Three fundamentally different technologies are used in "CD-ROM" type disks. Permanent products produced by vendors are read only disks with information impressed into them in the form of tiny pits in the surface. CDRs that can be written to, but only once, emulate the pits with a dye layer whose color can be altered by applying heat using the same laser beam used to read CDs, but at a higher level. CDRWs that can be erased and rewritten, emulate the pits using a phase change material whose optical properties can be manipulated by the heat from the laser.

Under normal conditions, CDROMs with pressed pits are pretty much permanent. They can be rendered unreadable by mechanical action -- cracking or scratching). There is a recorded case of a fungus growing on (in actually) a CDR in Belize and rendering it unreadable. Presumably CDRs or CDRWs could also be attacked. I have been unable to find any projections about the permanence of the polycarbonate disk material. Some estimates of lifetime of the reflective layer in pressed CDs are as low as 25 years due to projected oxidation of the reflective layer. (Pressed CDs use Aluminum reflectors. CDRs and CDRW use Silver Alloys or Gold because Aluminum reacts with the dyes used in writable CDs.) Manufacturers claim shelf lifetimes of 5 to 10 years for unrecorded CDR media and lifetimes after recording of 75 years or more. On the other hand at least one disk CDR is known to have developed so many bad bits in just two years of storage that it was unreadable.

All CD media need to be protected from heat, direct sunlight and excessive humidity.

Several different dyes are used in CDRs including cyanine(blue), Phthalocyanine (faint aqua), azo (dark blue) and formazan (light green). Reflective layers are usually thin metal films that are gold or silver colored. Since a gold reflector will shift the apparent colors of dyes toward yellow, quite a variety of colors may be encountered on the read/write side of the disk. The variety of dyes is largely due to the desire to avoid payment of patent royalties by developing and patenting another dye. It's probably true that some dyes work better than others and some have a longer lifetime. There is little data on which might be the best from the consumer point of view.

While scratches on a CD are never good, scratches on the non-laser side that go deep enough to affect the reflective layer can be more serious than those on the read/write side. Some CDs include additional scratch resistant layers. There is some evidence that some labeling materials -- including some of those used by manufacturers -- can damage the reflective layer over time. When writing on CDR media, the use of ball tip pens is strongly discouraged in favor of felt tipped pens. But even with felt tips, it is possible that some inks may attack some disk surfaces.

There are no objective standards for CD/CDR/CDRW readability. Pressed CDs will read in almost any drive. CDR and CDRW media may read (or write) better in some drives than others, but there is no guarantee that any given drive will or won't write or read any particular brand of disk. This is exclusive of Digital Rights Management technology that may attempt to protect against piracy. Even without DRM, a brand C CDR recorded in a brand M drive may or may not play back in a brand Y drive. As time passes, the probability of media and drives being compatible seems to be increasing.

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