediaware. April 2006.

[14] Rosenberger, R. "Archival Optical Disc System." Imation, Inc. Presentation to the 5 October 2005 meeting of the Government Information Preservation Working Group (GIPWoG).

[15] Byers, F. "Optical disks for archiving." National Institute for Standards and Technology (NIST). Presentation to the Optical Storage Technology Alliance (OSTA). 5 December 2004.

[16] "Optical Storage News." OSTA. September 2006.

[17] Lampson, D. L. "CD Bronzing." September 1995.

[18] Smith, C. "Myth of CD immortality is starting to rot away." The Scotsman. 15 May 2004.

[19] Shahani, et. al. "Longevity of CD Media: Research at the Library of Congress." The Library of Congress. Washington, DC. 15 December 2004.

[20] Labriola, D. "DVD Rot, or Not?" PC Magazine. Posted 22 June 2004.

[21] Byrnes, Rohan. "DVD Deterioration?" Viewed February 2007. [Dead link as of 2011.10.22]

[22] Svensson, Peter. "CDs and DVDs not so immortal after all." USA Today. Posted 5 May 2004.
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