| Introduction
to Light Emitters |
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| Samples of Emitters |
Unlike the limited number of useable light
detectors, there is a wide variety of light emitters that you can use for optical
through-the-air communications. Your communications system will depend much more on the
type of light source used than on the light detector. You should choose the light source
based on the type of information that needs to be transmitted and the distance you wish
cover to reach the optical receiver. In all cases the light source must be modulated
(usually turned on and off or varied in intensity) to transmit information.
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| The modulation rate will
determine the maximum rate information can be transmitted. You may have to make some
tradeoffs between the modulation rates needed, the distance to be covered and the amount
of money you wish to spend. |
| Many light sources listed below
are useful for low to medium speed modulation rates and can have ranges up to several
miles. A few others are ideal for low speed telemetry transmission that can reach beyond
50 miles. If you need high speed information transmission, there are only a few choices,
and those tend to be expensive. But, as the technology improves the prices should come
down. I have also described some of the latest devices that may become available to the
experimenter in a few years, but only demonstration devices exist today.
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| LIGHT
EMITTING DIODES (LEDS) |
| For most through-the-air
communications applications the infrared light emitting diode (IRLED) is the most common
choice. Although visible light emitting devices do exist, the infrared parts are generally
chosen for their higher efficiency and more favorable wavelength, especially when used
with silicon photo diode light detectors. |
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| GaAlAs
IR LED: GaAlAs (gallium, aluminum arsenic) infrared LEDs are the
most widely used modulated IR light sources. They have moderate electrical to optical
efficiencies, (at low currents 4%), and produce light that matches the common silicon PIN
detector response curve (900nm). Most devices can be pulsed at high current levels, as
long as the average power does not exceed the manufacturer's maximum power dissipation
specification (typically 0.25 watts). Some devices can be pulsed up to 10 amps, if the
duty cycle (ratio of on time to the time between pulses) is less than 0.2% (0.002:1
ratio). Some of the faster devices have response times that allow them to be driven with
current pulses as short as 100 nanoseconds but most devices require at least 900
nanoseconds. At a current level of about 6 amps a quality device can emit about 0.15 watts
of infrared light. However, at higher current levels their efficiency is generally poor,
dropping to less than 0.5% (See Figures 3a, 3b, 3c and 3d.) Many resemble the commonly
used visible LEDs and will typically be packaged in molded plastic assemblies that have
small 3/16" lenses at the end. The position of the actual LED chip within the package
will determine the divergence (spreading out) of the exiting light. The typical T-1 3/4
style device will have a half angle divergence ranging from 15 to 40 degrees. They are low
cost, medium speed (up to 1 million pulses per second) sources, with long operating
lifetimes (typically greater than 100,000 hours). They are a good choice for short and
medium distance control links and general communications applications. When used with a
large lens, a single device can be used for a communications system with a multi-mile
range. Multi-device arrays can also be constructed to transmit information over wider
areas or longer distances. They generally cost between $0.30 to $2.00 each and are
available from many manufacturers. |
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Figure 3a |
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Figure 3b |
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Figure 3c |
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Figure 3d
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| GaAs
IR LED: These devices are the older and less efficient
cousin to the GaAlAs devices. They come in all styles and shapes. The more useful devices
have smaller emitting surfaces than GaAlAs LED's, permitting narrow divergence angles with
small lenses. Also, the small emitting areas make them very useful for fiber optic
applications. Some commercial devices have miniature lenses cemented directly to the
semiconductor chip to produce a small exiting light angle (divergence angle). In
conjunction with a small lens (typically 0.5") such devices can launch light with a
narrow divergence angle (0.5 degrees). The most important feature of the GaAs LED is its
speed. They are generally 10 times faster than GaAlAs LED's but many only produce 1/6 as
much light. They are often picked when medium speed transmission over short distances is
required. Their price is typically a little more than the GaAlAs LED's, even though they
use an older technology. They will cost between $2.00 to $25.00. |
| GaAsP
Visible Red LEDs: Although not as efficient as the infrared
devices some visible red LEDs (Figure 3d-1)are now available, that might find limited use
in some short range through-the-air applications. Some so called "super bright"
LEDs boast high light output. However, even the brightest components will still produce
only 1/3 as much light as a quality infrared part. Also, since their light is a visible
red color, an automatic 2:1 penalty will be paid when the devices are used with a standard
silicon detector that has a weaker response to red light. The visible red LEDs are
generally faster (up to 2 million pulses per second) than IR components and can therefore
be used for medium speed applications. Also, since their light is visible, they are much
easier to align than invisible IR devices, especially when the devices are used with
lenses. TOP |
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Figure 3d-1 |
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| SOLID
STATE SEMICONDUCTOR LASERS |
| GaAs
(Hetrojunction) Laseras: These devices have been around since the 1960s
and can produce very powerful light pulses. Some devices are able to launch light pulses
in excess of 20 watts, which is some 200 times more powerful than a typical GaAlAs LED.
