LED lighting

LED lighting


A LED lamp is a light-emitting diode (LED) product that is assembled into a lamp (or light bulb) for use in lighting fixtures. LED lamps have a lifespan and electrical efficiency that is several times better than incandescent lamps, and significantly better than most fluorescent lamps. Like incandescent lamps and unlike most fluorescent lamps (e.g. tubes and CFL), LED lights come to full brightness without need for a warm-up time; the life of fluorescent lighting is also reduced by frequent switching on and off. Initial cost of LED is usually higher. Degradation of LED die and packaging materials reduces light output to some extent over time.

Research into organic LEDs (OLED) and polymer light-emitting diodes (PLED) is aimed at reducing the production cost of lighting products. Cost per lumen and output per device of LEDs has been improving rapidly according to what has been called Haitz's law, analogous to Moore's law for semiconductor devices.

Some LED lamps are made to be a directly compatible drop-in replacement for incandescent or fluorescent lamps. An LED lamp packaging may show the lumen output, power consumption in watts, color temperature or description ("warm white") and sometimes the equivalent wattage of an incandescent lamp of similar luminous output.

Electrical efficiency of LED devices is higher than other lighting and 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 light output of single LEDs is less that that of incandescent and compact fluorescent lamps; in most applications multiple LEDs are used to form a lamp, although high-power versions (see below) are becoming available.

LED chips need controlled direct current (DC) electrical power; an appropriate power supply is needed. LEDs are adversely affected by high temperature, 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 narrow 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.

One method (RGB or trichromatic white LEDs) uses multiple LED chips, each emitting a different wavelength, in close proximity to generate white light. This allows the intensity of each LED to be adjusted to change the overall color.

The second method uses LEDs in conjunction with a phosphor. 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.

Application

A significant difference from other light sources is that the light is more directional, i.e., emitted as a narrower beam. LED lamps are used for both general and special-purpose lighting. Where colored light is needed, LEDs that inherently emit light of a single color require no energy-absorbing filters.



White-light LED lamps have longer life expectancy and higher efficiency (more light for the same 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,[2] and the light output and spatial distribution of a LED array can be controlled with no efficiency loss.

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.[3] Unlike other lighting technologies, LED emission tends to be directional (or at least lambertian), which can be either advantageous or disadvantageous, 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.

Household LED lamps

Replacement for existing lighting

Lamp sizes and bases

LED lamps are made that replace screw-in incandescent or compact fluorescent light bulbs, mostly replacing incandescent bulbs rated from 5 to 60 watts. Such lamps are made with standard light bulb connections and shapes, such as an Edison screw base, an MR16 shape with a bi-pin base, or a GU5.3 (Bipin cap) or GU10 (bayonet fitting) and are made compatible with the voltage supplied to the sockets. They include circuitry to rectify the AC power and convert the voltage to an appropriate value.

As of 2010 some LED lamps replaced higher wattage bulbs; for example, one manufacturer claimed a 16-watt LED bulb was as bright as a 150 W halogen lamp.[4] A standard general-purpose incandescent bulb emits light at an efficiency of about 14 to 17 lumens/W depending on its size and voltage. According to the European Union standard, an energy-efficient bulb that claims to be the equivalent of a 60 W tungsten bulb must have a minimum light output of 806 lumens.[5]


Some models of LED bulbs ware compatible with dimmers as used for incandescent lamps. LED lamps often have directional light characteristics. The lamps have declined in cost to between US$10 to $50 each as of 2012. These bulbs are more power-efficient than compact fluorescent bulbs[6] and offer lifespans of 30,000 or more hours, reduced if operated at a higher temperature than specified. Incandescent bulbs have a typical life of 1,000 hours, and compact fluorescents about 8,000 hours. The bulbs maintain output light intensity well over their lifetimes. Energy Star specifications require the bulbs to typically drop less than 10% after 6,000 or more hours of operation, and in the worst case not more than 15%.[7] LED lamps are available with a variety of color properties. The purchase price is higher than most other, but the higher efficiency may make total cost of ownership (purchase price plus cost of electricity and changing bulbs) lower.[8]

Several companies offer LED lamps for general lighting purposes. The technology is improving rapidly and new energy-efficient consumer LED lamps are available.[9]

LED tube lamps

LED tube lights are designed to physically fit in fixtures intended for fluorescent tubes. Some LED tube lamps are intended to be a drop-in replacement into existing fixtures. Others require rewiring of the fixtures to remove the ballast. An LED tube lamp generally uses many individual LEDs and may be directional. Fluorescent lamps emit light all the way around the lamp. Most LED tube lights available can be used in place of T8, T10, or T12 tube designations, in lengths of 2, 4, and 8 feet.

