Optical Media

1
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)

2
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

3
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

4
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

5
Disc Structure CD vs. DVD
6
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

7
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

8
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

9
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 Groups Optical Media pen
  http//aic.stanford.edu/sg/emg (felt tip,
  water-soluble ink)

10
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

11
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)

12
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

13
Please Contact Us

• Weissman Preservation Center
• 617-495-8596
• http//preserve.harvard.edu
• Jane Hedberg jane_hedberg_at_harvard.edu
• Elizabeth Walters elizabeth_walters_at_harvard.edu
• Liz Coffey coffey_at_fas.harvard.edu