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Solid State LightingSolid State LightingSolid State LightingSolid State Lighting ( or LED Lamps)( or LED Lamps)( or LED Lamps)( or LED Lamps)

A LED lampLED lampLED lampLED lamp is a type of solid state lighting (SSL) that utilizes light-emitting

diodes (LEDs) as a source of illumination rather than electrical filaments or gas.

LED lamps (also called LED bars or Illuminators) are usually clusters of LEDs in asuitable housing. They come in different shapes, including the standard 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

History

The phenomenon of solid state junctions producing light was discovered in the crystal

detector era. In the 1960s commercial red LED's became available, and by the 1970s

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.

these were in widespread use as indicators in a very wide range of equipment. These

early LED's had much too small an output to be useful as lighting. They replaced the

previously widely used indicator types of filament lamps and neon. Compared to neon,indicator LED's have longer lifetimes and run on lower voltage; compared to miniature

filament lamps, indicator LED's have much longer lifetimes, such that they do not

require replacement, and consume less power. The lack of need for replacement also

eliminates the need for bulb sockets and a user access port.

Commercial amber (yellow) and orange LED's followed, and were used where

differentiation of multiple LEDs was required. For many years LED's came in infra-red,

red, orange, yellow, and green. Blue, cyan, and violet LEDs finally appeared in the

1990s.

To produce a white SSL device, a blue LED was needed. In 1993, Shuji

Nakamura of Nichia Corporation came up with a blue LED using gallium nitride (GaN).

With this invention, it was now possible to create white light by combining the light of

separate LED's (red, green, and blue), or by placing a blue LED in a package with an

internal light converting phosphor. With the phosphor type, some of the blue output

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becomes either yellow or red and green with the result that the LED light emission

appears white to the human eye.

In 2008, SSL technology advanced to the point that Sentry Equipment Corporation in

Oconomowoc, Wis. was able to light its new factory almost entirely with LEDs, both

interior and exterior. Although the initial cost was three times more than a traditionalmixture of incandescent and fluorescent bulbs, the extra cost will be repaid within two

years from electricity savings, and the bulbs should not need replacement for 20 years.

Pic:Pic:Pic:Pic: 200mm LED Traffic Light with Lifespan of 50,000 Hours and < 8W Power

Consumption

A typical LED lit Low consumption Torch can last several months with standarad cells

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Technology overview

A single LED diode can produce only a limited amount of light, and only a single color at

a time. To produce the white light necessary for SSL, light spanning thevisible

spectrum (red, green, and blue) must be generated in approximately correct

proportions. To achieve this, three approaches are used for generating white light with

LEDs: wavelengthwavelengthwavelengthwavelength conversionconversionconversionconversion, color mixingcolor mixingcolor mixingcolor mixing, and most recently HomoepitaxialHomoepitaxialHomoepitaxialHomoepitaxialZnSeZnSeZnSeZnSe.

Wavelength conversionWavelength conversionWavelength conversionWavelength conversion involves converting some or all of the LEDs output into visible

wavelengths. Methods used to accomplish this feat include:

BlueBlueBlueBlue LEDLEDLEDLED & yellow& yellow& yellow& yellow phosphorphosphorphosphorphosphor Considered the least expensive method forproducing white light. Blue light from an LED is used to excite a phosphor which

then re-emits yellow light. This balanced mixing of yellow and blue lights results in

the appearance of white light, but produces poor color rendition (i.e., has low CRI).

BlueBlueBlueBlue LEDLEDLEDLED & several& several& several& several phosphorsphosphorsphosphorsphosphors Similar to the process involved with yellowphosphors, except that each excited phosphor re-emits a different color. Similarly,

the resulting light is combined with the originating blue light to create white light.

The resulting light, however, has a richer and broader wavelength spectrum and

produces a higher color-quality light, albeit at an increased cost.

UltravioletUltravioletUltravioletUltraviolet (UV) LED & red, green, & blue phosphors(UV) LED & red, green, & blue phosphors(UV) LED & red, green, & blue phosphors(UV) LED & red, green, & blue phosphors The UV light is used toexcite the different phosphors, which are doped at measured amounts. The colorsare mixed resulting in a white light with the richest and broadest wavelength

spectrum.

Blue LED &Blue LED &Blue LED &Blue LED & quantum dotsquantum dotsquantum dotsquantum dots A process by which a thin layer of nanocrystalparticles containing 33 or 34 pairs of atoms, primarily cadmium and selenium, are

coated on top of the LED. The blue light excites the quantum dots, resulting in a

white light with a wavelength spectrum similar to UV LEDs.

