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1. INTRODUCTION
A rectifying semiconductor device, which converts electrical energy into
electromagnetic radiation. The wavelength of the emitted radiation ranges fromthe near-ultraviolet to the near-infrared, that is, from about 400 to over 1500
nanometers.
Most commercial light-emitting diodes (LEDs), both visible and infrared,
are fabricated from III–V semiconductors. These compounds contain elements
such as gallium, indium, and aluminum from column III (or group 13) of the
periodic table, as well as arsenic, phosphorus, and nitrogen from column V (or
group 15) of the periodic table. There are also LED products made of II–VI (or
group 12–16) semiconductors, for example ZnSe and related compounds. Taken
together, these semiconductors possess the proper band-gap energies to
produce radiation at all wavelengths of interest. Most of these compounds have
direct band gaps and, as a consequence, are efficient in the conversion of
electrical energy into radiation. With the addition of appropriate chemical
impurities, called dopants, both III–V and II–VI compounds can be made p- or n-
type, for the purpose of forming pn junctions. All modern-day LEDs contain pn
junctions. Most of them also have heterostructures, in which the pn junctions are
surrounded by semiconductor materials with larger band-gap energies. See also
Acceptor atom; Donor atom; Electroluminescence; Electron-hole recombination;
Junction diode; Junction transistor ; Laser ; Semiconductor ; Semiconductor diode.
Conventional low-power, visible LEDs are used as solid-state indicator lights in
instrument panels, telephone dials, cameras, appliances, dashboards, and
computer terminals, and as light sources for numeric and alphanumeric displays.
Modern high-brightness, visible LED lamps are used in outdoor applications such
as traffic signals, changeable message signs, large-area video displays, and
automotive exterior lighting. General-purpose white lighting and multielement
array printers are applications in which high-power visible LEDs may soon
displace present-day technology.
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1.1 Definition of: LED
(Light Emitting Diode) A display and lighting technology used in almost
every electrical and electronic product on the market, from a tiny on/off light to
digital readouts, flashlights, traffic lights and perimeter lighting. LEDs are also
used as the light source in multimode fibers, optical mice and laser-class printers.
1.2 LEDs Vs. LCDs
In the early 1970s, red LEDs were used in the first digital watches, but
were superseded by lower-power LCDs within a few years. LEDs still use more
power than LCDs, but less power than incandescent bulbs. They also last for
decades and are virtually indestructible.
LEDs and LCDs coexist on countless devices where the LEDs provide the
status lights, and the LCDs display data. In addition, white LEDs can provide the
backlight for LCD screens. See LCD.
1.3 Several Colors
LEDs are semiconductor diodes that typically emit a single wavelength of
light when charged with electricity. Originally red, today, several colors can be
generated based on the material used for the tips of the probes. Aluminum
indium gallium phosphide (AlInGaP) is used for red and yellow. Indium gallium
nitride (InGaN) is used for green and blue, and with the addition of phosphor, for
white light as well. See OLED, IRED, LED printer , fiber optics glossary and Nixie
tube.
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1.4 An LED Unit
The LED is the semiconductor die itself, which sits in a reflective cup that
acts as a heat sink and reflector. When voltage is applied to the LED, electrons
and holes in the two semiconductor layers are attracted to each other at the
junction. When they combine, they create photons.
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2. LED PROPERTIES
LEDs often have or can be used with small, inexpensive lenses and diffusers,
helping to achieve high light densities and very good lighting control andhomogeneity.
LEDs can be easily strobed (in the microsecond range and below) and
synchronized; their power also has reached high enough levels that
sufficiently high intensity can be obtained, allowing well lit images even with
very short light pulses: this is often used in order to obtain crisp and sharp
“still” images of fast moving parts.
LEDs come in several different colors and wave lengths, easily allowing to
use to best color for each application, where different color may provide better
visibility of features of interest. Having a precisely known spectrum allows
tightly matched filters to be used to separate informative bandwidth or to
reduce disturbing effect of ambient light.
LEDs usually operate at comparatively low workability temperature,
simplifying heat management and dissipation, therefore allowing plastic
lenses, filters and diffusers to be used. Waterproof units can also easily be
designed, allowing for use in harsh or wet environments (food, beverage, oil
industries).
