Inside the CD Media (Cont.)
In the figure below you can observe the scheme of layers on a CD. The drive reading head is there just to show which the reading side is.
The label layer shown in the figure can simply not exist on cheaper CDs, which makes possible to remove both the recording layer and the metallic layer when one sticks on and takes a label off the CD. Do not trust CDs with a very shiny gold or silver surface opposing the reading one. The big CD manufacturers insist on inserting adhesive layers with their name and information about the CD. A scratch on this surface, when it is not protected by any label, is a thousand times worse than a scratch on the reading side.
The first recordable CD has been produced to Mitsui by Taiyo Yuden, that developed an alloy of gold with cyanine resulting the green color. The process that produces the gold CDs with gold and phythalocyanine base has been developed by Toatsu Chemicals. But Verbatim developed the blue media, made up of silver layers and a cyanine layer. The silver CDs that only came on the market in 1998 were created by Ricoh and it consists of a gold layer over an advanced” phythalocyanine. This would explain the CD silver color in spite of a gold reflective base. Other manufacturers that appeared later on the market started producing silver media with the conventional phythalocyanine, but adding higher aluminum concentrations to the reflective alloy. They are the silver/green CDs, cheaper than the similar ones with Ricoh technology.
There are two basic reasons for this variety of media found on the market. The first reason is that as each one of these compounds is a commercial compound, no company that wishes to produce CDs can use the technology of another company without paying for it. It seems to be logical to believe that no company will want to sell its technology to a competitor, as expensive as it may seem. So, if a new company wants to produce CDs, it has to create its own new alloys and layers, in spite of the fact that all of them are based in fundamental concepts that allow the generalization of cyanine and phythalocyanine to all of them.
Some CDs, such as the “Infoguard” by Kodak, have yet extra layers that have the only function of increasing the lifetime of the CD. These layers are basically systems that protect against scratches and some that take scratches to a certain depth. Nowadays most media (except the extremely cheap ones) have systems like this, not necessarily with the same technology, but with similar purpose.
The CD-RW have a completely different composition, except for the plastic layer. In substitution to the cyanine compound and the reflective metallic alloy it is put a kind of crystal that becomes opaque or translucent, according to the wavelength that goes through it. Those CDs reflect only 30% of the incident ray, what makes it impossible for them to be read in any CD drive (the conventional CDs reflect from 70% to 80% of the incident ray). The reading of the CD-RW is only possible in drives that have a circuit called AGC (Automatic Gain Control), that makes up for this “bad reflection”. The manufacturers never said anything about the composition of those CDs and anything you find about this special recording material is mere speculation.