Science Fair Project Encyclopedia
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.
A standard CD-R has a storage capacity of 74 minutes of audio or 650MB of data (though MB is printed on CDs as the binary prefixes haven't caught on in the industry, MB will be used in this article). Non-standard CD-Rs are available with capacities of 80 minutes/703MB, which they achieve by 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/790MB and 99 minute/870MB discs, though they are rare.
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 a special 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.
A specially designed type of CD-ROM drive, called a CD-R drive, CD burner, or CD writer can be used to write CD-Rs. A laser is used to etch ("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.
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.
The CD-R was invented in 1988 by the Japanese company Taiyo Yuden (Taiyo Yuden made the discs. Yamaha made the first recorder.). First CD-Rs were produced in 1994. (First CD-Rs were produced in 1988.) Among the first manufacturers were the companies Taiyo Yuden, Kodak, Maxell, and TDK. Since then, the CD-R was further improved to allow writing speeds as fast as 54x (as of 2004) relative to the first 1x CD-Rs. The improvements were mainly due to optimisation of special dye compositions for CD-R, groove geometry, and the dye coating process. Low-speed burning at 1x is still used for special "audio CD-Rs", since CD-R audio recorders were standardized to this recording speed.
There are three basic formulations of dye used in CD-Rs.
- Cyanine dyes were the earliest ones developed, and their formulation is patented by Taiyo Yuden. Cyanine dyes are naturally green in color, and are chemically unstable. This makes cyanine discs unsuitable for archival use; they can fade and become unreadable in a few years. Many manufacturers use proprietary chemical additives to make more stable cyanine discs.
- Azo dye CD-Rs are blue in color, and their formulation is patented by Mitsubishi Chemicals . Unlike cyanine, azo dyes are chemically stable, and typically rated with a lifetime of decades.
- Phthalocyanine dye CD-Rs are usually silver or gold. The patents on pthalocyanine CD-Rs are held by Mitsui and Ciba Specialty Chemicals. These are also chemically stable, and often given a rated lifetime of hundreds of years.
Note that unfortunately, many manufacturers add additional coloring to disguise their cyanine CD-Rs, so you cannot determine the formulation of a disc based purely on its color. Similarly, a gold reflective layer does not guarantee use of phthalocyanine dye. Note also that rated CD-R lifetimes are estimates based on accelerated aging tests, and lifetime can vary considerably based on how you store the discs.
For optimum lifespan, CD-Rs should be stored vertically to prevent warping, inside archival plastic cases which use a ridged ring around the spindle which grips the disc. This ridge prevents the surface of the disc from coming into contact with anything during storage. Discs should be stored in cool, dark conditions, with controlled humidity. Avoid using any kind of label on the CD surface, and avoid use of printed inserts using anything other than water-based inks.
Although the CD-R was initially developed in Japan, most of the production of CD-R had moved to Taiwan by 1998. Chinese manufacturers supplied more than 70% of the worldwide production volume of 10.5 billion CD-Rs in 2003.
There was some incompatibility with CD-Rs and older CD-ROM drives. This was primarily due to the lower reflectivity of the CD-R disc. In general, CD drives marked as 8x or greater will read CD-R discs. Some DVD players will not read CD-Rs because of this change in reflectivity as well.
- The CD-R FAQ: http://www.cdrfaq.org/
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