LIGHT SYSTEMS CONFIGURATIONS:Opposed Configuration, Diffuse Reflective Configuration

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Chapter Four

LIGHT SYSTEM CONFIGURATIONS

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.

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

Figure 4a

use of the transmitted light.

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.

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

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.

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.

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

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

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.

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. 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

Figure 4c

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.

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

Figure 4d

nanoseconds or 10 microseconds.

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

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.

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.

Figure 4d-1

SMALL

When using the retro reflective technique you have

CORNER

CUBE

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 4d-3 shows a large corner cube reflector

LARGE FRESNEL LENS

you can make yourself. Gluing three glass tile

Figure 4d-2

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

Figure 4d-3

using triangulation techniques.

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

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