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The preservation of optical media is essential because it is a resource in libraries, and stores audio, video, and computer data to be accessed by patrons. While optical discs are generally more reliable and durable than older media types, (magnetic tape, LPs and other records) environmental conditions and/or poor handling can result in lost information. This article will introduce the different types of optical media discs and offer a discussion of strategies for preservation of these materials.〔Pohlmann, K. C. (1992). ''The compact disc handbook''. Madison, Wisconsin: A-R Editions INC.〕 ==Types of media== The two broad types of optical discs are Compact Discs (CDs) and DVDs. Data is retrieved by both CDs and DVDs by devices that focus a laser light beam against the reflective layer allowing data to be read. The data layer, supported by the polycarbonate substrate can be metallic or dye-based, depending on the disc type. The reflective and data layers of CDs are just below the label and a thin sheet of polycarbonate substrate. A much thicker layer of the substrate supports and protects the bottom of the disc. The reflective and data layers of double-sided DVDs are in the center of the disc structure, housed between two equal layers of polycarbonate substrate. Because the data layer of CDs is more exposed than double-sided DVDs, a thin lacquer layer is used to protect the surface of the CD. The top of a CD is delicate and fragile; the bottom is merely a transparent protective covering.〔Bennett, H. (2003). (Understanding CD-R and CD-RW. ) Cupertino California: Optical Storage Technology Association.〕 For preservation purposes: Gold CD-R (Compatible Disc-Recordable) and DVD-R (Digital Video Disc-Recordable or Digital Versatile Disc-Recordable) discs are preferred by experts over aluminium and silver for reliable long-term backup storage—the reflective layer of the optical disc is gold. Permanent and long-term storage are distinct. “()igital archiving experts commonly acknowledge that no carrier is permanent. Instead, one must maintain data transferred to storage and provide access and ensure integrity of the information for the long-term.” As technology changes data can be migrated from an older to a newer type of media to avoid media failure or “format obsolescence”—a real threat for technology when it is no longer supported. If the machine required to play and read the discs is not kept in working order and maintained, data loss may result.〔Bradley, Kevin. (2006) (''Memory of the World Programme: Sub-Committee on Technology: Risks Associated with the Use of Recordable CDs and DVDs as Reliable Storage Media in Archival Collections–Strategies and Alternatives'' ). pp. 3–4. Paris: UNESCO. Accessed on October 8, 2007.〕 Risks involved with optical media are covered below. Issues which affect data longevity of nominally archival-grade discs include the following: dye failure (discs with premium organic and pthalocyanine-based long-life dyes are more suitable); bonding failure (premium bonding agents and edge-to-edge coverage improve longevity); scratches,minimised by careful handling and a scratch-resistant coating; production quality (some factories have better quality control standards, and discs from a batch known to be good may be more reliable than another batch).〔Frazier, Ronald W. (December 6, 2006) (on Using CD’s and DVD’s for Long Term Storage.'' ) Online posting. The Archives & Archivists (A&A) List. Accessed on October 14, 2007.〕 Testing is required since conditions vary from machine to machine and from disc to disc; environmental control is required to prevent damaging conditions. When these conditions are met it is believed that the life of an archival-quality CD-R or DVD-R can be as long as 100 years, compared to the typical five to ten years for non-archival quality optical discs. The ISO 9660 standard specifies a stable room temperature of 18 - 23 °C with relative humidity of 30% to 50%.〔The University of the State of New York. ''Guidelines for Ensuring the Long-Term Accessibility and Usability of Records Stored as Digital Images: Government Records Technical Information Series''. p. 5. Albany: University of the State of New York, 1998.〕 Keeping multiple copies of discs is necessary for added protection. “One Master, stored under optimal conditions, one Working copy to be used for access purposes or copying, and one Safety copy to be stored at a different location.”〔Bradley, Kevin. (2006) (''Memory of the World Programme: Sub-Committee on Technology: Risks Associated with the Use of Recordable CDs and DVDs as Reliable Storage Media in Archival Collections–Strategies and Alternatives'' ). p. 11. Paris: UNESCO. Accessed on October 8, 2007.〕 According to the Institute of Conservation the container most suitable for storage is a rigid high-quality case made from inert polyester placed vertically.〔ICON Conservation Register. (2006) (''Caring for Digital and Electronic Media'' ) Accessed on October 12, 2007.〕 There is still speculation on how reliable optical media are. There are no accepted standards for blank discs and recording devices, so acceptable performance cannot be relied upon.〔Bradley, Kevin. (2006) (''Memory of the World Programme: Sub-Committee on Technology: Risks Associated with the Use of Recordable CDs and DVDs as Reliable Storage Media in Archival Collections–Strategies and Alternatives'' ). pp. 6–7. Paris: UNESCO. Accessed on October 8, 2007.〕 The longevity of rewritable formats—CD-RW, DVD-RAM, DVD-RW and DVD+RW—is less well known; they are also susceptible to accidental overwriting. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「optical media preservation」の詳細全文を読む スポンサード リンク
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