CDs have become more broadly used than floppy diskettes. With this, CDs themselves have to be a medium that can be mass produced. This is done today by stamping a pattern of pits and landings (discussed later) into the polycarbonate disc, called a substrate. Stamping can be done quite fast, and CD printers have complex machines that can product many CDs at the same time. But, stamping is not the only thing that does into creating a usable CD-ROM. Next, the clear disc is given a reflective coating so that it will reflect the laser light the drive shines on it. Silver is used as a basis in this coating because it spreads out evenly, coating all pits and landing uniformly. Next, a protective lacquer is coated onto the disc that will protect the reflective coating and help protect the CD from scratching. Finally, a label is printed onto the top side of the CD. .
Data is stored on the medium through the use of pits and landings. "Pits" refer to the tiny holes that are burned into the CD using a laser. Landing reflect to unburned portions of the disc. The landings are coated with the reflective medium that I discussed above, and being that there is no alteration in the surface to re-direct the laser light, the laser is reflected nicely off of the landings. The pits, though, refract the laser light into many different directions, and therefore no direct reflection is seen by the detector. The drive uses a highly focuses laser light to read data off of the CD. The laser is directed at the data side of the CD. It penetrates the protective plastic layer and then reaches the reflective surface. The pits and landings reflect the laser differently. The landings generate a strong reflection back, whereas the pits do not. These two transitions correspond to the 0's and 1's in the binary number system. The binary signal received by the light from the reflected laser is converted to computer logic and decoded.
