Transcript Optical Media

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
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Numerous format variants: CD-ROM, CD-R, CD-RW, DVDROM, 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)
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Metal reflective layer
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Polycarbonate (hygroscopic)
aluminum, silver, silver alloy, or gold
Data layer
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Molded, photosensitive dye, or
phase-changing metal alloy
Some discs have:
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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
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Read-Only/Replicated (ROM discs)
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Recordable/Write-Once (R discs)
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Data layer is molded into the base layer surface
Data layer is a photosensitive organic dye
(phthalocyanine, cyanine, or azo)
Rewritable/Erasable (RW discs)
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Data layer is a phase-changing metal alloy film
Disc Structure: CD vs. DVD
Damage and Deterioration (1)
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Oxidation or corrosion of metal reflective layer
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Dye fading
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Causes: exposure to light and/or heat, natural aging, repeated playback
Recorded areas become less distinguishable from unrecorded areas
Base layer damage or degradation
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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
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
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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)
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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
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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
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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
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AIC Electronic Media Group’s Optical Media pen
http://aic.stanford.edu/sg/emg (felt tip, water-soluble ink)
Environment and Storage
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Store in a clean, cool, dry, dark environment
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Minimize exposure to light, heat, and humidity
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Avoid dramatic changes in temperature or RH
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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
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Rigid, impact-resistant containers made
inert materials (PVC jewel cases may
emit corrosive gases)
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Locking mechanism to prevent accidental opening
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Good hub (preferably a self-releasing one that prevents
the disc from touching the container)
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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
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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 Center
617-495-8596
http://preserve.harvard.edu
Jane Hedberg [email protected]
Elizabeth Walters [email protected]
Liz Coffey [email protected]