CD-R

A CD-R (Compact Disc-Recordable) is a thin (1.2 mm) disc made of polycarbonate with a 120 mm or 80 mm diameter that is mainly used to store music or data. However, unlike conventional CD media, a CD-R has a core of dye instead of metal.

Related Topics:
Polycarbonate - Mm - Music - Data - CD - Dye - Metal

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A standard CD-R has a storage capacity of 80 minutes of audio or 700 MiB of data. Non-standard CD-Rs are available with capacities of 79 minutes and 59.74 seconds/736,966,656 bytes (702 MiB), which they achieve by slightly exceeding the tolerances specified in the Orange Book CD standards. Most CD-Rs on the market are of the latter capacity. There are also 90 minute/790 MiB and 99 minute/870 MiB discs, though they are rare.

Related Topics:
MiB - Orange Book

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The polycarbonate disc contains a spiral groove to guide the laser beam upon writing and reading information. The disc is coated on the side with the spiral groove with a very thin layer of organic dye and subsequently with a thin, reflecting layer of silver, a silver alloy or gold. Finally, a protective coating of a photo-polymerizable lacquer is applied on top of the metal reflector and cured with UV-irradiation.

Related Topics:
Laser - Silver - Alloy - Gold - UV

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A CD recorder is a special type of CD-ROM drive used to write onto blank CD-R media. A laser is used to "burn" small pits into the dye so that the disc can later be read by the laser in a CD-ROM drive or CD player. The laser used to write CD-Rs is an infrared laser which emits laser radiation at a wavelength of 780 nm. The reflectivity in the pit area is different (lower) than for the unchanged dye area, because the refractive index of the dye is lowered upon "burning" a pit. Upon reading back the stored information, the laser operates at a low enough power not to "burn" the dye and an optical pick-up records the changes in the intensity of the reflected laser radiation when scanning along the groove and over the pits. The change of the intensity of the reflected laser radiation is transformed into an electrical signal, from which the digital information is recovered ("decoded"). The decomposition of the dye in the pit area through the heat of the laser is irreversible (permanent). Therefore, once a section of a CD-R is written, it cannot be erased or rewritten, unlike a CD-RW. A CD-R can be recorded in multiple sessions.

Related Topics:
CD recorder - CD-ROM - Infrared - Laser radiation - Wavelength - Reflectivity - Refractive index - CD-RW

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When looked at by the naked eye, the part of the CD-R which contains data will be slightly darker than areas which are not written to. Using this method, you can get a rough estimation of the amount of data on a CD-R, and whether or not it has been written to.

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