The lighting is on the wall: Solid state is threatening to dismantle the dominion of incandescent and compact fluorescent light (CFL) sources. Illumination applications based on solid-state lighting are slowly replacing those previously ruled by the older technologies. Though very expensive to use, solid-state lighting has proven more efficient and in sync in a world where government initiatives are calling for a greener environment and less energy consumption.
High-brightness white LED lighting systems, particularly for large-area displays, are the wave of the future. A well-designed LED light source uses about 10 to 12 W to produce 900 lumens of light output. Comparatively, an incandescent light bulb requires about 60 W. LEDs also can be dimmed, which will ease their transition into present-day incandescent light-bulb sockets.
Presently, the cost of using LEDs for general-purpose indoor and outdoor lighting remains high compared to incandescent and fluorescent light sources. However, the average selling price for a high-brightness LED has dropped in the last few years. That drop can be attributed to LED manufacturers using 4-in. wafers to make the devices, instead of the conventional 2-in. wafers. Even 6-in. wafers are under consideration for mass production.
A few notable white LED products on the market compete directly with halogen and metal-halide lamps up to 80 W, in terms of lifetime and energy savings, leading to lower costs. For example, the LEDtronics PAR38-12X12WF warm-white light and pure-white frosted soft flood bulbs operate from 85 to 265 V ac, require no special adapters, and consume less than 18 W for energy savings of 77% to 85% (Fig. 1).
Bridgelux, a solid-state lighting company, is adopting the slogan “penny per lumen” to make white LEDs more affordable for many applications. “We’ve been increasing LED chip performance some 15% to 20% every six months,” says Brian Fisher, director of corporate marketing. Bridgelux expects to produce 100-lumen/W cool white light sources that, when integrated into lighting fixtures, will turn the slogan into reality.
Luminus Devices Inc., a developer and manufacturer of highbrightness LEDs, calls its SBM-160 PhatLight LEDs the industry’s brightest multi-color LEDs available in surface-mount technology (SMT) packages. They’re aimed at architectural and entertainment applications that require high light output and high efficiency levels. Available in a four-chip package consisting of individual red, green, blue, and white LEDs that generate more than 1500 lumens combined, they use a high-performance ceramic substrate to achieve a thermal resistance of less than 1.5°C/W.
A NEW ERA
Many expect LED illumination to revolutionize the lighting industry within the next five years (see “A Bright Future For LED Illumination”). That’s due to LED solid-state lighting’s efficiency edge over other light sources, and it’s one of the few illumination sources suitable for energy-efficient environments. Researchers are actively working on solutions for improved LED thermal-management methods, better color quality, higher lifetimes, and lower costs.
New global energy-conservation regulations are a driving force behind the adoption of LED lighting. In the U.S., the Energy and Security Act of 2007 restricts the sale of inefficient lamps. Europe is about to ban the clear 100-W incandescent lamp, followed by bans on other less efficient incandescent lamps. By 2012, the ban will be comprehensive.
China, which produces 70% of the world’s light bulbs, will cease producing incandescent bulbs in 10 years. Australia has mandated the use of energy-efficient lighting and already banned the importation of “non-compliant lighting,” which translates into a minimum energy standard for a light source of 15 lumens/W. Several other countries are planning to ban and phase out incandescent light bulbs, including Italy, Canada, the United Kingdom, Switzerland, and the Philippines.
More LED lighting products that meet industry and government standards for lifetime and output lumen depreciation levels are emerging. One such standard is the L70 Energy Star lifetime requirement, which means 70% lumen depreciation in an LED from the initial lumen output. Presently, the industry uses a figure of about 50,000 hours of operation before lumen depreciation sets in.
In collaboration with Samsung Electro-Mechanics, researchers at Rensselaer Polytechnic Institute developed and demonstrated a polarization-matched device that exhibits an 18% increase in light output and a 22% increase in ac wall-plug efficiency (Fig. 2). This new device notably reduces the “efficiency droop” that affects LEDs (largely due to electron leakage).
Cree, a major manufacturer of high-brightness LEDs, offers the most devices that meet stringent U.S. Department of Energy (DoE) Energy Star performance criteria for use in fixtures or luminaires. The DoE has accepted Cree’s LM-80 test data for the company’s XLamp XR-E, XP-E, and MCD LEDs, allowing such products to be submitted to fixture manufacturers (Fig. 3). The Illumination Engineering Society of North America (IESNA) adopted the LM-80 standard as an “Approved Method for Measuring Lumen Maintenance of LED Light Sources.”
