Description

This article is from the CD-Recordable FAQ, by Andy McFadden (fadden@fadden.com) with numerous contributions by others.

7-1] What kinds of media are there?

(2004/02/17)

The basic building blocks of CD-R media are organic dye and a reflective
layer. The dye types currently in use are:

- cyanine dye, which is cyan blue in color (hence the name);
- phthalocyanine and "advanced" phthalocyanine dye, which have
a faint aqua tinge;
- metalized azo, which is dark blue.

In addition, Kodak has patented a "formazan" dye, which is light green.
This has been reported to be a hybrid of cyanine and phthalocyanine.

The reflective layer is either a silver alloy, the exact composition of
which is proprietary, or 24K gold. Aluminum isn't used in CD-R media
because the metal reacts with the dyes.

Discs come in many different colors. The color you see is determined by
the color of the reflective layer (gold or silver) and the color of the dye
(light blue, dark blue, green, or colorless). For example, combining a
gold reflective layer with cyanine (blue) dye results in a disc that is
gold on the label side and green on the writing side.

Many people have jumped to the conclusion that "silver" discs are made with
pure silver, and have attempted to speculate on the relative reflectivity
and lifespan of the media based on that assumption. According to one source,
silver is susceptible to corrosion when exposed to sulfur dioxide (a common
air pollutant), so manufacturers use alloys of silver to inhibit corrosion.

Taiyo Yuden produced the original gold/green CDs, which were used during the
development of CD-R standards. Mitsui Toatsu Chemicals invented the process
for gold/gold CDs. Mitsubishi's NCC subsidiary developed the metalized azo
dye. Silver/blue CD-Rs, manufactured with a process patented by Verbatim,
first became widely available in 1996. According to the Ricoh web site,
the silver/silver "Platinum" discs, based on "advanced phthalocyanine dye",
were introduced by them in 1997. They didn't really appear on the market
until mid-1998 though. Kodak Japan holds the patent on formazan dye.

One reason why there are multiple formulations is that the materials and
processes for each are patented. If a new vendor wants to get into the
CD-R market, they have to come up with a new combination of materials that
conforms to the Orange Book specifications.

Some CDs have an extra coating (e.g. Kodak's "Infoguard") that makes the CD
more scratch-resistant, but doesn't affect the way information is stored.
The top (label) side of the CD is the part to be most concerned about,
since that's where the data lives, and it's easy to damage on a CD-R.
Applying a full circular CD label will help prevent scratches.

An EMedia Professional article discussing the composition of the newer
discs is online at http://www.emediapro.com/EM1998/starrett10.html.

CD-RW discs have an entirely different composition. The data side
(opposite the label side) is a dark silvery gray that is difficult to
describe.

Continue to:

• prev: 7] Media
• Index
• next: 7-2] Does the media matter?