Color mixingColor mixingColor mixingColor mixing involves using multiple colors of LEDs in a lamp to produce white light.

Such lamps contain a minimum of two LEDs (blue and yellow), but can also have three

(red, blue, and green) or four (red, blue, green, and yellow). As no phosphors are used,

there is no energy lost in the conversion process, thereby exhibiting the potential for

higher efficiency.

Homoepitaxial ZnSeHomoepitaxial ZnSeHomoepitaxial ZnSeHomoepitaxial ZnSe is a technology developed by Sumomito Electric where a LED is

grown on a ZnSe substrate, which simultaneously produces blue light from the active

region and yellow emission from the substrate. The resulting white light has a

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wavelength spectrum on par with UV LEDs. No phosphors are used, resulting in a higher

efficiency white LED.

To be considered SSL, however, a multitude of LEDs must be placed close together in a

lamp to add their illuminating effects. This is because an individual LEDproduces only a

small amount of light, thereby limiting its effectiveness as a replacement light source.In the case where white LEDs are utilized in SSL, this is a relatively simple task, as all

LEDs are of the same color and can be arranged in any fashion. When using the color-

mixing method, however, it is more difficult to generate equivalent brightness when

compared to using white LEDs in a similar lamp size. Furthermore, degradation of

different LEDs at various times in a color-mixed lamp can lead to an uneven color

output. Because of the inherent benefits and greater number of applications for

white LED based SSL, most designs focus on utilizing them exclusively.

Driving LEDs

LEDs have very low dynamic resistance, with the same voltage drop for widely varying

currents. Consequently they can not connect direct to most power sources without

causing self destruction. A current control ballast is normally used, which is sometimes

constant current.

Indicator LEDsIndicator LEDsIndicator LEDsIndicator LEDs

Miniature indicator LEDs are normally driven from low voltage DC via a current

limiting resistor. Currents of 2mA, 10mA and 20mA are common. Some low current

indicators are only rated to 2mA, and should not be driven at higher current.

Sub-mA indicators may be made by driving ultrabright LEDs at very low current. Efficacy

tends to reduce at low currents, but indicators running on 100uA are still practical. The

cost of ultrabrights is higher than 2mA indicator LEDs.

LEDs have a low max repeat reverse voltage rating, ranging from apx 2v to 5v, and this

can be a problem in some applications. Back to back LEDs are immune to this

problem. These are available in single color as well as bicolor types. There are various

strategies for reverse voltage handling.

In niche applications such as IR therapy, LEDs are often driven at far above rated

current. This causes high failure rate and occasional LED explosions. Thus many

parallel strings are used, and a safety screen and ongoing maintenance are required.

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Alphanumeric LEDsAlphanumeric LEDsAlphanumeric LEDsAlphanumeric LEDs

These use the same drive strategy as indicator LEDs, the only difference being the

larger number of channels, each with its own resistor. 7 segment and starburst LED

arrays are available in both common anode or common cathode forms.

Lighting LEDs on mainsLighting LEDs on mainsLighting LEDs on mainsLighting LEDs on mains

A CR dropper (capacitor & resistor) followed by full wave rectification is the usual

ballast with mains driven series-parallel LED clusters.

A single series string would minimise dropper losses, but one LED failure would

extinguish the whole string. Parallelled strings increase reliability. In practice usually 3

strings or more are used.

Operation on square wave and modified sine wave (MSW) sources, such as

many inverters, causes heavily increased resistor dissipation in CR droppers, and LED

ballasts designed for sine wave use tend to burn on non-sine waveforms. The non-sine

waveform also causes high peak LED currents, heavily shortening LED life.

Aninductor & rectifier makes a more suitable ballast for such use, and other options

are also possible.

Lighting LEDs on low voltageLighting LEDs on low voltageLighting LEDs on low voltageLighting LEDs on low voltage

LEDs are normally operated in parallel strings of series LEDs, with the totalLED voltage typically adding up to around 2/3 of the supply voltage,

and resistorcurrentcontrol for each string.

In resistor-drive devices, LED current is then proportional to power supply (PSU) voltage

minus total LED string voltage. Where battery sources are used, the PSU voltage can

vary widely, causing large changes in LED current and therefore color and light output.

