READING
Pits scatter, lands reflect
As the disc spins, light from a land is reflected back into a detector and light hitting a pit is scattered. The pattern becomes the ones and zeros of the music.
How it works
HOW THE DISC IS READ
Light, pits
and a spiral.
Every CD is read by a beam of light.
A focused laser follows a single spiral track of pits and lands, from the centre of the disc outwards. Anything that gets between the beam and that thin shiny layer can stop the music.
CENTRE OUT wipe in straight lines
Check a mark- Hold by the edge
- Wipe centre to rim
- Store upright, cool, dark
- Small habits, longer-lived discs
THE SHORT ANSWER
The data sits
near the label.
The laser reads through the clear plastic.
The fragile part is closer to the printed side.
HANDLING
Hold it by the edge
NIST's guide says to handle discs by the outer edge or the centre hole. Fingerprints and smudges can throw the laser out of focus.
Check a mark
STORAGE
Upright, like a book
Store discs upright in their cases in a cool, dry, dark place. Lying flat for a long time can let the plastic bend.
See the numbers
FOUR HABITS FROM THE ARCHIVES
Look after
your discs.
None of these needs special kit. They come from the advice NIST wrote for librarians and archivists, and work just as well for a shelf of albums.
Handle by the edge
Hold discs by the outer edge or the centre hole, and avoid bending them as you lift them off the case hub.
Wipe straight, not round
Clean with soft cotton from the centre straight to the edge, never round the disc. Isopropyl alcohol is a mild option for stubborn dirt.
Go easy on the label
Write only with a soft, non-solvent felt-tip, never a ballpoint, and do not try to peel off a stuck-on label.
Store upright, out of the sun
Keep discs in their cases, standing like books, somewhere cool, dry and dark. Sunlight is especially harmful to recordable discs.
TWO NUMBERS WORTH KNOWING
How long
a disc can last.
One figure from manufacturers' tests,
one storage setting archivists use.
100–200 years
Expected life of CD-R discs
Manufacturers that have tested CD-R discs agree on 100 to 200 years or more under recommended storage. Rewritable discs: 25 years or more. Little is known for pressed music CDs.
RECORDABLE CDs · NIST
18°C, 40% RH
A suitable long-term setting
The guide calls 18°C and 40% relative humidity suitable for long-term storage, and warns that sudden temperature changes stress a disc more than gradual ones.
STORAGE · NIST
In NIST tests, a CD left under clean water for 24 hours would not play at first, but played normally after drying for 24 hours at about room conditions.
A QUICK DISC CHECK
What kind of
mark is it?
Pick the mark on your CD to see how much it is likely to matter. The bar shows how likely it is to cause reading problems.
Based on the NIST guide Care and Handling of CDs and DVDs (2003). Players vary, so a disc may still play with some damage.
INSIDE THE DISC
A spiral of
tiny pits.
A pressed CD is moulded in clear polycarbonate plastic with a stamper that presses in pits and lands. A thin metal layer is added to reflect the laser, then a protective lacquer and the label go on top.
The laser reads from the centre outwards along a spiral track. The first CD player was launched on 1 October 1982, from a joint project between a Japanese and a Dutch electronics company.
Questions
Questions people ask
Why does a scratch on the label side matter so much?
On a CD the reflective metal and data layers sit just under the label. A sharp point can dent the metal or expose it, so the laser can no longer read that part from the other side.
Should I clean my CDs regularly?
No. NIST says discs do not need routine cleaning, and every wipe risks a new scratch. Clean only when there is dirt or a smudge to remove.
Which way should I wipe a disc?
Straight from the centre out to the edge. Scratches that run around the disc, along the track, are the most harmful kind.
Is it fine to put sticky labels on a CD?
NIST advises against adhesive labels on discs kept for more than five years, and warns that peeling one off can damage the lacquer and metal.
How long will a music CD last?
Nobody knows for certain. Manufacturer tests cover recordable discs; for pressed discs, including audio CDs, NIST says little information is available, so life depends on storage.
Sources
- NIST SP 500-252: Care and Handling of CDs and DVDs, a guide for librarians and archivists (PDF)
- CLIR pub121: Care and Handling of CDs and DVDs, by Fred R. Byers
- Molecular Expressions (Florida State University): Compact disk lasers tutorial
- Science Museum Group Collection: Compact disc player, 1983
Updated