Definition:
  LED technology   Physical function
   An LED is a special type of semiconductor diode. Like a normal diode, it consists of a chip of semiconducting material impregnated, or doped, with impurities to create a structure called 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 electron 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 as it does so.
   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 a n-type substrate, with 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 gets absorbed and turns into additional heat.
  Conventional LEDs are made from a variety of inorganic semiconductor materials, producing the following colors:

aluminum gallium arsenide (AlGaAs) - red and infrared
aluminum gallium phosphide (AlGaP) - green
aluminum 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
indium gallium nitride (InGaN) - near ultraviolet, bluish-green and blue
silicon carbide (SiC) as substrate - blue
silicon (Si) as substrate - blue (under development)
sapphire (Al2O3) as substrate - blue
zinc selenide (ZnSe) - blue
diamond (C) - ultraviolet
aluminum nitride (AlN), aluminum gallium nitride (AlGaN) - near to far ultraviolet

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

 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 Luminous efficiency and low heat emitting .The best LED in the market now emits 150LM/w, which is almost tenfold as efficient as an equivalent tungsten-filament light bulb.

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.
Unbreakable:
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.

Illumination
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.

Display
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.
 

Color
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.

Parameter of typical LEDs:

LED lighting terms

Luminance	A measure of the brightness or luminous intensity of light, usually expressed in units of Candelas per square meter (cd/m2) or foot Lamberts. 1 FL = 3.426 cd/m2. Unit: Gannet (nit)
Intensity	Refers to the bright level of light, the luminous intensity per unit area. Unit: Candlelight (cd)
Luminous flux	The total amount of visible light produced by lighting source per second. Unit: lumen [lm]
Wavelength	The changes of light color Strength can be described through data, we call this data wavelength. The light wavelength we can see is in the range between 380 to 780 nm. Unit: nanometer (nm)
Light efficacy	Refers to the luminous flux divided by the lighting power source. It is an important indicator to measure the energy source. Unit: Watts per lumen (Lm/w).
Color performance	Refers to the showing extent on objects of the lighting source, in other words, the real degree of color, usually called "color-rendering " Units : Ra
Glare	When the lighting source color is the same as the radiator at certain temperature, the temperature of the black body is the color temperature of the lighting source, it describes the color of a light source by comparing it to the color of a blackbody radiator at a given temperature. Unit: Kelvin (k).
Protection level	The IP level is described by two figures: the first one refers to the protection level against dust(from 0 to grade 6), the second against water(from 0 to grade8). Devices with a higher IP level keep the internal components safe and clean

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