02/16/99
 
 

A compact disc, or CD, is about 4.7 inches (12 centimeters) in diameter and 0.047 inches (1.2 millimeters) thick. Most of the CD consists of an injection-molded piece of clear polycarbonate plastic. During manufacturing, this plastic is impressed with microscopic bumps arranged as a single, continuous and extremely long spiral track of data.

 Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is placed onto the disc, covering the bumps. Then a thin acrylic layer is sprayed over the aluminum to protect it.

 The spiral data track begins at the disc's center. The track is about 0.5 microns wide - a micron equals one thousandth of a millimeter - and about 1.6 microns separate one portion of the track from the next parallel portion of the track. The track consists of elongated bumps 0.5 microns wide, at least 0.97 microns long and with a height of 125 nanometers (a nanometer equals a billionth of a meter).
 
 

SOURCE: www.howstuffworks.com/cd.htm
 
 

How a CD-RW is recorded
 
 

The recording layer of a CD-rewritable is polycrystalline. During recording, also called "burning," a laser selectively heats areas of the recording track above the layer's melting point. Melted crystals along the track flow into an amorphous phase, which is then "frozen-in" by quickly cooling the layer.

 The reflectance of the amorphous areas is much lower than that of the crystalline areas. During read-out, it gives rise to a signal similar to that produced from the pits and lands of a regular CD.

 During the rewrite process, some amorphous areas along the track are returned to the crystalline phase. Similarly, some crystalline areas are converted to the amorphous phase by heating above the melting point, then quenching as described above; others are rewritten as crystalline regions. 

This process can be repeated several thousand times per disc.
 
 

SOURCE: Philips Electronics
 
 

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