But, these devices can only be driven with duty cycles, less than 0.1% (off time must be
1000 times longer than on time). Also, their maximum pulse width must be kept short
(typically less than 200 nanoseconds) even under low pulse rate applications. However,
despite their limitations these devices can be used in some voice transmitter systems if
some careful circuit designs are used.
As in most semiconductor lasers, the GaAs laser does require a
minimum current level (typically 10 to 20 amps) before it begins emitting useable light.
Such high operating currents demand more complicated drive circuits. Despite a 10:1
sensitivity reduction, caused by the rather narrow emitted pulses (see receiver circuit
discussion), the more powerful light pulses available from GaAs lasers can increase the
useful range of a communications system by a factor of about 3, over a typical transmitter
using a single LED. In addition, since their emitting spot sizes are very small, they can
also be focused into very tight beams using rather small lenses. In addition, since their
spectral widths are very narrow the matching light detector circuit can use an optical
band pass filter to reduce the noise levels associated with ambient light (see receiver
circuit section). For low speed and long distance applications, the GaAs laser should be
considered. However, they do have some disadvantages. They typically cost much more than a
GaAlAs LED (up to $75). They have shorter lifetimes (may only last a few hundred hours)
and are sensitive to temperature. Therefore, they require a carefully designed transmitter
circuit that can switch 20 or more amps at high speeds and can compensate for changes in
operating temperature.
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| GaAlAs
(CW) Lasers: These are the latest in infrared light emitting
semiconductor devices and are rapidly maturing. The first wide spread application for
these devices was in audio compact disk players and CD-ROM computer disk drives. They are
also being used in some computer laser printers, bar code readers and FAX machines. They
have very small emitting areas, can produce peak power levels in excess of 0.2 watts and
have narrow spectral bandwidths (see Figure 3e.) The most important improvement over other
light sources is that they can be modulated at frequencies measured in gigahertz. However,
as in any new technology they are still rather expensive. Low power units that emit less
than 0.01 watts of 880nm infrared light, sell for about $20.00. Some of the more powerful
devices can cost as much as $20,000 each. Although the use of a laser in a communications
system might give a project a high tech sound, a much cheaper IR LED will almost always
out-perform a low power laser (typical LED will be able to emit 10 times more light at
1/10 the cost) in low to medium speed applications. But, when very high-speed modulation
rates (up to 1 billion pulses per second) are needed, these devices would be a good
choice. Although expensive now, these devices should come down in price over the next few
years. They will also most likely be available at higher power levels too. But, until
then, their advantages do not justify their expense and the more useful high power units
are beyond the reach of practical experimental designs. I suggest using these
devices only when necessary. |
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Figure 3e
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| Surface
Emitting Lasers (VCSEL): These devices are just now beginning to
appear in some catalogs. Many companies have been experimenting with these latest
semiconductor devices since about 1988. Their small size and high efficiency make
them very suitable for some applications. They are mostly used in optical fiber
communications. Instead of being grown as single chip emitters, these devices are
fabricated into large arrays of very small individual laser sources sharing a common
substrate. Since the individual laser diode emitters can be as small as one micron
(1/10,000cm) as many as 100 million separate devices could be placed into a 1cm X 1cm
area. The output efficiency (electrical power to
light power) has been reported to be about 40%, with each tiny device emitting about 0.003
watts. Although each device may emit only a small amount of light, when used as an array,
100 million such devices could launch some 100,000 watts of IR light from about 200,000
watts of electricity. Of course, cooling such a powerful array would be a real
challenge, if not impossible. But, perhaps smaller arrays could be placed into common
semiconductor packages for easy mounting and cooling. Maybe a 0.1-watt device would
be placed into inexpensive LED style packages. Other devices may be mounted in better heat
conducting metal packages to allow perhaps 100 watts of light to be emitted. Since their
maximum modulation rates have been measured in the multi-billion pulses per second rate,
surface-emitting lasers would be ideal for many future through-the-air communications
applications. They would especially be useful in broadcasting optical information over a
citywide area, where very powerful high-speed light sources are needed. A 10,000-watt
source, emitting light in a specially shaped 360-degree pattern, might be able to transmit
information over an area covering some 500 square miles. Such a broadcasting system might
be used to transmit library type information from large centralized databases.