Lighting designed for LEDs

Newer light fittings designed for LED lamps, or indeed with long-lived LEDs built-in, have been coming into use as the need for compatibility with existing fittings diminishes. Such lighting does not require each bulb to contain circuitry to operate from mains voltage.

Specialty uses

White LED lamps have achieved market dominance in applications where high efficiency is important at low power levels. Some of these applications include flashlights, solar-powered garden or walkway lights, and bicycle lights. Monochromatic (colored) LED lamps are now commercially used for traffic signal lamps, where the ability to emit bright monochromatic light is a desired feature, and in strings of holiday lights.

LED lights had also become popular in gardening and agriculture by 2010. First used by NASA to grow plants in space, LEDs came into use for home and commercial applications for indoor horticulture (aka grow lights). The wavelengths of light emitted from LED lamps have been specifically tailored to supply light in the spectral range needed for chlorophyll absorption in plants, promoting growth while reducing wastage of energy by emitting minimal light at wavelengths that plants do not require. The red and blue wavelengths of the visible light spectrum are used for photosynthesis, so these are the colors almost always used in LED grow light panels.

Commercial and industrial use

In 2008 Sentry Equipment Corporation in Oconomowoc, Wisconsin, USA, was able to light its new factory interior and exterior almost solely with LEDs. Initial cost was three times more than a traditional mix of incandescent and fluorescent lamps, but the extra cost was recovered within two years via electricity savings, and the lamps should not need replacing for 20 years.[10] In 2009 the Manapakkam, Chennai office of the Indian IT company, iGate, spent 3,700,000 (US$80,000) to light 57,000 sq ft (5,300 m2) of office space with LEDs. The firm expected the new lighting to pay for itself fully within 5 years.[11]

In 2009 the exceptionally large Christmas tree standing in front of the Turku Cathedral in Finland was hung with 710 LED bulbs, each using 2 watts. It has been calculated that these LED lamps paid for themselves in three and a half years, even though the lights run for only 48 days per year.[12]

In 2009 a new highway (A29) was inaugurated in Aveiro, Portugal, it included the first European public LED-based lighting highway.[13]

By 2010 mass installations of LED lighting for commercial and public uses were becoming common. LED lamps have also been used for a number of demonstration projects for outdoor lighting and LED street lights. The United States Department of Energy has available several reports on the results of many pilot projects for municipal outdoor lighting.[14] Many additional streetlight and municipal outdoor lighting projects have been announced.[15]

Comparison to other lighting technologies

See luminous efficacy for an efficiency chart comparing various 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 color quality depending on the filament operating temperature. Incandescent lamps emit 98% of the energy input as heat.[16] 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.[17] Incandescent lamps are relatively inexpensive to make. The typical lifespan of an AC incandescent lamp is 750 to 1,000 hours.[18][19] 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.
  • Fluorescent lamps work by passing electricity through mercury vapor, 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.[20] For comparison, general household LED bulbs available in 2011 emit 64 lumens/W,[21]
Cost Comparison
Incandescent Halogen CFL LED (Generic) LED (Philips)[22] LED (Philips L-Prize)
Purchase price $0.36[23] $4 $4 $10 $16 $30
Power used (watts) 60 42 13 13.5 9.5 10
Lumens 860 570 825 850 806 940
Lumens/Watt 14.3 13.6 63.5 63 84.8 94
Color Temperature Kelvin 2700 3000 2700 3000 2700 2700
CRI 100 100 82 >75 >80 92
Lifespan (hours) 1,000 3,500 8,000 25,000 15,000 30,000
Bulb lifetime in years @ 6 hours/day 0.5 1.6 3.7 >11.4 6.8 >13.7
Energy cost over 10 years @ 15 cents/kWh $197 $138 $43 $44 $31 $33
Total cost $206 $166 $55 $54 $55 $63
Total cost per 860 lumens† $206 $180 $56 $53 $57 $57
Comparison based on 6 hours use per day (21,900 hours over 10 yrs)

LED prices adjusted for life in this row, e.g. $10 lamp still has ~8,000 hours life left, so adjusted to $8.64

In keeping with the long life claimed for LED lamps, long warranties are offered. One manufacturer warrants lamps for professional use, depending upon type, for periods of (defined) "normal use" ranging from 1 year or 2,000 hours (whichever comes first) to 5 years or 20,000 hours[24]. A typical domestic lamp is stated to have an "average life" of 15,000 hours (15 years at 3 hours/day), and to support 50,000 switch cycles[22].

