Reading Information from the CD
Reading Information from the CD is done by illuminating it with a focused beam from a diode laser. The
diode laser radiation is focused by an optical system into a small spot on the surface of the CD. The
radiation is reflected back to a detector from both the upper metalic surface, and the bottom of the pit.
The CD today is based on the reflection of the laser light from the metal coating on the CD. The detector
is mounted on the same side of the CD as the laser
The light reflected from a valley is at a 180
0
phase difference
from the light reflected from the upper surface.
Every time that light is reflected from the border between pit
and CD surface, there is a destructive interference between the
two signals, and the reading is logic state zero ("0").
The total length of the track is about 5 kilometers (!).
The total number of pits is about 2*10
9
(
more than 2 Billion !).
The width of each pit is 0.6 [
µ
m].
The length of each pit is 0.83-3.05 [
µ
m], in 9 discrete steps.
The depth of each pit is: 0.11 [mm].
The distance between adjacent tracks is 1.6 [
µ
m].
The density of tracks is about 16,000 TPI (Tracks Per Inch),
For comparison: human hair covers about 60 tracks.
The wavelength of the Diode laser reading the CD is 780 [nm].
The spot size of the beam on the reflecting surface of the CD
is about 1 [
µ
m].
