CDs store data as tiny physical pits on a perfectly flat polycarbonate* disc. These pits are arranged in a spiral from the center of the disc outward. A very thin layer of reflective material applied to the disc allows the pits to be read from a laser beam passed through the polycarbonate material. A thin coating of lacquer and a printed label make up the rest of the disc.
The label side of a disc provides very little protection to the reflective layer which lies beneath it. Damage which penetrates down to the reflective layer will likely affect the disc's readability and is not repairable. Scratches or foreign material on the polycarbonate read side of the disc may cause the laser beam to be distorted during the reading process . . . causing pits to be read incorrectly. These scratches can be repaired as long as they don't penetrate down to the encoded information.
DVDs are essentially made the same as CDs, except the reflective layer is sandwiched between two polycarbonate discs. Because of this, the reflective layer and data encoding of these discs are better protected than those of CDs and CD-ROMs.
Double-side DVD Layers
Double-sided DVDs have data encoded on both of the ‘sandwiched’ polycarbonate discs, allowing two play-sides. Instead of a label, a small ring surrounding the disc hub is provided to display disc information.