Background: Recordable optical discs can be convenient media for access and temporary storage. Recordable disc formats include CD-R, CD-RW, DVD-R, DVD-RW, DVD+R, and DVD+RW. Unfortunately, these media are machine-dependent, and continued access to the digital content is contingent on the availability of compatible hardware and software. Additionally, these media are subject to deterioration just like any other material.
Optical discs are composed of layers, based on the original CD-ROM construction, which consists of a polycarbonate substrate that has been stamped with data from a master, a metal reflective layer, and a protective top coating. Recordable formats, "R" media, are a type of Write Once, Read Many (WORM) technology. These media use a photosensitive organic dye as the data layer rather than stamping of the polycarbonate. Rewritable formats, "RW" media, use a phase-changing metal-alloy film sandwiched between two dielectric layers as the data layer. The chemical composition of these layers varies depending on when and where the disc was manufactured. Aging, storage environment, and handling can adversely affect disc materials, which can lead to loss of data. Research has shown that recordable media tend to degrade faster than ROM media
The Library of Congress has long been committed to gaining an understanding of the longevity of optical storage media. A primary objective of this research is to assess the preservation needs of the Library's collections of digital content stored on recordable discs to devise strategies to minimize the loss of data that may result as the collection ages. An essential first step is to gain an understanding of the failure modes and mechanisms of the various recordable disc formats by monitoring errors that may accumulate upon aging. Identifying compositional characteristics of these media that can be linked to failure mechanisms would enable curators of digital media to flag more vulnerable discs for replication or backup to preserve the data before any of it is lost.
Contributing Study: Slattery, O.T. 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, Vol. 109, No. 5, 2004. [PDF: 654 KB / 8 p]
Iraci, J. "The Relative Stabilities of Optical Disc Formats; " Restaurator, Vol. 26, No.5, 2005. [PDF: 511 KB / 18 p]
Zheng, J., Slattery, O.T. NIST/Library of Congress Optical Disc Longevity Study: Final Report. [PDF: 404 KB / 32 p.]
Project Description: Several different complementary studies have been undertaken or are continuing.
In 2004, as a complement to the Library's research studies on the longevity of CD-ROM discs, the Preservation Directorate initiated a joint project between the Library of Congress and The Information Access Division of the National Institute of Standards and Technology (NIST) to perform a detailed investigation of the longevity of recordable Compact Disc (CD) and Digital Versatile Disc (DVD) media. This project used accelerated aging techniques and statistical analysis to estimate the life expectancy (LE) of currently available recordable DVD and CD media. The researchers at NIST based the study on the ISO 18927 test method for CD-R discs, making modifications to the stress conditions and the exposure intervals to accommodate shorter-lived media. Only the effects of temperature and relative humidity on the media were considered. The testing did not attempt to model degradation due to exposure to light, corrosive gases, contaminants, or mishandling, nor did it account for variations in the playback subsystem.
The Library of Congress Preservation Research & Testing Division (PRTD) is currently working on a research plan to continue the work on recordable optical disc formats started in the joint LC/NIST study. In 2005 initial error testing was conducted by NIST on a collection of test samples of CD-R, DVD-R/RW and DVD+R/RW discs. These discs were transferred to LC in 2010 and will be re-tested at various intervals in conjunction with the Library's other on-going natural aging studies on CD-ROM media. These discs will serve as a baseline for future accelerated aging studies using the most up-to-date aging protocols.
In 2011 PRTD began conducting experiments to develop a rapid identification method to distinguish the most fugitive materials used in recordable discs from those that have been shown to be more stable according to accelerated aging studies. The initial focus will be distinguishing the organic dyes used in recordable CDs followed by developing a more thorough understanding of the chemical composition and variables in disc manufacture that affect degradation and readability of the data.
Outcomes/Findings: The results of these efforts are summarized below:
- The accelerated aging study conducted by NIST revealed significant variations in the predicted longevity for the different DVD products tested, although most life expectancies were greater than 30 years. All of the CD media tested had life expectancies greater than 30 years. The standardized life expectancy estimated using this model is defined for discs maintained at 25ºC and 50% RH, but can be applied to give an estimate of the life expectancy at any moderate storage conditions. Discs exposed to more severe conditions of temperature and humidity would be expected to experience a shorter life.
- A greater percentage of CD products were predicted to achieve a long life expectancy relative to the DVD products. The reasons for these differences in longevity are related to the different size of the bit markings between the two media, the stability of the dyes used, and the maturity of the technology for CD media compared to DVD at the start of the study.
- Different optical media products react to various exposure conditions in very different ways, depending on the stability of the materials used in the recording and substrate layers, and the relative sensitivity of the recording layer to the effects of heat, humidity, and light. Other factors that affect longevity are the quality of the manufacturing process; the amount and type of markings or labels attached; the compatibility of the media with the recording device and the quality of the initial recording; the age of the media at the time of recording; the storage conditions and the handling of the media during use. The relative effects of these multiple factors make predictions of the actual lifetime, or even average life expectancy, of these products difficult. This type of artificial aging testing only represents a broad estimate of the actual longevity of the media.
- The difference in quality between the CD and DVD products is expected to change as the technology further matures and stabilizes. At the time that the samples were collected for the NIST study (2004), products aimed at the long-term archival storage market were still under development. Continued testing of new products as they arrive on the market may show significant changes in the longevity estimates and relative stabilities of all types of recordable optical discs.
Update and Images:
Error testing of optical discs
May 2009: Presentation, American Institute for Conservation, Electronic Media Group: Characterizing Optical Disc Longevity at the Library of Congress, published in AIC post-prints, 2010.
April 2010: Presentation, Library of Congress, Topics in Preservation Science 50th Symposium: Update on LC's Electronic Media Research.
April 2010: Presentation, Andrew W. Mellon Symposium, Technical Conservation Issues of Time-Based Media: CD-R, CD-ROM and Other Time Based Media Research at the Library of Congress.
November 2010: Presentation, Eastern Analytical Symposium, Digital Media in Cultural Heritage: New Preservation Technologies Session: Research Studies on the Longevity of Digital Optical Media at the Library of Congress.
