optical media signal is written to and read from a rotating disc by means of a focused laser beam...
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Optical Media
Signal is written to and read from a rotating disc by means of a focused laser beam
Formats include Laserdiscs (introduced in 1978), CDs (1982), pre-recorded MiniDiscs (1992), and DVDs (1995)
Optical Media — CDs and DVDs
Numerous format variants: CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-R, DVD-RW, DVD-RAM, Blu-Ray DVD, and many more
Standard size is 12cm (4.75”) in diameter and 1.2mm thick, but structurally there is wide variation
Complex laminate structures vulnerable to damage from light, temperature, humidity, and poor handling
Recordable media (e.g., CD-Rs and DVD-Rs) are the variants most likely to be found in Special Collections, and are also the most vulnerable
Optical Media — Structure
All optical discs have: Substrate (base layer)
Polycarbonate (hygroscopic) Metal reflective layer
aluminum, silver, silver alloy, or gold Data layer
Molded, photosensitive dye, or phase-changing metal alloy
Some discs have: Protective lacquer coating on top and sides (CDs) Adhesive layer (DVDs) Label or printable surface
ALL of these components are vulnerable to multiple types of damage and deterioration
Three Types of CDs and DVDs
Read-Only/Replicated (ROM discs) Data layer is molded into the base layer surface
Recordable/Write-Once (R discs) Data layer is a photosensitive organic dye
(phthalocyanine, cyanine, or azo)
Rewritable/Erasable (RW discs) Data layer is a phase-changing metal alloy film
Damage and Deterioration (1)
Oxidation or corrosion of metal reflective layer Causes: incursion of oxygen or moisture via scratches or other surface
damage Diminishes reflectivity, making disc unreadable Appears as pin holes when disc is held up to the light and/or brownish
discoloration
Dye fading Causes: exposure to light and/or heat, natural aging, repeated playback Recorded areas become less distinguishable from unrecorded areas
Base layer damage or degradation Causes: poor handling, solvents (crazing), natural aging, darkening
caused by light Loss of base transparency leads to read errors or tracking problems
Lacquer layer damage or degradation Causes: scratches, solvents, natural aging, label adhesives and inks,
fingerprints Can expose or damage the vulnerable metal reflective layer
Damage and Deterioration (2)
Deformation (delamination, shrinkage, warping, cracking) Causes: Heat, temperature and RH fluctuations, mishandling, physical
stress (flexing, bending, etc.), horizontal storage, removal or expansion/contraction of adhesive labels
Deformed discs cannot spin properly, and can even damage playback equipment
Care and Handling
Avoid touching disc surfaces Hold discs by gently grasping the outer edges or
or placing a finger in the center hole Never bend or flex discs Remove a disc from its container by pressing down on
the center hub to release Never leave a disc sitting inside playback equipment Clean discs with a soft, non-abrasive, lint-free cloth;
use distilled water only if absolutely necessary Wipe discs radially (from center to outer edge) Inspect discs periodically for signs of deterioration
Labelling
Do not apply adhesive labels to disc surfaces (labels can warp or unbalance discs, inks may damage lacquer)
Do not attempt to peel off adhesive labels (this may delaminate the disc)
If you must write on an optical disc, write only on the clear inner hub on the label side of the disc
Never write on discs with a writing implement that could scratch the disc (possibly damaging the data below) or that uses ink containing acids that could damage the lacquer protective layer
AIC Electronic Media Group’s Optical Media pen http://aic.stanford.edu/sg/emg (felt tip, water-soluble ink)
Environment and Storage
Store in a clean, cool, dry, dark environment
Minimize exposure to light, heat, and humidity
Avoid dramatic changes in temperature or RH
Orient discs vertically in rigid enclosures specifically designed for optical media; never stack discs horizontally or allow them to lean
Do not pack discs too tightly or loosely on shelves
Return discs to storage containers immediately after use to avoid surface damage and exposure to light
Preferred Containers
Rigid, impact-resistant containers made of inert materials (PVC jewel cases may emit corrosive gases)
Locking mechanism to prevent accidental opening
Good hub (preferably a self-releasing one that prevents the disc from touching the container)
Avoid paper or plastic sleeves, thin jewel cases, and other flexible containers (which provide little physical protection, may interact chemically with the disc, and can scratch disc surfaces)
If possible, remove liner notes or other materials from the container (paper attracts moisture and creates dust)
Requires Immediate Attention
Visually discernable evidence of damage or deterioration such as delamination, oxidation or other discoloration, chips, or gouges
Early date of manufacture: Pre-1995 for CD-Rs, Pre-1999 for DVD-Rs
Please Contact Us
Weissman Preservation Center617-495-8596
http://preserve.harvard.edu
Jane Hedberg [email protected] Walters [email protected] Coffey [email protected]