Marc McClear, director of Cree’s business development for solid-state lighting, says we shouldn’t expect to see much direct replacement of incandescent or fluorescent bulbs with LEDs in the same socket, at least not in the short term. It’s simply not practical to place a modern high-brightness LED into the same socket, since it dissipates a lot more heat than the bulb it’s replacing.
Instead, McClear sees more of a growth market in replacing light bulbs in existing can and overhead fixtures at the junction box in the ceiling, emphasizing that designers should think in terms of fixtures and heat and photon management, as well as light sources. He also foresees a large market for using highbrightness LEDs in outdoor facilities, street lighting, and architectural lighting.
“Two years ago, it simply would not have been practical to use LEDs for street lighting,” McClear says (see “High-Brightness White LEDs Light The Way To Greener Illumination”). “A year ago, the idea was barely into the trial stage. A few cities would replace the fixtures on one block of one street. This year, it’s full steam ahead around the world.”
Reflecting this fact is the growing LED City program initiative supported by Cree, the Lighting Science Group, Amtech Lighting Services, and Progress Energy. Launched in 1997 with a parking-garage light pilot program in Raleigh, N.C., the LED City community provides a guide to accelerating the deployment of high-efficiency LED lighting for cities worldwide. The growing number of participating cities includes Ann Arbor, Mich.; Austin and Fairview, Texas; Anchorage, Alaska; Tianjian, China; Torraco, Italy; Toronto and Welland, Ontario, Canada; Indian Well, Calif.; Chapel Hill, N.C.; and Gwangjiu, South Korea.
NEEDED: A SYSTEMS APPROACH
When using high-brightness LEDs for illuminating large areas, lamp designers must follow a systems approach that considers not only the light source and its package, but also the fixture it’s mounted in as well as optical and thermal issues. The issues of driving and powering LEDs are particularly important for applications that require many high-brightness LEDs for large-area illumination. It is more desirable to use multiple LED strings in parallel, which requires large current-capacity power supplies.
Often, disciplines other than familiar electronics, such as optics, photometry, and colorimetry, come into play. Factors like light wavelength (color) and the solid angle at which the light is dispersed call for a better understanding of how to measure optical power properties.
A good understanding of the color rendering index (CRI), sometimes called the color rendition index, is also important. CRI is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. Light sources with a high CRI are desirable in color-critical applications. The CRI by itself doesn’t indicate the color temperature of the reference light source. As a result, it’s customary to also cite the color-corrected temperature (CCT).
Commercially available LED products can be independently tested and compared using the DoE’s CALIPER (Commercially Available LED Product Evaluation and Reporting) program, which is available on the DoE Web site. This helps both consumers and manufacturers better evaluate products for their intended applications.
For a white light output, color temperature is paramount. As more LEDs find applications, users are becoming aware that these devices have a range of color temperatures from blue to yellow, within the white light spectrum emitted. Tighter binning by LED manufacturers helps minimize such artifacts, which can sometimes be undesirable.
Fortunately for LED luminaire designers, design tools are available to simplify their work. Future Lighting Solutions offers four comprehensive reference designs that reduce the time and cost of developing luminaires using Luxeon Rebel LEDs from Philips Lumileds. The applications include 4- and 6-in. recessed down lighting, low-bay lighting, and commercial refrigeration systems.
Cree’s interactive Product Characterization Tool (PCT) simplifies the task of translating LED performance to real-world conditions. Accessible via Cree’s Web site, it characterizes any of the company’s XLamp LED products over a wide range of operating conditions.
The adoption of energy-efficient, high-brightness LED illumination of large areas continues to expand worldwide. Besides a slew of outdoor applications, they’re also now found in indoor decorative and office lighting.
Typical of large-scale illumination projects that are using highbrightness LEDs, South Korea’s Wonju City employs 520 LEDbased street lamps with 37,440 Osram Opto Semiconductors Golden Dragon Plus LEDs (cool white) along the road from the Wongju interchange to the South Wonju interchange. These LEDbased lamps replaced traditional metal-halide 250- and 400-W street lamps.
The Golden Dragon Plus products are also used in mainland China’s Yangzi River tunnel for illumination. The luminaires, installed by Guangzhou Zhonlong Communications Technology Co. Ltd., illuminate the 8.9-km long twin-tunnel complex with an internal diameter of 12.7 m. The structure is one of the world’s largest tunnel projects.
