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Re: Gold v. Green Debian CDs

The CD-R process attempts to retrofit a write-once disk into the
pre-existing CD system. It's not completely successful. CD-Recordable
disks aren't readable in some drives. Apparently some drives can read
the "gold" ones and not the "green" ones.

The original CD uses a thin aluminum layer over a thick acrylic layer.
The signal is encoded as pits in the aluminum surface in a process
very much like the one that had been used to duplicate vinyl phonograph
records. A metal "stamper" disk is used to stamp the pits into the
aluminum surface of the CD. Another thick acrylic layer is then placed
on top of the aluminum surface. The label is then painted on this layer.

CD-recordables have different layers in the "sandwich". The data
surface starts with _one_ thick layer of acrylic that provides all of the
mechanical rigidity of the CD, rather than two layers. On top of this
layer is a thin layer of organic dye, then a layer of elemental gold only
a few molecules thick, a _very_thin_ acrylic protective layer, and the
paint of the label.

Recording uses a laser with 10-30 times the power used to read the
CD. Pits are created in the gold surface like the pits that would have
been pressed into aluminum CDs. To create a pit, the dye is heated by
the laser until it forms a bubble. The bubble pushes a pit into
the gold. Gold is used rather than aluminum because it is more mallable
and denser. The mallebility means that gold will deform more easily than
aluminum when the dye bubbles, and the density means that even a very
thin layer of gold will reflect most light.
There are two forms of dye used, one looks "gold", but of
course you are actually seeing the gold layer behind the dye. The other
one looks green. Both kinds of dye absorb a substantial amount of light
at the wavelength of the laser.

There are several deficiencies of the CD-R process. The disks are
physically fragile on the label side - writing on them with a ballpoint
pen will destroy them. Their life may be limited (nobody really knows).
They are less optically transmissive than regular CDs, and thus
difficult to read.

Don't be surprised if some drives can't read a CD-R. In many cases
you can't solve the problem except by changing to a different drive.


	Bruce Perens
Bruce Perens K6BP   Bruce@Pixar.com
Finger bruce@master.Debian.org for PGP public key.
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