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.
General-purpose lighting needs white light. LEDs emit light in a very small band of wavelengths, emitting light of a colour characteristic of the energy band gap 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 colours.
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 colour 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 manufacturers to add a fourth, amber LED, marketing the product as RGBA LED (not to be confused with the RGBA colour space) or tetrachromatic white LED. It can be expected that the number of colours 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 (colour rendering index) value can range from less than 70 to over 90, and colour 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 colour-mixing method, a uniform colour distribution can be difficult to achieve, while the arrangement of white LEDs is not critical for colour balance. Further, degradation of different LEDs at various times in a colour-mixed lamp can lead to an uneven colour output. LED lamps usually consist of clusters of LEDs in a housing with driver electronics, a heat sink, and optics.
Comparison to Other Technologies
- Incandescent lamps (light bulbs) generate light by passing electric current through a resistive filament, thereby heating the filament to a very high temperature so that it glows and emits visible light over a broad range of wavelengths. Incandescent sources yield a “warm” yellow or white colour quality depending on the filament operating temperature. Incandescent lamps emit 98% of the energy input as heat. A 100 W light bulb for 120 V operation emits about 1,180 lumens, about 11.8 lumens/W; for 230 V bulbs the figures are 1340 lm and 13.4 lm/W. Incandescent lamps are relatively inexpensive to make. The typical life span of an AC incandescent lamp is 750 to 1,000 hours. They work well with dimmers. Most older light fixtures are designed for the size and shape of these traditional bulbs. In the U.S. the regular sockets are E26 and E11, like E27 and E14 in some European countries.
- Compact fluorescent lamps life span may vary from 6,000 hours to 15,000 hours.
- Fluorescent lamps work by passing electricity through mercury vapour, which in turn emits ultraviolet light. The ultraviolet light is then absorbed by a phosphor coating inside the lamp, causing it to glow, or fluoresce. Conventional linear fluorescent lamps have life spans around 20,000 and 30,000 hours based on 3 hours per cycle according to lamps NLPIP reviewed in 2006. Induction fluorescent relies on electromagnetism rather than the cathodes used to start conventional linear fluorescent. The newer rare earth triphosphor blend linear fluorescent lamps made by Osram, Philips, Crompton and others have a life expectancy greater than 40,000 hours, if coupled with a warm-start electronic ballast. The life expectancy depends on the number of on/off cycles, and is lower if the light is cycled often. The ballast-lamp combined system efficacy for then current linear fluorescent systems in 1998 as tested by NLPIP ranged from 80 to 90 lm/W. For comparison, general household LED bulbs available in 2011 emit 64 lumens/W, with the best LED bulbs coming in at about 100 lumens/W.