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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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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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2) LED wireless Remote control – 1 Channel

High quality unit! Single Channel

High power Instructions

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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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REFERENCES

1. www.wikipedia .org

2. www.electronicsforyou.com

3. www.seminartopics.com

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