Optical Storage - Upgrading and Repairing PCs - page 654

of the disc, and a photosensitive receptor detects when the light is reflected back. When
the light hits a land (flat spot) on the track, the light is reflected back; however, when the
light hits a pit (raised bump), no light is reflected back.
As the disc rotates over the laser and receptor, the laser shines continuously while the re-
ceptor sees what is essentially a pattern of flashing light as the laser passes over pits and
lands. Each time the laser passes over the edge of a pit, the light seen by the receptor
changes in state from being reflected to not reflected, or vice versa. Each change in state
of reflection caused by crossing the edge of a pit is translated into a 1 bit digitally. Micro-
processors in the drive translate the light/dark and dark/light (pit edge) transitions into 1
bits, translate areas with no transitions into 0 bits, and then translate the bit patterns into
actual data or sound.
The individual pits on a CD are 0.125 microns deep and 0.6 microns wide. Both the pits
and lands vary in length from about 0.9 microns at their shortest to about 3.3 microns at
their longest. The track is a spiral with 1.6 microns between adjacent turns (see Figure
11.2 ) .
Figure 11.2 Pit, land, and track geometry on a CD.
The height of the pits above the land is especially critical because it relates to the
wavelength of the laser light used when reading the disc. The pit (bump) height is exactly
1/4 of the wavelength of the laser light used to read the disc. Therefore, the light striking
a land travels 1/2 of a wavelength of light farther than light striking the top of a pit (1/4 +
1/4=1/2).Thismeansthelightreflectedfromapitis1/2wavelengthoutofphasewiththe
rest of the light being reflected from the disc. The out-of-phase waves cancel each other
out, dramatically reducing the light that is reflected back and making the pit appear dark
even though it is coated with the same reflective aluminum as the lands.
ThereadlaserinaCDdriveisa780nm(nanometer)wavelengthlaserofabout1milliwatt
inpower.Thepolycarbonateplasticusedinthedischasarefractiveindexof1.55,solight
travels through the plastic 1.55 times more slowly than through the air around it. Because
the frequency of the light passing through the plastic remains the same, this has the effect
of shortening the wavelength inside the plastic by the same factor. Therefore, the 780nm