Research and development

US Department of Energy

In May 2008, the U.S. Department of Energy (DOE) announced details of the Bright Tomorrow Lighting Prize competition, known as the "L Prize". This is the first government-sponsored technology competition designed to spur lighting producers to develop high quality, high efficiency solid-state lighting products to replace the common light bulb. The competition will award cash prizes, and may also lead to opportunities for federal purchasing agreements, utility programs, and other incentives for winning products.

The Energy Independence and Security Act (EISA) of 2007 authorized the DOE to establish the Bright Tomorrow Lighting Prize competition. The legislation challenges industry to develop replacement technologies for the most commonly used and inefficient products, 60 W incandescent lamps and PAR 38 halogen lamps. The L Prize specifies technical requirements for these two competition categories. Lighting products meeting the competition requirements would use just 17% of the energy used by most incandescent lamps in use today. A future L Prize program announcement will call for developing a new “21st Century Lamp”, as authorized in the legislation.

The EISA legislation establishes basic requirements and prize amounts for each category. The legislation authorizes up to $20 million in cash prizes.[25] On September 24, 2009 the DOE announced that Philips Lighting North America was the first to submit lamps in the category to replace the standard 60 W A-19 "Edison screw fixture" light bulb,[8] with a design based on their earlier "AmbientLED" consumer product. On August 3, 2011, DOE awarded the prize in the 60 W replacement category to Philips' bulb after 18 months of extensive testing.[26]

National Institute of Standards and Technology

In June 2008, NIST announced the first two standards for solid-state lighting in the United States. These standards detail performance specifications for LED light sources and prescribe test methods for solid-state lighting products.

The Illuminating Engineering Society of North America (IESNA) published a documentary standard LM-79, which describes the methods for testing solid-state lighting products for their light output (lumens), efficacy (lumens per watt) and chromaticity.

The solid-state lights being studied are intended for general illumination, but white lights used today vary greatly in chromaticity. ANSI C78.377-2008 specifies the recommended color ranges for solid-state lighting products using cool to warm white LEDs with various correlated color temperatures.[27]

DOE launched the Energy Star program for solid-state lighting products in 2008.

Environmental Protection Agency

In the United States and Canada, the Energy Star Light Bulbs for Consumers is a resource for finding and comparing Energy Star qualified lamps.

Other venues

In the United Kingdom a program is run by the Energy Saving Trust to identify lighting products that meet energy conservation and performance guidelines.[29]

Philips Lighting has ceased research on compact fluorescents, and is devoting the bulk of its research and development budget, 5 percent of the company's global lighting revenue, to solid-state lighting.[10]

In January 2009, it was reported that researchers at Cambridge University had developed an LED bulb that costs £2 (about $3 U.S.), is 12 times as energy efficient as a tungsten bulb, and lasts for 100,000 hours.[30] Honeywell Electrical Devices and Systems (ED&S) recommend world wide usage of LED lighting as it is energy efficient and can help save the Climate.[31]

Limitations

Color rendition is not identical to incandescent lamps. A measurement unit called CRI is used to express how the light source's ability to render the eight color sample chips compare to a reference on a scale from 0 to 100.[32] LEDs with CRI below 75 are not recommended for use in indoor lighting.[33]

LED efficiency and life span drop at higher temperatures, which limits the power that can be used in lamps that physically replace existing filament and compact fluorescent types. Thermal management of high-power LEDs is a significant factor in design of solid state lighting equipment.

The long life of LEDs, expected to be about 50 times that of the most common incandescent bulbs and significantly longer than fluorescent types, is advantageous for users but will affect manufacturers as it reduces the market for replacements.[10]

"Feedback" reports on some sellers' Web sites report occasional manufacturer-dependent issues with some units, including failure shortly after purchase (faults, rather than short working life), buzzing and noise, flickering, excessive size and weight (due to power and heatsinking components rather than the LED devices), and unsatisfactory color[34]. Although manufacturers try to provide lamps with wide angular cover, users comment that they are significantly more directional than the incandescents replaced.

Possible hazard to vision

Tests performed at the Complutense University of Madrid indicate that prolonged exposure to the shorter blue band spectrum LED lights may permanently damage the pigment epithelial cells of the retina. The test conditions were the equivalent of staring at a 100 watt blue incandescent source from 20 cm for 12 hours; researchers say additional testing is required to ascertain what intensities, wavelengths, and exposure times of LED lighting devices are lethal and non-lethal for retinal tissue.[35][36]

See also

Energy portal

References

Further reading

  • ISBN 0-521-86538-7

External links

  • e-lumen.eu – a website from the European Commission about the second generation of energy-saving lightbulbs
  • U. S. Department of Energy, Using Light Emitting Diodes
  • U. S. Department of Energy, Solid-State Lighting GATEWAY Demonstration Results
  • Efficient LED lighting in conjunction with low-voltage domestic solar PV and mains, earth.org.uk

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