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| EXTERNALLY
EXCITED SOLID STATE LASERS |
| Some of the very first lasers
made were the Ruby and YAG lasers. Most of these lasers are excited externally using large
xenon flash tubes that are positioned around the central glass laser rod. A small portion
of the light from the xenon flash excites the specially positioned rod material, forming
short coherent light pulses. Although these lasers are capable of emitting very power
light pulses, with very narrow divergence angles, they are generally much too expensive
and too complicated for the average experimenter. They would therefore find very limited
use in earth-bound optical communications. However, some scientists believe that the
extremely powerful light pulses that these devices are capable of producing, might be
useful in transmitting information into very deep space. Since some pulsed lasers have
been reported to launch light pulses approaching one terawatt (1000 billion watts), low
speed communications might be possible to a range of several light years (one light year =
6 trillion miles). Such a feat would be very difficult to accomplish with microwave
techniques. |
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| GAS
LASERS |
| Helium-neon, carbon dioxide and
argon are the more common types of gas lasers. The light emitted from a gas arc, inside a
glass tube, is bounced back and forth through the excited gas using specially fabricated
mirrors. A portion of the light is allowed to escape through one of the mirrors and
emerges as very monochromatic (one wavelength) and highly coherent (same phase) light.
Such lasers have narrow divergence angles (typically less than 0.1 degrees) but have very
low conversion efficiencies (much less than 0.1%). They are also expensive and bulky that
makes them impractical for most optical communications applications. Some published
designs that did provide experimental optical communications using helium-neon lasers were
designed to transmit voice audio information over a range of only a few miles. The
modulation technique was to vary the gas arc current that then produced a light intensity
modulation. However, the extra cost and relative low power that resulted usually did not
warrant the trouble. A properly designed system using a single LED will usually out
perform any short-range helium-neon laser communications system at a fraction of the cost.
Although too expensive for the experimenter, some
gas lasers have been used by the military for many years. In particular, carbon dioxide
lasers, that emit long infrared wavelengths (10,000 nanometers), have been used in some
military targeting systems. The long infrared wavelength can penetrate smoke and fog
better than visible or near IR lasers. Also, the Navy has been experimenting with some
blue-green laser light to attempt to provide communications to submarines deep under
water. But, overall gas lasers fall short of the ideal for practical through-the-air
communications.
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| FLUORESCENT
LIGHT SOURCES |
| Fluorescent
Lamps: Fluorescent lamps work on the principle of
"fluorescence" and because of their low cost have many through-the-air
applications. An electrical current passed through a mercury vapor inside a glass tube
causes the gas discharge to emit ultraviolet "UV" light. The UV light causes a
mixture of phosphors, painted on the inside wall of the tube, to glow at a number of
visible light wavelengths (see Figure 3f.) The electrical to optical conversion efficiency
of these light sources is fairly good, with about 3 watts of electricity required to
produce about 1 watt of light. A cathode electrode at each end of the lamp that is heated
by the discharge current, aids in maintaining the discharge efficiency, by providing rich
electron sources. By turning on and off the electrical discharge current, the light being
emitted by the phosphor, can be modulated. Also, by driving the tubes with higher than
normal currents and at low duty cycles, a fluorescent lamp can be forced to produce
powerful light pulses. However, like the pulse techniques used with LEDs, the fluorescent
lamp pulsing techniques must use short pulse widths to avoid destruction of the lamp.
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Figure 3f
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| To modulate a fluorescent lamp
to transmit useful information, the negative resistance characteristic of the mercury
vapor discharge within the lamp must be dealt with. This requires the drive circuit to
limit the current through the tube. The two heated cathode electrodes of most lamps also
require the use of alternating polarity current pulses to avoid premature tube darkening.
