How CDs are made?
A compact disc is a thin, circular disc of plastic about 12cm in diameter and 1.2mm thickness. It's actually made of four layers.
- A disc is created from injection moulded clear polycarbonate plastic and which is then engraved using a die with microscopic bumps. These bumps are known as "pits" and flats are called "lands".
- A reflective aluminium coating is applied. This layer reflects the laser back to the player, so it's integrity is extremely important.
- A clear lacquer coating is applied to seal the reflective layer and prevent oxidation.
- Finally the artwork is screen-printed on the top of the disc.
CD's consist of 99% clear polycarbonate plastic. The reflective layer, protective layer and screen print comprise the remaining 1% of the disc.
How does CDs work?
- CDs store information digitally, i.e. with the help of millions of 1s and 0s.
- Data on a CD is "burned" with a laser beam that engraves bumps (called pits) into its surface. A bump represents the number zero. The lack of a bump (which is a flat, unburned area on the disc, called a land) represents the number one.
- Thus, the laser can store all the information sampled from the original data by burning some areas (to represent zeros) and leaving other areas unburned (to represent ones).
- The disc holds this information in a tight, continuous spiral of about 3-5 billion pits. If you could unwrap the spiral and lay it in a straight line, it would stretch for about 5 to 6 km.