A good example of large-area outdoor illumination can be seen at Raymond James Stadium in Tampa, Fla., where the National Football League’s Super Bowl XLIII championship game was played in February 2009. The stadium uses LEDs supplied by Philips Color Kinetics (Fig. 4a). The same LEDs are also found in decorative indoor illumination at the Mohegan Sun Casino and Resort in Pocono Downs, Pa. (Fig. 4b).
Other stadiums are planning to use large-scale LED displays. Argentina’s Multiled Corp. will soon install a 15- by 17-m fullscreen video display that uses LEDs in the country’s Rosario Central Football Stadium. It will measure over 100 square meters and become Argentina’s largest high-resolution screen. It consists of 1-m2, 65-kg SMT modules, each with a 12.5-m pitch and providing 1600-pixel/m2 resolution.
After a $27 million makeover last year, the Frederick Douglas Memorial Bridge in Washington, D.C., commonly known as the South Capitol Street Bridge, began using LEDs for illumination (Fig. 5a). The LEDtronic Beacon Illuminaire BSD-1928-001 LED fixtures consume 15 W each (a savings of 70% to 90% over conventional light sources) and offer 100,000-hour lifetimes.
LEDtronic also supplies the LEDs that light up Buckingham Palace’s center room (Fig. 5b). All 32 25-W tungsten lamps were removed and a low-voltage system controlling 2.8-W STL610- 48-XIW-024M LEDs was installed, initiating energy savings in excess of 80%.
THE THERMAL-MANAGEMENT CHALLENGE
Of all the technical hurdles facing the development of LED light sources, none is as challenging as proper thermal management with resultant long lifetimes. While a traditional incandescent bulb radiates heat outward in the form of infrared (IR) energy, an LED does the opposite. Its heat must be conducted away from the LED die, necessitating the use of heatsinks and other thermal management methods. The IR spectrum is not part of an LED’s output, nor is an LED’s ultraviolet (UV) spectrum.
This has very important implications for an LED’s lifetime. Maintaining an LED die’s temperature at 60°C allows it to emit at least 70% of its initial light output after 100,000 hours, an uncommon temperature for most high-brightness lighting applications. And if the die’s temperature reaches 100°C, that lifetime is cut down to just 30,000 hours or less. If anything, it’s more common for a high-brightness LED’s die to reach 120°C.
The type of package used, the materials and type of construction of the light source, and the heatsinking employed can influence how heat is managed and the light source’s lifetime. Fortunately, new materials like Honeywell’s LTM6300-SP are coming to the rescue. According to the company, this family of phase-changing thermal-management materials improves LED lifetimes by changing phase during use to more efficiently transfer heat away from the device. It replaces silicone-based pastes, which pump out and degrade at high temperatures, with a polymer matrix containing thermally conductive metals.
“This material enhances the performance of energy-efficient LEDs, extending their lifetimes and making sure their colors do not become muted due to overheating,” says Brian Daniels, CTO for Honeywell’s Electronic Materials sector.
The LED industry’s hype about 100,000- hour lifetimes is a bit exaggerated. Technically speaking, an LED might be able to emit light for 100,000 hours. However, its output would become so degraded after such time that it would not be considered a useful light source, especially for highbrightness applications.
“The technology for 100,000-hour lifetimes for high-brightness LEDs does not exist,” says Pat Goodman, an application engineer at Philips Lumiled Lighting Co. “We have published what is probably the longest database on lifetime data and it only goes out to 9000 hours.”
Nevertheless, the U.S. Department of Energy (DoE) is providing incentives for LED lighting improvements. Its Bright Tomorrow Lighting Competition will award a $10 million “L Prize” for the best LED replacement of a common 60-W A-19 incandescent bulb (see “$10 Million Light Bulb Prize Remains Unfunded” and “Beyond The $10 Million Light Bulb”). This competition was founded by the Energy Independence and Security Act of 2007. In fact, Philips Electronics has already submitted an entry.
There is a proviso, though. The winning device must perform similarly to the incandescent lamp it’s intended to replace in terms of color appearance, light output and distribution, lamp shape and size, form factor, and operating environment. It must also be reliable and available through normal market channels. And, it must be competitively priced.
Recently, the DoE’s National Technology Laboratory (NETL) selected four projects for solid-state lighting in its sixth round of funding. These awards will be used to examine high-priority R&D activities that will advance state-of-the-art solidstate lighting used for general illumination applications. Three of the projects are funded under the mandate of the American Recovery and Reinvestment Act, and one is funded by appropriation.