LEDs source can be shaped in several main configuration (spot lights for
reflective illumination; ring lights for coaxial illumination; backlights for contour
illumination; linear assemblies; flat, large format panels; dome source for
diffused, omnidirectional illumination).
Very compact designs are possible, allowing for small LED illuminators to be
integrated within smart cameras and vision sensors.
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3. HISTORY
The first known report of a light-emitting solid-state diode was made in
1907 by the British experimenter H. J. Round. However, no practical use wasmade of the discovery for several decades. Independently, Oleg Vladimirovich
Losev published "Luminous carborundum [silicon carbide] detector and detection
with crystals" in the Russian journal Telegrafiya i Telefoniya bez Provodov
Losev's work languished for decades.
The first practical LED was invented by Nick Holonyak, Jr., in 1962 while
he was at General Electric Company. The first LEDs became commercially
available in late 1960s, and were red. They were commonly used as
replacements for incandescent indicators, and in seven-segment displays, first in
expensive equipment such as laboratory and electronics test equipment, then
later in such appliances as TVs, radios, telephones, calculators, and even
watches. These red LEDs were bright enough only for use as indicators, as the
light output was not enough to illuminate an area. Later, other colors became
widely available and also appeared in appliances and equipment. As the LED
materials technology became more advanced, the light output was increased,
and LEDs became bright enough to be used for illumination.
Most LEDs were made in the very common 5 mm T1-3/4 and 3 mm T1
packages, but with higher power, it has become increasingly necessary to get rid
of the heat, so the packages have become more complex and adapted for heat
dissipation. Packages for state-of-the-art high power LEDs bear little
resemblance to early LEDs (see, for example, Philips Lumileds).
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4. WORKING PRINCIPLE
Light-emitting diode
.
Blue, green, and red LEDs; these can be combined to produce any color,
including white. Infrared and ultraviolet (UVA) LEDs are also available.
A light-emitting diode, usually called an LED is a semiconductor diode
that emits incoherent narrow-spectrum light when electrically biased in the
forward direction of the p-n junction, as in the common LED circuit. This effect isa form of electroluminescence.
A LED is usually a small area light source, often with extra optics added to
the chip that shapes its radiation pattern. LEDs are often used as small indicator
lights on electronic devices and increasingly in higher power applications such as
flashlights and area lighting. The color of the emitted light depends on the
composition and condition of the semiconducting material used, and can be
infrared, visible, or ultraviolet. LEDs can also be used as a regular household
light source. Besides lighting, interesting applications include sterilization of water
and disinfection of devices.
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4.1 Multi-touch sensing
Since LEDs share some basic physical properties with photodiodes, which
also use p-n junctions with band gap energies in the visible light wavelengths,
they can also be used for photo detection. These properties have been known for
some time, but more recently so-called bidirectional LED matrices have been
proposed as a method of touch-sensing. In 2003, Dietz, Yerazunis, and Leigh
published a paper describing the use of LEDs as cheap sensor devices.
In this usage, various LEDs in the matrix are quickly switched on and off.
LEDs that are on shine light onto a user's fingers or a stylus. LEDs that are off
function as photodiodes to detect reflected light from the fingers or stylus. The
voltage thus induced in the reverse-biased LEDs can then be read by a
microprocessor, which interprets the voltage peaks and then also uses them
elsewhere.
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5. WIRELESS LED TECHNOLOGY
Physical function
Like a normal diode, the LED consists of a chip of semiconducting
material impregnated, or doped , with impurities to create a p-n junction. As in
other diodes, current flows easily from the p-side, or anode, to the n-side, or
cathode, but not in the reverse direction. Charge-carriers—electrons and holes—
flow into the junction from electrodes with different voltages. When an electron
meets a hole, it falls into a lower energy level, and releases energy in the form of
a photon.
The wavelength of the light emitted, and therefore its color, depends on
the band gap energy of the materials forming the p-n junction. In silicon or
germanium diodes, the electrons and holes recombine by a non-radiative
transition which produces no optical emission, because these are indirect band
gap materials. The materials used for the LED have a direct band gap with
energies corresponding to near-infrared, visible or near-ultraviolet light.
LED development began with infrared and red devices made with gallium
arsenide. Advances in materials science have made possible the production of
devices with ever-shorter wavelengths, producing light in a variety of colors.
