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

Disc Structure: CD vs. DVD

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 jane_hedberg@harvard.eduElizabeth Walters elizabeth_walters@harvard.eduLiz Coffey coffey@fas.harvard.edu