Light-emitting diodes (LEDs) used for illumination are solid-state devices that produce light by passing electric current across layers of semiconductor chips that are housed in a reflector, which in turn is encased in an epoxy lens. The semiconductor material determines the wavelength and subsequent color of the light. The lens converts the LED into a multidirectional or unidirectional light source based on specification.
Traditionally used since the 1970s for indicators and numeric displays, colored LED technology has recently made inroads into a range of mainstream applications such as exit signs, traffic lights, task lights, accent lights, wall washing, signage, decorative lighting, display lighting, cove lights and other tight spaces, wall sconces, outdoor lighting, automobile lights, downlighting and custom lighting.
LEDs offer a number of benefits vs. traditional light sources, including small size, which increases flexibility with fixture designs and ability to light tight spaces; greater reliability, with no filaments or moving parts; greater energy efficiency, with 70 percent less energy being consumed; safer and environmentally friendly operation, with less waste and no mercury; color-changing; and increased quality, color and strength of light.“LEDs are already an important technology because of their advantages -- life, controllability, both spatial and temporal, and the ability to create rugged, unique fixtures,” says Dr. Nadarajah Narendran, Director of Research, Lighting Research Center, Rensselaer Polytechnic Institute. Researchers like Dr. Narendran have been studying LEDs and promoting their technological development for years.
“While they still have limitations compared with the traditional light sources that everyone is used to seeing, LEDs are the latest rage in the illumination world because of their tremendous flexibility and potential for innovative solutions,” says Dr. Makarand “Chips” Chipalkatti, director of OSRAM North American LED Lamp Modules.
Colored LEDs currently dominate the exit sign market, are making inroads into the traffic signal market (with current penetration estimated at 15-20%), show significant promise for automobile lighting, and are being sold in a variety of consumer products such as flashlights and light wands. In commercial applications, colored LEDs, which can be combined in custom arrays, can also be controlled to produce dynamic color-changing effects for contour lighting, wall washing, accent lighting, signage, advertising, display, public space and “architainment” applications.
“With LEDs, colors can be dynamically controlled,” says Narendran. “For example, a dawn to dusk scene can be created in an office space, if needed. Similar scenes can be created for night shift workers. Quantus Airlines already has such a system in some of their aircraft.”
The first white light LED was developed in the mid 1990s, a blue indium gallium nitride (InGaN) LED with a phosphor that converts some of the light output into yellow, producing a cool, bluish-white light with a color temperature ranging from 4000K to 11000K. White light can also be produced by combining red, blue and green LEDs in an array, or using an LED that produces UV light that is converted into white light by an RGB phosphor, similar to a fluorescent lamp. Suitable applications include cove lighting, display lighting and undershelf lighting. To address the demands of illumination applications, LEDs have evolved to larger sizes and wattages, now including 3W and 5W units in addition to 1W, and with heat-sinking materials to dissipate heat and extend useful life. White LEDs have generated a lot of excitement as they continue to evolve towards a competitive alternative to popular general light sources.
Unfortunately, white LED technology is still too immature for prime time in many major general lighting applications due to light output, cost and color stability issues. While LEDs are approaching the light output of some low-wattage incandescent lamps (see Table 1) and becoming more efficacious (20-24 LPW for LEDs vs. 16 LPW for incandescent), the current cost per kilolumen is about $200 versus $0.40 for incandescent. However, experts such as Dr. Narendran predict that these roadblocks will be overcome within five to 10 years, including predictions that efficacy will reach 160 LPW and cost per thousand lumens will fall to $2.
“The popularity of LEDs will grow rapidly as scientists are able to make them brighter, while emitting a consistent white light,” says Dr. Chipalkatti of OSRAM.
Table 1. LED Light Output. Source: LightingResearchCenter, 2003
Color
Light Output (lm)
White
18-87 lm
Blue
7-30 lmGreen
25-120 lm
Yellow
20-69 lm
Red
25-55 lm
These predictions are believable when one considers the intensive research effort going into the LED technology race by such as organizations as the U.S. Department of Energy and partnerships between lamp and semiconductor companies, who are used to thinking in terms of six-month product cycles. In fact, by the time you read this article, the labs may already be raising the bar with new discoveries. LED light output, for example, has doubled every 18-24 months since 1970.
As the roadblocks are overcome, the lighting industry may experience a complete transformation in the future. Besides doubling the efficiency of standard lighting systems, the small size of LEDs provides new possibilities in fixture sizes and shapes, as well as color-changing. A related technology, organic LEDs (OLEDs), offers the possibilities of using “lighting wallpaper” cut to specification. OLEDs can also be integrated into architectural and room materials -- imagine a curtain by day that becomes a light source by night.
No other technology than LEDs and OLEDs, called “solid-state lighting” as a family, has such high energy-savings potential. Solid-state lighting has the potential to save enough energy to power the states of Arizona, Colorado and Mississippi and reduce the nation’s electric bill by nearly $100 billion over the next 20 years.
“It is becoming clear that LED technology will be key to the progress of lighting in coming years,” says Alan Marble, marketing manager for LED drivers for Advance Transformer.