LEDs are usually built on an n-type substrate, with an electrode attached to the
p-type layer deposited on its surface. P-type substrates, while less common,
occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire
substrate. Substrates that are transparent to the emitted wavelength, and backed
by a reflective layer, increase the LED efficiency. The refractive index of the
package material should match the index of the semiconductor, otherwise the
produced light gets partially reflected back into the semiconductor, where it may
be absorbed and turned into additional heat, thus lowering the efficiency. This
type of reflection also occurs at the surface of the package if the LED is coupled
to a medium with a different refractive index such as a glass fiber or air. The
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refractive index of most LED semiconductors is quite high, so in almost all cases
the LED is coupled into a much lower-index medium. The large index difference
makes the reflection quite substantial (per the Fresnel coefficients), and this is
usually one of the dominant causes of LED inefficiency. Often more than half of
the emitted light is reflected back at the LED-package and package-air
interfaces. The reflection is most commonly reduced by using a dome-shaped
(half-sphere) package with the diode in the center so that the outgoing light rays
strike the surface perpendicularly, at which angle the reflection is minimized. An
anti-reflection coating may be added as well. The package may be cheap plastic,
which may be colored, but this is only for cosmetic reasons or to improve the
contrast ratio; the color of the packaging does not substantially affect the color of
the light emitted. Other strategies for reducing the impact of the interface
reflections include designing the LED to reabsorb and reemit the reflected light
(called photon recycling ) and manipulating the microscopic structure of the
surface to reduce the reflectance, either by introducing random roughness or by
creating programmed moth eye surface patterns.
Conventional LEDs are made from a variety of inorganic semiconductor
materials, producing the following colors:• Aluminium gallium arsenide (AlGaAs) — red and infrared
• Aluminium gallium phosphide (AlGaP) — green
• Aluminium gallium indium phosphide (AlGaInP) — high-brightness
orange-red, orange, yellow, and green
• Gallium arsenide phosphide (GaAsP) — red, orange-red, orange, and
yellow
• Gallium phosphide (GaP) — red, yellow and green
• Gallium nitride (GaN) — green, pure green (or emerald green), and blue
also white (if it has an AlGaN Quantum Barrier)
• Indium gallium nitride (InGaN) — 450 nm - 470 nm — near ultraviolet,
bluish-green and blue
• Silicon carbide (SiC) as substrate — blue
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• Silicon (Si) as substrate — blue (under development)
• Sapphire (Al2O3) as substrate — blue
• Zinc selenide (ZnSe) — blue
• Diamond (C) — ultraviolet
• Aluminium nitride (AlN), aluminium gallium nitride (AlGaN), aluminium
gallium indium nitride (AlGaInN) — near to far ultraviolet (down to 210 nm)
With this wide variety of colors, arrays of multicolor LEDs can be designed
to produce unconventional color patterns.
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6. TYPES OF LEDS
There are 3 main types of LEDs:
miniature LED,
alphanumeric LEDs
lighting LEDs.
6.1 Miniature LEDs
These are mostly single die LEDs used as indicators, and come in various
size packages:
• surface mount
• 2 mm
• 3 mm (T1)
• 5 mm (T1-3/4)
• Other sizes are also available, but less common.
Common package shapes:
• Round, dome top
• Round, flat top
• Rectangular, flat top (often seen in LED bargraph displays)
• Triangular or square, flat top
The encapsulation may also be clear or semi opaque to improve contrast
and viewing angle.
There are 3 main categories of miniature single die LEDs:• Low current - typically rated for 2 mA at around 2 V (approximately 4 mW
consumption).
• Standard - 20 mA LEDs at around 2 V (approximately 40 mW) for red,
orange, yellow & green, and 20 mA at 4-5 V (approximately 0.1 W) for
blue, violet and white.
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• Ultra high output - 20 mA at approximately 2 V or 4-5 V, designed for
viewing in direct sunlight.
6.1.1 Multicolor LEDs
Bicolor LEDs contain 2 dice of different colors connected back to back,
and can produce any of 3 colors. Current flow in one direction produces one
color, current in the other direction produces the other color, and bidirectional
current produces both colors mixed together.
