Principles of Laser Disc Operation
The Optical System
VIDEO DISC
COLLIMATOR
LENS
\
MOVING
PARTS
I
FOLDING
MIRROR
1/4~ PLATE
I
RADIAL
MIRROR
TANGENTIAL ---------
MIRROR
SPOT
LENS
GRATI NG
LASER
Atari FIREFOX, the coin video game, in-
troduced the most advanced disc player
used in the video game industry. Some
features in the Philips Laservision disc
player were custom-designed for Atari to
make the player fast enough and interac-
tive enough for our technical require-
ments. One of the key features of the
Philips player is its ability to instantly jump
a large number of tracks, thus eliminating
dead spots in game play.
Laser Disc
The laser disc contains 54,000 tracks and
has a total playing time of approximately
30 minutes. The tracks, similar to grooves
on a record , are microscopic pits; the
length and the spacing between the pits
determines the information stored on the
disc. The separation between tracks is 1.6
microns (a micron is one millionth of an
inch), the width of the pits is 0.4 micron,
and the depth is 0.1 micron.
Making a laser disc is much more in-
volved than making a record. First a mas-
ter recording is made, consisting of a
glass plate with a photosensitive coating
on one side. The information to be record-
ed modulates the beam of a laser which
"writes" the information in the coating .
After a photographic development pro-
cess, a pattern of pits appears. A master
stamp of the pits is made, which then
transfers the pattern into a transparent
plastic base which becomes the disc. A
thin, reflective layer of aluminum is added,
and a protective coating is placed over
the aluminum layer.
Optical Reading of the Disc
A laser beam penetrates the highly reflec-
tive flat disc and focuses onto the surface
of the aluminum coating. Light reflected
from a pit is less bright than the areas
without pits. This modulation of light (light
vs. dark) is focused on a photodiode
which converts the light to a frequency-
modulated electrical signal, correspond-
ing to the information on the disc.
Why does a pit produce less light? The
depth of the pit, 0.1 micron, is exactly a
quarter wavelength of a laser beam. So as
the beam t"ravels into the pit, it must travel
an extra half wavelength to get back to the
outer surface of the disc. This puts it 180°
out of phase to the light reflected from the
surface, and thus they cancel each other
out. Because the width of the pit is almost
the same size as the wavelength of the
beam, what little light remains reflects
back over a wide angle.
Each of the 54,000 tracks has a number
encoded on it, which is read by the disc
player and the logic boards, letting the
player and microprocessor know where
the laser beam is presently located on the
track. Each track contains a picture to
which the game microprocessor may
want to direct the laser. For instance, in
FIREFOX you have a choice of flying in
the sky or close to the ground. If you were
in the sky and chose to go to the ground,
the microprocessor would tell the disc
player which track contained the appropri-
ate ground scene. The disc player would
then move the laser beam to that track.
The heart of the video disc player is the la-
ser and optical components, located on a
slide assembly. To understand the optical
system, one must understand the charac-
teristics of light.
For this purpose, a light beam can be
thought of as electromagnetic energy
whose wavelength is visible to the human
eye, or a high-frequency sine wave pro-
pagating through space. Ordinarily, light
is non-polarized , or constantly changing
its polarity. Th rough optics we can force
the light waves to move only in certain di-
rections, forming horizontally, vertically, or
circularly polarized light.
Perhaps the most well-known applica-
tion of this phenomenon is in polarized
sunglasses. The special lenses will not
pass light that is horizontally polarized .
Since most light from glare and reflections
is horizontally polarized, it won't pass
through the lenses. The video disc player
also uses optical components which are
affected by the polarity of light.
The red light beam from the laser is ver-
tically polarized. The diagram shows the
path traveled by the light beam on the
player's slide assembly. The beam passes
through a series of lenses, a prism and
mirrors, and is directed onto the disc ac-
cording to the messages given by the mi-
croprocessor. The reflected light returns
through the mirrors, prism, and lenses to
the photodiodes, creating the electrical
signals.
It is a good thing that the game player
doesn't have to know all the technical as-
pects in order to enjoy playing the game!
But it's interesting to have some under-
standing of the technology that puts so
much fun into our leisure-time activities.
The next time you enjoy playing FIREFOX
or one of the other laser disc games, you'll
have an appreciation of the research and
development that takes place before a
new product reaches your location.