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                                                                         Your Position: Home > Articles > LED/Lighting Knowledge > Led Lamp

Led Lamp

Monika / 2013-03-02

An LED lamp (or LED light bulb) is a solid-state lamp that uses light-emitting diodes (LEDs) as the source of light. LED lamps offer long service life and high energy efficiency, but initial costs are higher than those of fluorescent and incandescent lamps. Chemical decomposition of LED chips reduces luminous flux over life cycle as with conventional lamps.

Commercial LED lighting products use semiconductor light-emitting diodes. Research into organic LEDs (OLED), or polymer light-emitting diodes (PLED) is aimed at reducing the production cost of lighting products. Diode technology currently improves at an exponential rate.

LED lamps can be made interchangeable with other types of lamps. Assemblies of high power light-emitting diodes can be used to replace incandescent or fluorescent lamps. Some LED lamps are made with bases directly interchangeable with those of incandescent bulbs. Since the luminous efficacy (amount of visible light produced per unit of electrical power input) varies widely between LED and incandescent lamps, lamps are usefully marked with their lumen output to allow comparison with other types of lamps. LED lamps are sometimes marked to show the watt rating of an incandescent lamp with approximately the same lumen output, for consumer reference in purchasing a lamp that will provide a similar level of illumination.

Efficiency of LED devices continues to improve, with some chips able to emit more than 100 lumens per watt. LEDs do not emit light in all directions, and their directional characteristics affect the design of lamps. The efficiency of conversion from electric power to light is generally higher than for incandescent lamps. Since the light output of many types of light-emitting diodes is small compared to incandescent and compact fluorescent lamps, in most applications multiple diodes are assembled.

Light-emitting diodes use direct current (DC) electrical power. To use them on AC power they are operated with internal or external rectifier circuits that provide a regulated current output at low voltage. LEDs are degraded or damaged by operating at high temperatures, so LED lamps typically include heat dissipation elements such as heat sinks and cooling fins.

Technology overview

General-purpose lighting needs white light. LEDs emit light in a very small band of wavelengths, emitting light of a color 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 some of the light to other colors.

The first method (RGB- or trichromatic white LEDs) uses multiple LED chips, each emitting a different wavelength, in close proximity to generate the broad spectrum of white light. The advantage of this method 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 character of the light can be changed dynamically by adjusting the power supplied to the different LEDs.

The color rendering of RGB LEDs, however, is worse than one would expect; the wavelength gap between red and green is much larger than that between green and blue, resulting in an uneven spectral density. An orange fruit, for example, does reflect some red and it does reflect some green, but not in a ratio that the human retina interprets as orange. Neglecting to poll the orange line makes most orange objects appear reddish. RGB LEDs are therefore suitable for display purposes, but less so for illumination, which prompted some manufacterers to add a fourth, amber LED, marketing the product as RGBA LED (not to be confused with the RGBA color space) or tetrachromatic white LED. It can be expected that the number of colors will be further increased to six or more, equally-tempered wavelengths.

The second method, phosphor converted LEDs (pcLEDs) uses one short-wavelength LED (usually blue, sometimes ultraviolet) in combination with a phosphor which absorbs a portion of the blue light and emits a broader spectrum of white light. (The same mechanism—the Stokes shift—is used in a fluorescent lamp emitting white light from a UV-illuminated phosphor.) The major advantage is the low production cost. The CRI (color rendering index) value can range from less than 70 to over 90, and color temperatures in the range of 2700 K (matching incandescent lamps) up to 7000 K are available. The character of the light cannot be changed dynamically. The phosphor conversion absorbs some energy, but most of the electrical energy is still wasted as heat within the LED chip itself. The low cost and adequate performance makes this the most widely used LED technology for general lighting today.

A single LED is a low-voltage solid-state device and cannot be directly operated on AC power without circuitry to control the current flow through the lamp. In principle a series diode and resistor could be used to limit the current and to control its direction, but this would be very inefficient since most of the applied power would be dissipated by the resistor. A series string of LEDs would minimize dropped-voltage losses, but one LED failure would extinguish the whole string. Paralleled strings increase reliability by providing redundancy. In practice, three or more strings are usually used. To be useful for illumination, a number of LEDs must be placed close together in a lamp to combine their illuminating effects. As of 2012, white LED assemblies emitting 10,000 lm are available. When using the color-mixing method, a uniform color distribution can be difficult to achieve, 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 driver electronics, a heat sink, and optics.

Application

The main difference to other light sources is the directed light. Thus illuminating a flat defined area requires less Lumen compared to light sources which would need reflectors or lenses to do the same. For illuminating an 360° orbit, the benefits of LED are much smaller. LED lamps are used for both general and special-purpose lighting. Where colored light is needed, LEDs that inherently emit single colored light require no energy-absorbing filters. This improves the energy efficiency over a white light source that generates all colors of light then discards some of the visible energy in a filter.

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, and that lifetime is unaffected by cycling on and off, so that they are well suited for light fixtures where bulbs are often turned on and off. LED light bulbs are also mechanically robust; most other artificial light sources are fragile.

White-light light-emitting diode lamps have longer life expectancy and higher efficiency (the same light for less electricity) than most other lighting. LED sources are compact, which gives flexibility in designing lighting fixtures and good control over the distribution of light with small reflectors or lenses. Because of the small size of LEDs, control of the spatial distribution of illumination is extremely flexible, and the light output and spatial distribution of a LED array can be controlled with no efficiency loss.

LED lamps have no glass tubes to break (some models have a decorative glass bulb, however), and their internal parts are rigidly supported, making them resistant to vibration and impact. With proper driver electronics design, an LED lamp can be made dimmable over a wide range; there is no minimum current needed to sustain lamp operation.

LEDs using the color-mixing principle can emit a wide range 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 requirements. For applications where non-directional light is required, either a diffuser is used, or multiple individual LED emitters are used to emit in different directions.

 

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