The typical household fluorescent lighting uses an inductive ballast method to limit the
lamp current. Although such a method is efficient, the inductive current limiting scheme
slows the rise and fall times of the discharge current through the tube and thus produces
longer then desired light pulses. To achieve a short light pulse emission, a resistive
current limiting scheme seems to work better. In addition, there seems to be a
relationship between tube length and the maximum modulation rate. Long tubes do not
respond as fast as shorter tubes. As an example, a typical 48" 40 watt lamp can be
modulated up to about 10,000 pulses per second, but some miniature 2" tubes can be
driven up to 200,000 pulses per second. The main factor that ultimately limits the
modulation speed is the response time of the phosphor used inside the lamp. Most visible
phosphors will not allow pulsing much faster than about 500,000 pulses per second. The
visible light emitted by the typical "cool white" lamp is also not ideal when
used with a silicon photo diode. However, some special infrared light emitting phosphors
could be used to increase the relative power output from a fluorescent lamp, which may
also produce faster response times. (see Figure 3g.) |
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Figure 3g
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If a conventional "cool white" lamp is used, a 2:1 power
penalty will be paid due to the broad spectrum of visible light being emitted (see Figure
3f.) This results since the visible light does not appear as bright to a silicon light
detector as IR light (see section on light detectors). Also, light detectors with built-in
visible filters should not be used, since they would not be sensitive to the large amount
of visible light emitted by the lamps. Although the average fluorescent lamp is not an
ideal light source, the relative low cost and the large emitting surface area make it
ideal for communications applications requiring light to be broadcasted over a wide area.
Experiments indicate that about 20 watts of light can be launched from some small 9-watt
lamps at voice frequency pulse rates (10,000/sec). Such power levels would require about
100 IR LEDs to duplicate. But, the large surface emitting areas of fluorescent lamps makes
them impractical for long-range applications, since the light could not be easily
collected and directed into a tight beam. (For additional information see section on
fluorescent lamp transmitter/receiver circuits.)
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| Cathode
Ray Tubes (CRT): CRTs work somewhat like fluorescent lamps, since
they too use fluorescence emission techniques. Electrons, emitted from a heated cathode
end of the cathode ray vacuum tube, are accelerated toward the anode end by the force of a
high voltage applied between the cathode and anode electrodes. Before hitting the anode
screen, the electrons are forced to pass through a phosphor painted onto the inside of the
screen. In response to the high-speed electrons, the phosphor emits light at various
wavelengths. A voltage applied to a special metal grid near the tube's cathode end is used
to modulate the electron beam and can thus produce a modulation in the emitted light. This
principle is used in most computer and TV screens. Since the electron beam can be
modulated at very high rates, the light source modulation rate is limited only by the
response time of the phosphor used. Depending on the type of phosphor, the electrical to
optical efficiency can be as high as 10%. Some specially made cathode ray tubes produce
powerful broad (unfocused) electron beams that illuminate the entire front screen of the
CRT instead of a small dot. Such tubes can yield powerful light sources, with large flat
emitting areas. A variation on the usual television type CRT design positions a curved
phosphor screen at the back of the vacuum tube and places the cathode electrode at the
front or side of a clear glass screen (some portable Sony TVs use such CRTs). This
technique increases the overall efficiency, since it allows the light from the phosphor to
exit from the same side as the electron source. With the aid of external cooling, such
techniques could create very powerful light sources that might be able to launch tens of
thousands of watts of light, pulsed at rates exceeding tens of millions of light pulses
per second. Although the typical experimenter may not be interested in such light power
levels it does raise some interesting possibilities for use in city wide optical
communications. |
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| GAS
DISCHARGE SOURCES |
| Xenon
Gas Discharge Tubes |
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| Samples of Xenon Lamps |
The most common form of this class of light source
is the electronic camera flash. These devices are some of the most intense light sources
available to the experimenter and have many interesting applications. The discharge lamps
are typically made from a glass tube with a metal electrode installed at each end. They
are filled with xenon gas at about one atmosphere of pressure (14psi). The gas inside the
tube can be made to glow with very high intensity when an electrical current is passed
through it. |
| As illustrated in Figure 3h, the
xenon arc emits light over a broad spectrum with some large peaks in the near infrared
range. The electrical to optical conversion is fairly good. A typical camera flash can
produce about 2,000 watts of light from about 10,000 watts of electrical power (20%
efficiency). Some specially made discharge tubes can generate flashes that exceed one
million watts of light power. As in fluorescent lamps, the minimum flash duration is
somewhat dependent on the length of the discharge tube. A typical camera flash tube has an
electrode gap of about 15mm (0.6") and will usually produce a flash, which lasts
about one millisecond. The energy used to produce the short flash comes from discharging a
special capacitor, charged to several hundred volts. By decreasing the size of the
capacitor (say to 6 microfarads) and increasing the voltage (say to 300 volts) the camera
flash tube can be made to produce flashes as short as 20 microseconds. Shorter discharge
flashes are only possible by using specially made discharge tubes with very narrow
electrode gaps (0.5mm). These narrow gap lamps can produce flashes as short as one half
microsecond. However, the physics of the xenon gas arc prevents flashes much shorter.
