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Preface -- Introduction -- Light
Theory -- Light Detector --
Light Emitter
Light System Configuration -- Light Processing -- Receiver Circuits --
Transmitter Circuits |
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Chapter Four
- Light
System Configurations |
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| Whether you are sending a simple on and
off signal or high-speed computer data, some kind of light path must be establish between
the light transmitter and the distant receiver. The three basic ways the information can
be transferred are: "Opposed", "diffused reflective" and "retro
reflective". Every communications system will use one or more of these methods.
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| Opposed
Configuration |
| As illustrated in Figure 4a an
"opposed" or "through beam" configuration points the light transmitter
and the receiver directly at each other. Although much of the light launched by the
transmitter may never reach the distant receiver assembly, sufficient light is detected to
pass information. Since there is only air between the transmitter and receiver, it is the
most commonly used configuration to transmit information over long distances. Most optical
communications systems rely on this configuration. Remote controllers for televisions,
VCRs, audio systems and computers all rely on this direct light link method, since it
makes the most efficient use of the transmitted light. |
Figure 4a |
| As the light emerges from the end of the
transmitter it immediately begins spreading out. The light forms a cone shaped pattern of
illumination. The spreading out of the light beam means the area being illuminated at the
distant receiver will always exceed the receiver's light collecting area. The light that
does not actually strike the receiver assembly is therefore lost. If you tried to design a
system so all the launched light hit the receiver, you would soon discover that it would
be impossible to maintain proper alignment. Small vibrations, building sway and even air
disturbances could bend the light beam enough to miss the receiver assembly altogether. An
intentional over-illumination scheme works the best, since it allows for some misalignment
without the complete loss of the light signal. When designing a system using an opposed
configuration you can use the range equation discussed in the last section as a way of
predicting how much light will strike the receiver, how much light power needs to be
launched and what kind of divergence angle is needed to establish a communications link
over a specified distance.
TOP
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| Diffuse
Reflective Configuration |
| When you look at the stars at night, car
headlights or at the sun, your eyes collect the light that is coming directly from the
light source. When you look at the moon, a movie screen or when you look at the light
reflected off walls from a table lamp, you don't see the source of the light, but the
light that happens to reflect off the object being illuminated by the source. Unless the
object has a mirrored surface, the light that strikes the object spreads out in all
directions. The light that you see is only a very small portion of the total light that
actually illuminates the object. This "diffuse reflective" configuration, as
shown in Figure 4b is a technique that is very useful in some communications systems. It
is especially good for short distances when multiple reflections allow the light receiver
to be aimed, not at the light source directly, but at objects being illuminated by the
source. Some cordless stereo headsets use such a method to give a person some freedom of
movement as he listens to music. These systems bounce the light off the walls, ceilings
and floors with sufficient power that enough light finds its way to a light detector
attached to the headset, no matter how the headset detector is oriented.
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Figure 4b |
| The amount of light detected by the
receiver is very dependent on the nature of object's surface that reflects the light. As
an example, walls painted with white paint will reflect more light than those painted with
dark paint. Also, rough surfaces will tend to reflect less light than smooth surfaces.
Most surfaces reflect the light in a hemispherical pattern with more light being bounced
straight back toward the light source then off to the sides. When you are trying to
predict the behavior of such reflections it is best to think of the area of illumination
as an independent light source that has a 90-degree half-angle divergence pattern. Then,
if you know the acceptance angle of the light receiver and its collection area, you can
use the range equation to calculate how much of the total light reflected will be
collected by the light receiver. |
| If a single surface reflection is to be
used, it is best to try to illuminate the smallest area possible. This concept can be
illustrated by imagining how your eyes respond better to a brightly lit spot reflected off
a wall than to a broad floodlight. By concentrating most of the light onto a small area
more light will be reflected back to a nearby receiver that is aimed at the illuminated
area. However, when multiple reflections are desired, such as done with the stereo
headsets, a small or large illuminated area will work just about the same. In detecting
light from single reflections you should plan to use a large collection area, with a small
acceptance angle. The receiver would be aimed directly at the illuminated spot. However,
for multiple reflection applications it is best to use a detector with a very wide
acceptance angle. Detectors using large lens collectors will have little effect in
multiple reflection cases, since they would have narrow acceptance angles.
