Optical media longevity
The X Lab regularly fields questions concerning the longevity of optical media, such as:
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:
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
The following terminology is used in this FAQ:
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." 
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.
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. 
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. 
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:
The subject of Accelerated Life Testing is rich and complex. Readers interested in a deeper understanding of this subject should consult the following references:
The Optical Storage Technology Association (OSTA) has published the following longevity estimates for recordable optical media:
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:
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. 
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.
At present, no certified, archival-quality recordable optical media available in the market. This is despite interest in such from a number of users:
The following reasons help to explain the unavailability of certified, archival-quality recordable optical media:
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. 
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.  OSTA has two groups focused on archival storage via optical media, one of which is working on a faster testing standard:
OSTA has also established a Photo Archiving Roundtable to address the issue of digital photo backup.  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.
CD rot, also known as CD bronzing, has been an issue with certain discs manufactured from the late 1980s to the early 1990s.  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.  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. 
DVD rot has been debunked as a chronic problem, yet it remains a persistent urban legend.  While there have been documented cases of deterioration in specific discs, they appear to be the result of poor manufacturing.  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. 
To promote the longevity of one's optical media, adhere to the advice in the following NIST publications:
 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.
 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.
 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."
 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.
 Stinson, et. al. "Lifetime of KODAK Writable CD and Photo CD Media." Eastman Kodak Company. 1995. Viewed February 2007.
 "TDK CD-R Technology." Viewed February 2007.
 Hartke, J. "Recordables CD-R Longevity Claims: Fact or Fiction?" MediaLine. August 2001. [Dead link as of 2011.10.22]
 Hartke, J. "Measures of CD-R Longevity." Media Sciences, Inc. Posted 17 July 2001.
 Hartke, J. "Why CD-Rs Fail." Media Sciences, Inc. October 2002.
 Trustworthy Storage and Management of Electronic Records The Role of Optical Storage Technology. Cohasset Associates, Inc. Chicago, Il. April 2003.
 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).
 English, D. "Live Long and Prosper: Defining Disc Longevity." Mediaware. April 2006.
 Rosenberger, R. "Archival Optical Disc System." Imation, Inc. Presentation to the 5 October 2005 meeting of the Government Information Preservation Working Group (GIPWoG).
 Byers, F. "Optical disks for archiving." National Institute for Standards and Technology (NIST). Presentation to the Optical Storage Technology Alliance (OSTA). 5 December 2004.
 "Optical Storage News." OSTA. September 2006.
 Lampson, D. L. "CD Bronzing." September 1995.
 Smith, C. "Myth of CD immortality is starting to rot away." The Scotsman. 15 May 2004.
 Shahani, et. al. "Longevity of CD Media: Research at the Library of Congress." The Library of Congress. Washington, DC. 15 December 2004.
 Labriola, D. "DVD Rot, or Not?" PC Magazine. Posted 22 June 2004.
 Byrnes, Rohan. "DVD Deterioration?" Viewed February 2007. [Dead link as of 2011.10.22]
 Svensson, Peter. "CDs and DVDs not so immortal after all." USA Today. Posted 5 May 2004.