Tricolor LEDs contain 2 dice of different colors with a 3 wire connection,
available in common anode or common cathode configurations. The most
common form of both the bicolor and tricolor LEDs is red/green, producing
orange when both colors are powered.
RGB LEDs contain red, green and blue emitters, generally using a 4 wire
connection with one common (anode or cathode).
6.1.2 5 and 12 volt LEDs
These are miniature LEDs incorporating a series resistor , and may be
connected directly to 5 volt or 12 volt.
6.1.3 Flashing LEDs
These miniature LEDs flash when connected to 5 V or 12 V. Used as
attention seeking indicators where it is desired to avoid the complexity of external
electronics.
6.2 Alphanumeric LEDs
LED displays are available in 7 segment and starburst format. 7 segment
displays handle all numbers and a limited set of letters. Starburst displays can
display all letters.
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7 segment LED displays were in widespread use in the 1970s and 1980s,
but increasing use of LCD displays, with their lower power consumption and
greater display flexibility, has reduced the popularity of numeric and
alphanumeric LED displays.
6.3 Lighting LEDs
LED lamps (also called LED bars or Illuminators) are usually clusters of
LEDs in a suitable housing. They come in different shapes, among them the light
bulb shape with a large E27 Edison screw and MR16 shape with a bi-pin base.
Other models might have a small Edison E14 fitting, GU5.3 (Bipin cap) or GU10
(bayonet socket). This includes low voltage (typically 12 V halogen-like) varieties
and replacements for regular AC mains (120-240 V AC) lighting. Currently the
latter are less widely available but this is changing rapidly.
Seoul Semiconductor Co., Ltd produces LEDs that can run directly from
mains power without the need for a DC converter. For each half cycle part
of the LED diode emits light and part is dark, and this is reversed during
the next half cycle. Current efficiency is 80 lumens per watt.
Ultraviolet and blue LEDs
6.3.1 Ultraviolet GaN LEDs.
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Blue LEDs are based on the wide band gap semiconductors GaN (gallium
nitride) and InGaN (indium gallium nitride). They can be added to existing red
and green LEDs to produce the impression of white light, though white LEDs
today rarely use this principle.
The first blue LEDs were made in 1971 by Jacques Pankove (inventor of
the gallium nitride LED) at RCA Laboratories. However, these devices were too
feeble to be of much practical use. In the late 1980s, key breakthroughs in GaN
epitaxial growth and p-type doping by Akasaki and Amano (Nagoya, Japan) [16]
ushered in the modern era of GaN-based optoelectronic devices. Building upon
this foundation, in 1993 high brightness blue LEDs were demonstrated through
the work of Shuji Nakamura at Nichia Corporation.
Wavelengths down to 210 nm were obtained in laboratories using
aluminium nitride.
While not actually LEDs as such, ordinary NPN bipolar transistor will emit
violet light if its emitter-base junction is subjected to non-destructive reverse
breakdown. This is easy to demonstrate by filing the top off a metal-can transistor (BC107, 2N2222 or similar) and biasing it well above emitter-base breakdown (≥
20 V) via a current limiting resistor.
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7. USES OF WIRELESS LEDS
Considerations in use
Close-up of a typical LED in its case, showing the internal structure.
Unlike incandescent light bulbs, which light up regardless of the electrical
polarity, LEDs will only light with correct electrical polarity. When the voltage
across the p-n junction is in the correct direction, a significant current flows and
the device is said to be forward-biased . If the voltage is of the wrong polarity, the
device is said to be reverse biased , very little current flows, and no light is
emitted. Some LEDs can be operated on an alternating current voltage, but they
will only light with positive voltage, causing the LED to turn on and off at the
frequency of the AC supply.
While the only definitive way to determine the polarity of the LED is to
examine its datasheet, these methods are usually reliable:
sign: + -terminal: anode (A) cathode (K)
leads: long Short
exterior: round Flat
interior: small Large
wiring: red Black
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Less reliable methods of determining polarity are:
sign: + -
marking: none stripe
pin: 1 2
PCB: round square
While it is not an officially reliable method, it is almost universally true that
the cup that holds the LED die corresponds to the cathode. It is strongly
recommended to apply a safe voltage and observe the illumination as a test
regardless of what method is used to determine the polarity.