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| As food for thought, it may be possible
to use fluffy white clouds as diffuse reflectors to link two distant light transceivers.
Some preliminary test results indicate that such a scheme may be possible if a
transmitter, using a narrow light beam, launches sufficient light power and an equally
efficient light receiver with a large light collector is used. Such a method may be very
useful in allowing one powerful transmitter to be received by multiple light receivers
that do not have a direct line-of-sight path to the transmitter. The imagined scheme might
resemble the bright search lights often used to attract people to some gala event. Even
the tiny amount of light reflected off dust particles in the air allow you to see the
search light beam moving up toward the clouds many miles away. This concept would be a
great area for an experimenter to try to see if such a system could actually be made to
work. TOP |
| Retro
Reflective Configuration |
| As illustrated in Figure 4c if a special
mirror reflector, called a "corner cube" reflector, is used to bounce light from
a transmitter to a nearby light receiver, the light transmitter and receiver are said to
be linked using a "retro reflective" configuration. |
Figure 4c |
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A corner cube reflector can be made from a specially
ground piece of glass, as shown in figure 4d or from positioning three mirrors at right
angles to each other as shown in figure 4d-3. Some plastic reflectors often used on
bicycles and roadside indicators are actually large arrays of miniature molded corner cube
reflectors (see figure 4d-1). A corner cube has the unique characteristic that will return
much of the light striking the assembly directly backs to the light source in a parallel
path, independent of the position of the emitter. However, because of the parallel path,
the light transmitter and receiver must positioned very close to each other. Some very
accurately made corner cube reflectors send the light back in a path that is so parallel
that the light receiver must actually be placed inside the light transmitter to properly
detect the light being returned. |
Figure 2d |
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Corner cube reflectors have a wide
variety of applications. Several highly accurate corner cube arrays were left on the moon
during some of the Apollo moon missions in the early 1970s. Scientists have been using
powerful lasers and specially modified telescopes to bounce light off of the reflectors.
By measuring the time the light pulses take to make the round trip from the earth, to the
moon and back, the distance can be measured down to inches. Electronic distance
measurement devices (EDMs), used by survey crews, also use corner cubes and "time of
flight" techniques to measure distances accurate to inches. Some systems have
effective ranges of several miles. Remember, light travels about one foot in one
nanosecond, so for a round trip of 10,000 feet would cause a pulse delay of 10,000
nanoseconds or 10 microseconds.
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Some alarm systems also use the
retro-reflective technique. Pulsed light is bounced off a distant plastic reflector and is
collected by a nearby light receiver. Objects moving between the light transmitter and the
reflector break the established light path, setting off the alarm. Some industrial systems
also use the technique to monitor products moving down a production line. |
Figure 2d-1 |
You can increase the
effective corner cube size by placing a fresnel lens in front of the corner cube as shown
in figure 4d-2. Using the technique, you can make a one inch diameter glass corner
cube appear to be several feet in diameter. This technique can dramatically
lower the overall cost |
| When using the retro reflective technique you have
to treat the reflector as a distant light source with its own emitting area and divergence
angle. The amount of light sent back by the reflector will depend on the ratio of the
illuminated area and the reflector's area. A typical plastic reflector has an equivalent
divergence angle of about 0.5 degrees. For long-range applications a large reflector will
be needed. |
Figure 2d-2 |
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Figure 4d-3 shows a large corner cube
reflector you can make yourself. Gluing three glass tile mirrors together makes it. A
sturdy cardboard box will help position the mirrors. One mirror is positioned at the
bottom of the box and the other two converge at the box sides. You would align such an
assembly so the light would enter at a 30-degree angle relative to the bottom. The target
for such an assembly would be the point where the three mirrors converge. I have used such
a simple mirror for some experiments and was able to detect reflections over a distance of
10 miles. Larger mirror assemblies or even multi-reflector arrays are also possible to
increase the effective range. Perhaps you might experiment with your own large reflector
to see if a long range distant measuring systems could be devised. Using two such
reflectors it might be possible to pinpoint your location using triangulation techniques.
TOP |
Figure
4d-3 |
