DIGITAL REPOSITORIES AND STRUCTURES

DIGITAL REPOSITORIES AND STRUCTURESOPTICAL MEDIA

1OPTICAL DISCS are high-density direct access storage medium consisting of specially coated discs on which data are encoded in a pattern of tiny pits burned into the surface with a laser, to be read by a device that reflects a laser beam off the pitted surface, which is then decoded by a microprocessor into digital signals

-- Online Dictionary of Library and Information ScienceDefinition2

Basic Mechanism of OperationOptical discs consist of similar basic materials and layers but different types of discs have differences in detail.Disc StructureSubstrate layer this makes up most of the disc. It provides necessary disc depth for the layer to focus on the reflective and data layers. It also gives the disc enough strength to remain flat.Disc Structure

Data layer as the name implies, this is the layer that contains the optical discs data. The data appears as pits that either absorb or reflect light back to the sensor.Disc Structure

Reflective layer this is the layer that reflects the laser beam back to the sensor. Typically, this layer is made out of gold, silver or silver alloy.Disc Structure

Protective layer this optional layer is applied on the label side of optical discs to protect the metals in the reflective layer from the environment.Certain types of optical discs cannot have this layer.Disc Structure

Surface layer this optional layer is added to provide a labeling surface for the optical disc. In certain types of optical discs, this layer only exists in the hub part of the disc.Disc Structure

The results of a study by the LoC and the NIST in 2007 indicate the following:Life expectancy of CD-Rs: 30-45 years, many samples have results of more than 45 yearsLife expectancy of DVD-Rs: Variable, only 47% of samples have results of more than 15 years.

Life ExpectancyData from the OSTA gave the following:Life Expectancy

Technological obsolescence is a bigger issue than longevity of the material.Life ExpectancyHistory of Optical Disc Mediafirst generationsecond generationthird generationFirst GenerationUsed infrared lasersCDs and its derivatives

Second GenerationUsed red (visible light) lasersDVDs and its derivativesHistory of Optical disc MediaThird GenerationUsed blue (visible light) lasersHD DVD and Blu-rayStacked Volumetric Optical Discs (SVODs) rely on stacking wafer-thin layers of data, hundreds of which are in one disc, for data storage capacity reaching 100GB per discHolographic Versatile Discs (HVDs) rely on a different way of storing and accessing data, enabling it to read 60,000 bits per laser pulse which achieves at least 100GB for a 10cm discArchival discs, and M-Discs on the other hand try to achieve reliability and longevity by making sure that the data layer is secure and stable while ensuring reasonably high data storage capabilitiesHistory of Optical Disc MediaLaserdiscLDLD-ROMVideo Single Disc (VSD)MiniDiscMDHi-MDCompact Disc (CD)CD-DACD-ROM,CD-R,CD-RW5.1 Music DiscSuper Audio CD (SACD)Photo CD, CD Video (CDV)Video CD (VCD)Super Video CD (SVCD)CD+GCD-TextCD-ROM XACD-iVCDHDGD-ROMDVDDVD-RDVD+RDVD-R DLDVD+R DLDVD-R DSDVD+R DSDVD-RWDVD+RWDVD-RAMDVD-DDVD-AHVDEcoDiscEnhanced Versatile Disc (EVD)Forward Versatile Disc (FVD)Universal Media Disc (UMD)Nintendo optical disc (NOD)HD DVDHD DVD-R,HD DVD-RWHD DVD-RAMBlu-ray Disc (BD)BD-RBD-REChina Blue High-definition Disc (CBHD)High definition Versatile Multilayer Disc (HD VMD)Holographic Versatile Disc (HVD)Ultra Density Optical (UDO)Stacked Volumetric Optical Disk (SVOD)Five dimensional disc (5D DVD)Archival DiscKinds and Variety of Optical Media

Kinds and Variety of Optical MediaKinds and Variety of Optical Media

Kinds and Variety of Optical Media

Kinds and Variety of Optical MediaDouble-sided vs. Double-layered

Kinds and Variety of Optical Media

Record information onto many discs made out of different materials.Open packaging in a clean environment with similar conditions to the place where the burning will take place.Use appropriate software and hardware to burn the data.Check and recheck the data burned.Store archival discs in the appropriate environment.Recording Information into Optical Media Discs (for Archiving)Appropriate environment for the storage of optical discs: TEMPERATURE:4C to 20C RELATIVE HUMIDITY:40% to 50%Recording Information into Optical Media Discs (for Archiving)DOs Handle discs by the outer edge or the center hole.Use a non-solvent-based felt-tip permanent marker to mark the label side of the disc.Keep dirt or other foreign matter from the disc.Store discs upright (book style) in plastic cases specified for CDs and DVDs.Return discs to storage cases immediately after use.Leave discs in their packaging (or cases) to minimize the effects of environmental changes.Care and Handling of Optical DiscsDOs (continued...)Open a recordable disc package only when you are ready to record data on that disc.Store discs in a cool, dry, dark environment in which the air is clean.Remove dirt, foreign material, fingerprints, smudges, and liquids by wiping with a clean cotton fabric in a straight line from the center of the disc toward the outer edge.Use CD/DVD-cleaning detergent, isopropyl alcohol, or methanol to remove stubborn dirt or material.Check the disc surface before recording.Care and Handling of Optical DiscsDONTsTouch the surface of the disc.Bend the disc.Use adhesive labels.Store discs horizontally for a long time (years).Open a recordable optical disc package if you are not ready to record.Expose discs to extreme heat or high humidity.Care and Handling of Optical DiscsDONTs (continued...)Expose discs to extremely rapid temperature or humidity changes.Expose recordable discs to prolonged sunlight or other sources of ultraviolet light.Write or mark in the data area of the disc (the area the laser reads).Clean by wiping in a direction going around the disc.Care and Handling of Optical DiscsDONTs (when handling CDs)Scratch the label side of the disc.Use a pen, pencil, or fine-tip marker to write on the disc.Write on the disc with markers that contain solvents.Try to peel off or reposition a label.Care and Handling of Optical DiscsCD-RCD-RWDVDRDVDRW

Unrecorded shelf life (years)5 to 10unspecified

Manufacturers estimated recorded life span (years)50 to 20020 to 10030 to 10030

Maximum rewritesn/a1,000n/a1,000