Most LEDs have low reverse breakdown voltage ratings, so they will also
be damaged by an applied reverse voltage of more than a few volts. Since some
manufacturers don't follow the indicator standards above, if possible the data
sheet should be consulted before hooking up the LED, or the LED may be tested
in series with a resistor on a sufficiently low voltage supply to avoid the reverse
breakdown. If it is desired to drive the LED directly from an AC supply of more
than the reverse breakdown voltage then it may be protected by placing a diode
(or another LED) in inverse parallel.
LEDs can be purchased with built in series resistors. These can save PCB
space and are especially useful when building prototypes or populating a PCB in
a way other than its designers intended. However the resistor value is set at the
time of manufacture, removing one of the key methods of setting the LED's
intensity. To increase efficiency (or to allow intensity control without the
complexity of a DAC), the power may be applied periodically or intermittently; so
long as the flicker rate is greater than the human flicker fusion threshold, the LED
will appear to be continuously lit.
Multiple LEDs can be connected in series with a single current limiting
resistor provided the source voltage is greater than the sum of the individual LED
threshold voltages. Parallel operation is also possible but can be more
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problematic. Parallel LEDs must have closely matched forward voltages (Vf) in
order to have equal branch currents and, therefore, equal light output. Variations
in the manufacturing process can make it difficult to obtain satisfactory operation
when connecting some types of LEDs in parallel.
Bicolor LED units contain two diodes, one in each direction (that is, two
diodes in inverse parallel ) and each a different color (typically red and green),
allowing two-color operation or a range of apparent colors to be created by
altering the percentage of time the voltage is in each polarity. Other LED units
contain two or more diodes (of different colors) arranged in either a common
anode or common cathode configuration. These can be driven to different colors
without reversing the polarity, however, more than two electrodes (leads) are
required.
LEDs are usually constantly illuminated when a current passes through
them, but flashing LEDs are also available. Flashing LEDs resemble standard
LEDs but they contain an integrated multivibrator circuit inside which causes the
LED to flash with a typical period of one second. This type of LED comes most
commonly as red, yellow, or green. Most flashing LEDs emit light of a singlewavelength, but multicolored flashing LEDs are available too.
Generally, for newer common standard LEDs in 3 mm or 5 mm packages,
the following forward DC potential differences are typically measured. The
forward potential difference depending on the LED's chemistry, temperature, and
on the current (values here are for approx. 20 milliamperes, a commonly found
maximum value).
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Color Potential Difference (Vf)
Infrared 1.6 V
Red 1.8 V to 2.1 V
Orange 2.2 V
Yellow 2.4 V
Green 2.6 V
Blue 3.0 V to 3.5 V
White 3.0 V to 3.5 V
Ultraviolet 3.5 V
Many LEDs are rated at 5 V maximum reverse voltage.
LEDs also behave as photocells, and will generate a current depending on
the ambient light. They are not efficient as photocells, and will only produce a few
microamps, but will put out a surprising voltage level, as much as 2 or 3 volts.
This is enough to operate an amplifier or CMOS logic gate. This effect can be
used to make an inexpensive light sensor, for example to decide when to turn on
the LED illuminator.
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8. ADVANTAGES & DISADVANTAGES OF USING
WIRELESS LEDS
ADVANTAGES
LED schematic symbol
LEDs produce more light per watt than incandescent bulbs; this is useful in
battery powered or energy-saving devices.
LEDs can emit light of an intended color without the use of color filters that
traditional lighting methods require. This is more efficient and can lower
initial costs.
The solid package of the LED can be designed to focus its light.Incandescent and fluorescent sources often require an external reflector to
collect light and direct it in a usable manner.
When used in applications where dimming is required, LEDs do not
change their color tint as the current passing through them is lowered,
unlike incandescent lamps, which turn yellow.
LEDs are ideal for use in applications that are subject to frequent on-off
cycling, unlike fluorescent lamps that burn out more quickly when cycled
frequently, or HID lamps that require a long time before restarting.
LEDs, being solid state components, are difficult to damage with external
shock. Fluorescent and incandescent bulbs are easily broken if dropped
on the ground.
LEDs can have a relatively long useful life. One report estimates 35,000 to
50,000 hours of useful life, though time to complete failure may be longer.