Until that time, LEDs will continue to be used in thousands of applications around the world, already providing much of the lighting used in applications such as retail stores, casinos, museums, houses of worship, bridges, public spaces, restaurants and bars, hotels, building facades and other spaces. LEDs are being adopted because of the captivating color-changing effects they offer in addition to advertising claims of long service life. Fixture manufacturers are also beginning to incorporate LEDs into new designs either as a distinguishing accent or as a primary light source.
Perhaps the most dramatic contribution of LEDs to lighting so far, however, has actually not been the displacement of standard light sources, but the development of entirely new applications and creative possibilities.
Manufacturers of LEDs include Cree, Lumileds, Nichia and OSRAM Opto Semiconductors. Manufacturers of LED lighting systems include Color Kinetics, GELcore, Lumileds and OSRAM Opto Semiconductors. Manufacturers of LED drivers include Advance Transformer, Color Kinetics, Maxim, Microsemi, OSRAM SYLVANIA/OSRAM Opto Semiconductors, Rohm and Toshiba.
Although LED technology has made significant advances in recent years, relatively, its applications are not widespread. The Alliance for Solid-State Illumination Systems and Technologies (ASSIST) is working towards overcoming hurdles and identifying suitable applications for LED technology. ASSIST is a program formed by the LightingResearchCenter in collaboration with leading manufacturers and system benefit administration groups. OSRAM Opto Semiconductors, Gelcore, Nichia, Boeing Aircraft Co., the New York State Energy Research and Development Authority, and the California Energy Commission have currently signed up for the ASSIST program. (For more information about ASSIST, visit http://www.aboutlightingcontrols.org/click.php?url=www.lrc.rpi.edu/programs/solidstate/ASSISTHome.htm.)
Figure 1. The BenFranklinBridge received a colorful makeover for its 75th birthday, courtesy of Grenald Waldron Associates, a lighting design firm. Nearly 400 LED fixtures from Color Kinetics line the bridge, producing dynamic light shows to commemorate holidays and celebrate city events that can be seen from as far as 10 miles away. A “Chasing Rainbow” effect makes it appear that the lights are “chasing” the trains across the bridge. Photo credit: Carlton Read. Photo courtesy of Color Kinetics.
Figure 2. The Harrah’s Atlantic City Resort & Casino wanted a unique look for its new 28-story BayviewTower expansion, and used LEDs to get it. Stone Mountain Lighting Group conceptualized bands of programmable LED lights that wrap each level of the building, utilizing 4,100 linear feet of Color Kinetics fixtures. The result is a wide variety of specialized lighting effects, from color chases and sunburst patterns to rainbow effects, dramatic color wipes and a large-scale simulation of the American flag. A custom DMX show controller was used for LED control. Photo courtesy of Color Kinetics.
Figure 3. Focus Lighting lighted this luxurious restaurant designed by Karim Rashid. Color Kinetics’ iColor Cove fixtures were used to light glass partitions between each of the restaurant’s booths, which shift in color from subtle white and pale tints to dramatic saturations.
Table 2. Capabilities, advantages and things to
watch out for when choosing LED lighting products.
Capabilities & Advantages
Watch Out For …Color. LEDs are available in a wide range of colors. Colors can be combined in arrays and controlled to produce color-changing effects. Available colors include white, deep blue, blue, green, yellow, amber, orange, red, bright red and deep red. These can be combined to produce light in millions of colors.
Color. Color constancy is uncertain with white LEDs. It may experience color shift over time, when viewing the LED from different angles, and when viewing its light impacting a wall. In addition, mixing red, blue and green LEDs to produce white light may be a flexible solution and result in higher efficacy than phosphor-converted white LEDs, but may require tuning of each individual fixture, and different LED materials degrade at different rates, which can result in color shift.
Size. LEDs are very small light sources, typically 5mm units or T1-3/4 but also including larger “surface-mounted” types. The size of the light source provides opportunities to be flexibly packaged into desired shapes, colors, sizes and lumen packages. They can also be integrated into architectural materials and used to edge-light glass and plastic panels. The distribution of the LED’s light can be controlled by an epoxy lens to make the light source directional, simplifying the construction of fixtures designed to use LEDs.
Light Output. Most 5mm white LEDs produce about one or two lumens each, while newer high-power LEDs produce as much as 20 lumens and are becoming more commonly used for illumination rather than indication purposes. Light output degrades over time, fairly rapidly if ambient heat builds up around the LED.
Other Advantages. LEDs are durable and shock-resistant, and start at temperatures as low as -40oC. They produce no UV energy and little infrared. They operate on a low-voltage DC power supply, which enables LEDs to be easily adaptable to different power supplies, permits longer stand-by power, and increases safety.
Service Life and Heat. LightingResearchCenter tests on white 5mm LEDs discovered that light output decreased by 50% after 10,000 hours of operation. Some LEDs can be operated a higher current to produce up to 65% more light, but the degradation of light output over time occurs even faster. The problem is heat, which builds up around the LED. New high-power white LEDs with heat-sin |
