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LED (light emitting diodes)

Author£ºAdmin Hit£º225 Time£º2012/4/6 9:27:09

LED (light emitting diodes) are solid-state semiconductor devices that can convert electrical energy directly into light. The heart of a led is a semiconductor chip of which one side is attached on the top of an anvil that is the negatie power lead, and the other side is connected with a whisker to a positive power lead. It is the most important part of the LED.and it is entirely encapsulated in an epoxy resin enclosure.

A semiconductor chip consists of two parts, one is called P-type semiconductor where holes dominate the region, and the other part is N-type semiconductor where is dominate by electrons. When they are sandwiched together, there is a PN junction in between. And when an electric current is applied to the chip through the lead, electrons are pushed across the junction into the P-region, there, electrons and holes meet and recombine, and then release energy in the form of photons of light. That is why LED can shine. And the wavelength of the light, therefore its color,depends on th materials forming the PN junction, which is,the materials used to make the LED chip.

Generally , LED chips are made from gallium-based crystals that contain one or more additional materials such as phosphor to produce a distinct color.

Features and Benefits of LED
The inherent features of LED decide it to be the best alternative to conventional light source, and provide a wide range of application.

l.Small size:
A LED is essentially a tiny chip ,about 4.5mm, encapsulated in an epoxy resin enclosure, so it can be extremely small and light.

2.Low Power Consumption:
Generally a LED is designed to operated at 2-3.6v,0.02-0.03A current, that mean, it needs no More than 0.1w to operate.

3.Long lifetime:
Operating at a desired rated voltage, current and environment, LEDS enjoys a long life of up to 100,000hours.

4.High luminous efficiency and low heat emitting :
Due to the special materials that used to make LEDs, during electrons transition, LEDs mainly emit electromagnetic energy in the visible parts of spectrum, unlike the incandescent filaments heated, which emit a lot of electromagnetic energy in the infrared pat of the spectrum which can’t be seen but is felt as heat. That is to say .LEDs are of high illuminus efficiency and low heat emitting .The best LED in the market now emits 321m/w, which is almost twice as efficient as an equivalent tungsten-filament light bulb,and it is reported that LED emit 601m/w, which is as bright as 20w fluorescent lights has been found in the market.

5.Environmental protection:
LEDs are made from non-toxic material, unlike the fluorescent lights with mercury that will pose a pollution danger. LEDs can be recycled too.

The light-emitting device of a LED is completely embedded in an epoxy resin enclosure which is much more sturdy than the bulb and fluorescent tube;there are no loose and moving parts within the solid epoxy enclosure. This makes LEDs virtually indestructible.

The application of LED
LED’s proprietary characteristics above mentioned determines their ideal suitability in a wide range of application.

Decorative lighting
Due to the rich variety in colors, small size, durability, energy savings, LEDs are perfect light source for decorative application. Attached on a PCB, flexible cable, or other desired materials; LEDs can be used for light source of channel letter, signs, border light, light tube and so on.

With the advance of the white light LED technology, an array of white light LEDs assembled in a certain way are now bright enough to be used for domestic illumination, such as light, table lamp, and garden lamp etc.

A typical application of LEDs is for message display or large-scale display screen, which are popularity used in sport fields, airports, business centers and so on. The largest LED display in the world is 36mm high, at Times Square,Manhattan.

LCD backlight
Because of LED’s small size and low power consumption, they are perfect light source for LCD backlight.

Monochromaticity is the inherent property of LEDs. They emit a pure color in a narrow frequency range. The color emitted from a LED is identified by peak wavelength(Ipk) and measured in nanometers(nm).Peak wavelength is a function of the LED chip material, the composition of the different materials determines the wavelength and therefore the color of light emitted.

As we know, white color is a combination of all colors. But eye does not require a mixture of all the colors of the spectrum to perceive white light.A mixture of three primary colors(red, green, blue) can be well enough.

Based on this principle, many solutions have been proposed to fabricate white color LEDs. The typical one is to bury a GaN type blue LED having an InGaN active layer into a YAG pond emitting yellow fluorescene.

Intensity an View Angle
LED light output varies with the type of chip, encapsulation, and other variables .But there is no consistent international criterion for LED brightness measurement. Generally, the amount of light emitted from a LED is quantified by a single point, on-axis luminous intensity value(IV), and is specified in millicandela(mcd).

A LED with higher luminous intensity value does not mean that it has higher total light output. To measure the total light output, the viewing angle must be taken into account too. LED viewing angle is also a function of the LED chip type an the epoxy lens that distributes the light, LEDs with different chips and epoxy lens will have different viewing angle. If two LEDs have the same luminious intensity value, the lamp with the larger viewing angle will have the higher total light output.

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