Fluorescent tubes typically are rated at about 30,000 hours, and
incandescent light bulbs at 1,000–2,000 hours LEDs mostly fail by
dimming over time, rather than the abrupt burn-out of incandescent bulbs.
LEDs light up very quickly. A typical red indicator LED will achieve full
brightness in microseconds; Philips Lumileds technical datasheet DS23
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for the Luxeon Star states "less than 100ns." LEDs used in
communications devices can have even faster response times.
LEDs can be very small and are easily populated onto printed circuit
boards.
LEDs do not contain mercury, unlike compact fluorescent lamps.
LEDs are produced in an array of shapes and sizes. The 5 mm cylindrical
package (red, fifth from the left) is the most common, estimated at 80% of world
production. The color of the plastic lens is often the same as the actual color of
light emitted, but not always. For instance, purple plastic is often used for infrared
LEDs, and most blue devices have clear housings. There are also LEDs in
extremely tiny packages, such as those found on blinkies and on cell phone
keypads. (not shown).
Disadvantages
LEDs are currently more expensive, price per lumen, on an initial capital
cost basis, than more conventional lighting technologies. The additional
expense partially stems from the relatively low lumen output and the drive
circuitry and power supplies needed. However, when considering the total
cost of ownership (including energy and maintenance costs), LEDs far
surpass incandescent or halogen sources and begin to threaten compact
fluorescent lamps. In December 2007, scientists at Glasgow University
claimed to have found a way to make Light Emitting Diodes brighter and
use less power than energy efficient light bulbs currently on the market by
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imprinting holes into billions of LEDs in a new and cost effective method
using a process known as nanoimprint lithography.
LED performance largely depends on the ambient temperature of the
operating environment. Over-driving the LED in high ambient
temperatures may result in overheating of the LED package, eventually
leading to device failure. Adequate heat-sinking is required to maintain
long life. This is especially important when considering automotive,
medical, and military applications where the device must operate over a
large range of temperatures, and is required to have a low failure rate.
LEDs must be supplied with the correct current. This can involve series
resistors or current-regulated power supplies.
The spectrum of some white LEDs differs significantly from a black body
radiator, such as the sun or an incandescent light. The spike at 460 nm
and dip at 500 nm can cause the color of objects to be perceived
differently under LED illumination than sunlight or incandescent sources,
due to metamerism.[29] Color rendering properties of common fluorescent
lamps are often inferior to what is now available in state-of-art white LEDs.
LEDs do not approximate a "point source" of light, so cannot be used in
applications needing a highly collimated beam. LEDs are not capable of providing divergence below a few degrees. This is contrasted with
commercial ruby lasers with divergences of 0.2 degrees or less. This can
be corrected by using lenses and other optical devices.
There is increasing concern that blue LEDs and white LEDs are now
capable of exceeding safe limits of the so-called blue-light hazard as
defined in eye safety specifications such as ANSI/IESNA RP-27.1-05:
Recommended Practice for Photobiological Safety for Lamp and Lamp
Systems.
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W IRELESS LED S
9. APPLICATIONS
1) Wireless LED Candle Light
2) LED wireless Remote control – 1 Channel3) Remote control – 4 channel
1) Wireless LED Candle Light
Illuminate the night with candle light. Our 9" wireless LED candle™
is battery operated. Insert two AA batteries and turn on the switch
and an automatic timer will keep the candle lit for 8 hours, then turn
off for 16 hours. This decorative candle will repeat this cycle for the
life of the batteries. (Batteries not included.) A realistic flickering
flame makes this candle light a safe and cozy alternative in your
home décor! Bronner #1094868.
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W IRELESS LED S
2) LED wireless Remote control – 1 Channel
High quality unit! Single Channel
High power Instructions
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W IRELESS LED S
10. CONCLUSION
Most commercial light emitting diodes (LEDs) both visible and infrared are
fabricated from III-V semi conductor. There compounds contain elements such
as gallium, indius and alluminium from column III of the periodic table and
nitrogen from column V of the periodic table.
All modern day LEDs contain PN junctions. Most of them also have
heterostructure, in which the on junctions are surrounded by semiconductor
materials with larger band-gap energies.
LEDs and LCDs coexists on countless device where the LEDs provide the
status light and LCD display data.
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