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Figure 3h
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| Flash rates up to 10,000 per
second are possible with the short gap lamps, but the typical camera flash tube can't be
pulsed much faster than about 100 flashes per second. Since some special high speed lamps
can dissipate up to 75 watts of average power, it is possible to design an optical voice
information transmitter which could launch as much as 1000 watts of light with a narrow
divergence. Such a transmitter would certainly have some long-range possibilities.
However, most xenon discharge lamps are more useful for low speed and long-range
applications, requiring very powerful light pulses. Many years ago, I constructed a
demonstration telemetry system that launched very powerful light pulses at a low data rate
that had a useable range of 50 miles. (See discussion on long-range telemetry transmitters
using xenon flash sources.)
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| Nitrogen
Gas (air) Sparks: For very powerful and very short light pulse
applications, a simple electrical spark in air can be used. Some simple systems use two
closely spaced (0.5mm) electrodes (usually made of tungsten) in open air. With sufficient
voltage, the air between the electrodes can be made to ionize briefly, forming a small
spark. Some gas barbecue grill igniters that use piezoelectric crystals to produce the
needed high voltage, can be modified to produce useful sparks for some experiments.
Commercially made nitrogen spark sources claim to generate light flashes that pack about
100,000 watts of light power into short 5 nanosecond pulses.
The nitrogen (air) arc emits a broad spectrum of light with large
peaks in the visible blue and invisible ultraviolet (see Figure 3i.) Such a spectrum is
not ideal when used with silicon detectors. But the small emission areas of the sparks
allow simple lenses or mirrors to be used to form very tight divergence angles. But, the
air ionization (sparking) can be become very unstable at high pulse rates, without using
specially made discharge tubes and drive circuits. Therefore, the sparks are best used for
powerful, very short pulse applications that demand only low pulse rates. Optical radar,
electronic distance measurements, air turbulence monitors and wind shear analysis are some
possible uses for such a light source. You shouldn't be fooled by the seemingly dim
appearance of these light emitters. To our human eyes the tiny flashes may not seem very
bright, but to a fast detector they can be very powerful. However, to take advantage of
these unique pulses, a fast light detector and an equally fast amplifier must be used.
Since few experiments have been conducted with these unique light sources, it is a great
area for the experimenter to see what can be done. |
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Figure 3i
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| Other
Gas Discharge Sources: Glass discharge tubes filled with
Cesium, Krypton or Rubidium will all produce lots of infrared light. Krypton behaves much
like Xenon and has a very similar emission output. Cesium and Rubidium are both
semi-liquids at room temperatures and can be operated under high or low pressures in a
discharge lamp. Such lamps might be constructed in a similar manner to the more common
yellow color sodium vapor street lamp. Cesium, in particular, appears to be a good
candidate for some experimentation in developing some powerful light sources with high
peak power outputs. Since kilowatt size sodium vapor street lamps are being manufactured,
perhaps similar lamps using cesium could be made. Such lamps might be able to produce
multi-kilowatts of modulated infrared light using pulse methods.
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| EXTERNAL
LIGHT MODULATORS |
| Ferroelectric light valves,
modulated mirror arrays, piezoelectric shutters, Kerr cells, Pockels cells, Bragg cells
and liquid crystals are all light modulators. They can be used to intensity modulate light
being emitted by an external source as it passes through them or reflects off them. The
light can originate from incandescent lamps, CW xenon gas arc lamps, light from a gas
laser or even focused sunlight. Although usually very expensive, some of the devices can
be used to produce powerful modulated light signals at high pulse rates.
Liquid crystal modulators are
perhaps the slowest of the group. Most can't be driven much faster than about 100 flashes
per second. Ferroelectric light valves and piezoelectric shutters are a little faster and
can be pushed to perhaps 10,000 flashes per second. Kerr cells, Bragg cells and Pockel
cells, on the other hand, are known to be very fast. However, they work best when used
with laser light at a specific wavelength and at narrow angles. Some of these devices can
modulate the light from a laser at rates beyond 100 million pulses per second. But, most
of these devices are very expensive, are complicated and are therefore impractical for the
average experimenter.
A new device developed by Texas Instruments (Figure 3j)
has some interesting possibilities. The technology was originally developed for flat panel
computer and TV displays, but the techniques might be useful for optical communications.
TI's process fabricates a large array of very small mirrors that can be moved using a
voltage difference between the mirror and an area behind the mirror. Like tiny fans, each
mirror would wave back and forth in response to the drive voltage. Because the mirrors are
very small, the modulation rates might be pushed to perhaps 100,000 activations per
second. If the mirrors were used to reflect light from an intense light emitter, a nice
source of modulated light could be produced.
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Texas Instruments
Micromirror Array |
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