Compact Disk – Recordable
Media type that can be recorded at home by end users, also called "blank CD". CD-R recording is done through a CD-R recorder, also known as CD-R burner.
CD-R media vary a lot depending on the manufacturer, since the manufacturers can use a lot of different materials to make the CD-R layers, specially the reflecting layer and the organic dye layer. Thus there are different media quality on the market.
The CD-R disk has four layers:
The first layer is the plastic substrate layer with the tracks already shaped. These tracks guide the laser beam;
The second layer is the organic dye layer, which can be cianine, phthalocyanine e metalic azo;
The third layer is responsible for reflecting the laser beam. This layer can be made of gold, platinum, aluminium or silver;
The fourth layer is a protective lacquer layer.
When the CD-R is blank, i.e. without any information stored, the organic dye layer is translucid, allowing the laser beam to cross it and reflect back on the metal layer. During the recording process, the laser beam "burns" the organic dye layer, making it opaque on the exact spot targeted by the beam. That spot won't reflect the laser beam anymore during the reading process.
Since CD-R uses the same standard as the regular CD, CD-R can be read by CD-ROM drives, which automatically understand opaque and translucid spots as pits and lands, respectivelly. Once data is written to a CD-R they cannot be erased, since it is impossible to get the organic dye layer to its original state (it is impossible to "fix" the burning caused by the laser beam).
Some CD players cannot play CD-R discs. This occurs because the CD-R reflection rate (which measures how much of the light sent to the disc returns to the sensor when it strikes a reflection spot) is smaller than regular commercial CDs ("silver CDs"). While industrially manufactured CDs have a reflection rate of 70%, CD-R discs have a reflection rate of 65%.