For such applications, a constant current regulator is preferred to resistor control. Low

drop-out (LDO) constant current regs also allow the total LED string voltage to be a

higher percentage of PSU voltage, resulting in improvedefficiency and reduced power

use.

Torches run one or more lighting LEDs on a low voltage battery. These usually use

a resistor ballast.

In disposable coin cell powered keyring type LED lights, the resistance of the cell itself

is usually the only current limiting device. The cell should not therefore be replaced with

a lower resistance type, such as one using a different battery chemistry.

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Finally, an LED can be run from a single cell by use of a constant current switched

mode inverter. While adding additional expense, this method provides a high level of

color and brightness control, and ensures longer LED lifetime.

Comparison to other lighting technologiesComparison to other lighting technologiesComparison to other lighting technologiesComparison to other lighting technologies

Incandescent lamps (light bulbs) create light by running electricity through a thinfilament, thereby heating the filament to a very high temperature so that it glows

and produces visible light. A broad range of visible frequencies are naturally

produced, yielding a pleasing warm yellow or white color quality. The incandescing

process, however, is highly inefficient, as over 98% of its energy input is emitted as

heat. A standard 100 watt 120 VAC light bulb produces about 1700 lumens, about

17 lumens per watt. Incandescent lamps are relatively inexpensive to produce. The

typical lifespan of a mains incandescent lamp is around 1,000 hours. They work

well with dimmers. Most existing light fixtures are designed for the size and shape

of these traditional bulbs.

Fluorescent lamps (light bulbs) work by passing electricity through mercuryvapor, which in turn produces ultraviolet light. The ultraviolet light is then absorbed

by a phosphor coating inside the lamp, causing it to glow, or fluoresce. While the

heat generated by fluorescent lamps is much less than its incandescent

counterpart, energy is still lost in generating the ultraviolet light and converting this

light into visible light. If the lamp breaks exposure to mercury can occur. Linearfluorescent lamps are typically five to six times the cost of incandescent lamps but

have life spans around 10,000 and 20,000 hours. Lifetime varies from 1,200 hours

to 20,000 hours for compact fluorescent lamps.

The efficacy of fluorescent tubes with modern electronic ballast commonly

averages 50 to 67 lm/W overall. Most compact fluorescents 13 watts or more

with integral electronic ballasts achieve about 60 lumens/watt. They should be

recycled rather than disposed to prevent mercury release. Some flicker at 100

or 120 Hz, and the quality of the light tends to be a harsh white due to the lack

of a broad band of frequencies. Most are not compatible with dimmers.

Neon lampNeon lampNeon lampNeon lamp (light bulbs) used like night-lamp in children's room. Typically a 230V (in Europe) is rated 0.5 W of power.

SSL/LEDsSSL/LEDsSSL/LEDsSSL/LEDs LEDs come in multiple colors, which are produced without the needfor filters. A white SSL can be comprised of a single high-power LED, multiple

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white LEDs, or from LEDs of different colors mixed to produce white light.

Advantages include:

High efficiencyHigh efficiencyHigh efficiencyHigh efficiency - LEDs are now available that reliably offer over 100lumens from a one-watt device, or much higher outputs at higher drive

currents

Small sizeSmall sizeSmall sizeSmall size - provides design flexibility, arranged in rows, rings, clusters, orindividual points

High durabilityHigh durabilityHigh durabilityHigh durability - no filament or tube to break Life spanLife spanLife spanLife span - in properly engineered lamps, LEDs can last 50,000 - 60,000

hours

Full dimmabilityFull dimmabilityFull dimmabilityFull dimmability unlike fluorescent lamps, LEDs can be dimmedusing pulse-width modulation (PWM - turning the light on and off very

quickly at varying intervals). This also allows full color mixing in lamps with

LEDs of different colors.[1]

MercuryMercuryMercuryMercury----freefreefreefree - unlike fluorescent and most HID technologies, LEDs

contain no hazardous mercury or halogen gases

However, some current models are not compatible with standard dimmers. It is

not currently practical to produce high levels of room lighting. As a result,

current LED screw-in light bulbs offer either low levels of light at a moderate

cost, or moderate levels of light at a high cost. In contrast to other lighting

technologies, LED light tends to be directional. This is a disadvantage for most

general lighting applications, but can be an advantage for spot or flood lighting.

Because individual LEDs are low-voltage DC devices, implementing SSL to

operate from mains AC requires well designed circuitry and a thermal case to

dissipate the heat.