Led Lamp or Led bulb (Technology and Application)

An LED lamp (LED lightbulb) is a solid-state lamp that uses light-emitting diodes (LEDs) as the source of light. The LEDs involved may be conventional semiconductor light-emitting diodes, to organic LEDs (OLED), or polymer light-emitting diodes (PLED) devices, although OLED and PLED technologies are not commercially available in 2010.

Since the light output of light-emitting diodes is small compared to incandescent and compact fluorescent lamps, multiple diodes are often used together.




In last years, as led light technology has improved, high power light-emitting diodes with higher lumen output are making it possible to replace other lamps with LED lamps. One high power LED chip used in the commercial LED lights can emit 7,527 lumens while using only 100 watts. LED lamps can be made interchangeable with other types of lamps but the most are non dimmable.

Diodes use direct current (DC) electrical power, so LED lamps must also include internal circuits to operate from standard AC voltage. LED's are damaged by being run at higher temperatures, so LED lamps typically include heat management elements such as heat sinks, cooling fins. LED lamps have long service life and high energy efficiency, but the first costs are higher than those of fluorescent lamps.

Technology overview

General purpose lighting needs white light. LEDs emit light in a tiny band of wavelengths, emitting strongly colored light. The color is characteristic of the energy bandgap of the semiconductor material used to make the LED. To emit white light from LEDs requires either mixing light from red, green, and blue LEDs, or using a phosphor to convert a quantity of the light to other colors.

The first technique (RGB-LEDs) makes use of multiple LED chips each emitting a different wavelength in close nearness, to form the broad white light spectrum. The advantage of this technique is that the intensity of each LED can be adjusted to "tune" the character of the light emitted. The major disadvantage is high production cost.

The second technique, phosphor converted LEDs (pcLEDs) makes use of one short wavelength LED (usually blue or ultraviolet) in combination with a phosphor, which absorbs a portion of the blue light and emits a broader spectrum of white light. (The mechanism is similar to the way a fluorescent lamp emits white light from a UV-illuminated phosphor.) The major advantage here is the low production cost, and high CRI (color rendering index), while the disadvantage is the inability to dynamically modify the character of the light and the fact that phosphor conversion reduces the efficiency of the tool. The low cost and adequate performance makes it the most widely used expertise for general lighting today.

A single LED is a low-voltage solid state tool and cannot be directly operated on standard AC current without some circuitry to control the voltage applied and the current flow through the lamp. A series diode and resistor could be used to control the voltage polarity and to limit the current, but this is inefficient since most of the applied voltage would be dropped as wasted heat in the resistor. A single series string of LEDs would minimize dropped-voltage losses, but one LED failure could extinguish the whole string. Paralleled strings increase reliability by providing redundancy. In practice, two strings or more are usually used. To be useful for illumination for home or work spaces, a considerable number of LEDs must be placed close together in a lamp to merge their illuminating effects. This is because individual LEDs emit only a fraction of the light of traditional light sources. When using the color-mixing technique, a uniform color distribution can be difficult to accomplish, while the arrangement of white LEDs is not critical for color balance. Further, degradation of different LEDs at various times in a color-mixed lamp can lead to an uneven color output. LED lamps usually consist of clusters of LEDs in a housing with both driver electronics, a heat sink and optics.

Application

Compared to fluorescent bulbs, introduced at the 1939 World's Fair, advantages claimed for LED light bulbs are that they contain no mercury (unlike a Compact fluorescent lamp or CFL), that they turn on instantly, & that lifetime is unaffected by running on & off, so that they are well suited for lighting fixtures where bulbs are often turned on & off. LED light bulbs are also less apt to break.

LED lamps are used for both general & special-purpose lighting. Where colored light is necessary, LEDs come in multiple colors, which are emitted with no need for filters. This improves the energy efficiency over a white light source that generates all colors of light then discards a quantity of the visible energy in a filter.

White-light light-emitting diode lamps have the traits of long life expectancy & comparatively low energy use. The LED sources are compact, which gives flexibility in designing lighting fixtures & lovely control over the distribution of light with tiny reflectors or lenses. Because of the tiny size of LEDs, control of the spatial distribution of illumination is flexible, & the light output & spatial distribution of a LED array can be controlled with no efficiency loss.

LED lamps have no glass tubes to break, & their internal parts are rigidly supported, making them resistant to vibration & impact. With proper driver electronics design, an LED lamp can be made dimmable over a wide range; there is no maximum current needed to maintain lamp operation.

LEDs using the color-mixing principle can emit a variety of colors by changing the proportions of light generated in each primary color. This allows full color mixing in lamps with LEDs of different colors. In contrast to other lighting technologies, LED emission tends to be directional (or at least lambertian). This can be either an advantage or a disadvantage, depending on the requirements of the application. For applications where non-directional light is necessary, either a diffuser is used, or multiple individual LED emitters are used to cover different directions.