Hardware Reference
In-Depth Information
CD Construction and Technology
A CD is made of a polycarbonate wafer, 120mm in diameter and 1.2mm thick, with a 15mm hole in
the center. This wafer base is stamped or molded with a single physical track in a spiral configuration
starting from the inside of the disc and spiraling outward. The track has a pitch, or spiral separation,
of 1.6 microns (millionths of a meter, or thousandths of a millimeter). By comparison, an LP record
has a physical track pitch of about 125 microns. When viewed from the reading side (the bottom), the
disc rotates counterclockwise. If you examined the spiral track under a microscope, you would see
that along the track are raised bumps, called
pits
, and flat areas between the pits, called
lands
. It
seems strange to call a raised bump a
pit
, but that is because when the discs are pressed, the stamper
works from the top side. So, from that perspective, the pits are actually depressions made in the
plastic.
The laser used to read the disc would pass right through the clear plastic, so the stamped surface is
coated with a reflective layer of metal (usually aluminum) to make it reflective. Then the aluminum is
coated with a thin protective layer of acrylic lacquer, and finally a label or printing is added.
Caution
Handle optical media with the same care as a photographic negative. The disc is an optical
device and degrades as its optical surface becomes dirty or scratched. Also, it is important to
note that, although discs are read from the bottom, the layer containing the track is actually
much closer to the top of the disc because the protective lacquer overcoat is only 6-7 microns
thick. Writing on the top surface of a disc with a ballpoint pen, for example, easily damages the
recording underneath. You need to be careful even when using a marker to write on the disc.
The inks and solvents used in some markers can damage the print and lacquer overcoat on the
top of the disc, and subsequently the information layer right below. Use only markers designed
for or tested as being compatible with optical media. The important thing is to treat both sides
of the disc carefully, especially the top (label) side.
Mass-Producing CDs
Commercial mass-produced optical discs are stamped or pressed and not burned by a laser as many
people believe (see
Figure 11.1
)
. Although a laser is used to etch data onto a glass master disc that
has been coated with a photosensitive material, using a laser to directly burn discs would be
impractical for the reproduction of hundreds or thousands of copies.
