LEDs may be the first real technological revolution in lighting in one hundred years, but can they provide enough optical power to be truly practical?![](/ProductImages/0405/Fig-1a-high-res.jpg")
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The OSRAM 6-lead MULTILED (left) is a single package with individually addressable red, green, and blue LEDs. By controlling the output of each source, any color of the visible light spectrum can be synthesized. A section of LINEARlight LED tape from OSRAM (right) demonstrates the exceptional flexibility surrounding fixture design with LEDs. |
The lighting industry represents an extremely diverse application space, but can be thought of as having two broad classifications: informational, in which a light source is directly viewed (e.g. signage, indicator lights, displays, etc.), and illumination, wherein the source provides light by which something else is viewed (e.g. task lighting, general illumination, and street lights). For roughly 40 years, light-emitting diodes (LEDs) have been used almost exclusively for informational applications, such as indicator and status lights, while advances in the last decade have made it feasible to use LEDs in some illumination applications as well.
An LED is a semiconductor device wherein electrical energy is directly converted into optical energy, within a very narrow wavelength range. Until 1993, LEDs were available in only a limited number of colors: red, yellow, and green. In that same year, while at Nichia Corp., Tokushima, Japan, Shuji Nakamura developed a practical method of fabricating a blue LED. According to Makarand Chipalkatti, Director, North America LED Lamp Modules for OSRAM Opto Semiconductors, Danvers, Mass., "the development of the blue LED was a key advance in solid-state illumination. It now became possible to create virtually any color in the visible spectrum by combining separate, narrow-spectrum red, green, and blue sources." Bill Ryan, LED Group Project Manager at Philips Lighting, Somerset, N.J., agrees. "It enables us to provide light over the entire range of daylight from sunrise to sunset."
Control of the emission spectrum has literally been turned inside out. A conventional light source has an emission spectrum that is hundreds of nanometers wide centered on a characteristic wavelength. Phosphor coatings are added to convert emission from outside the visible spectrum into light within the visible spectrum to achieve improvement in either color temperature or color rendering. "Since LED lighting is a synthetic process in which you insert just the light you use, thus there is no wasted light," says Chipalkatti.
Scotty, we need more power!LED sources have several inherent advantages over conventional light sources such as incandescent, fluorescent, and halogen bulbs. Typically, they have much longer lifetimes and higher efficiency than conventional light sources, as well as higher optical power density, lower voltage and power requirements, much smaller form factor, flexible packaging, and greater robustness. These attributes have all contributed to their incredible growth in informational applications. But due to low luminosity and a higher cost, their adoption into illumination markets has been somewhat limited. "For LED illumination to begin to displace conventional sources in illumination applications, the amount of light produced by LEDs needs to increase and the cost per lumen needs to drop," says Philips' Ryan.
Using a comparison of $/lm/hour, the long lifetime characteristic of an LED will help to defray some of this cost since it is replaced far less often than a conventional light source.
As total luminosity continues to improve over the next decade, the LED has the potential to be to the conventional light source, what the transistor was to the vacuum tube. Moreover, according to Ryan, "there exists a clear path to higher luminosity in the future. The goal of a 100 lm/W LED by 2008 looks very achievable." The keys to higher output power include improved wafer starting material, better epitaxial growth, improved packaging, and most importantly, more efficient heat sinking of the LEDs.
OSRAM is also pursuing an alternative approach to higher luminosity, although at the expense of efficiency and color temperature agility. Light from blue LEDs is used to excite yellow phosphors, and the blue and yellow light combines to yield white light. Automobile headlights based on this approach have been included in the HCD-8 concept car from Hyundai Motor America, Irvine, Calif.
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LED sources can be used to provide virtually any color temperature, which enables the user to achieve the desired effect for any setting.
(Photos: Lumileds Corp., www.lumileds.com) |
LED illumination"Although not there yet, the LED has real potential to become a disruptive technology, because of its high lumens per cubic millimeter, control of directionality, and the simplicity they can bring to fixture design," says Tom Russello, VP of Engineering and Product Development for Stonco/ Crescent/Exceline, Union, N.J. The impact of LED technology depends on the application in which they are used, such as high-intensity discharge (HID) systems (roadway, security, and landscape lighting), fluorescent (retail spaces and display lighting), or signage (neon signs and decorative). Currently, replacement of neon sources in signage applications is a high-growth area for LEDs.
While for HID systems the optical power output isn't high enough to be feasible yet, the potential of being able to replace complicated, bulky fixtures with simple, light panels of discrete LED sources is very attractive. "It is likely that there will need to be change in the way we design these systems," says Russello. "Currently the question is how they may be used in replacement systems for the fixtures already out there, but given the freedom offered with LED packaging, we may have to reconsider the way we think about fixture design."
City Lights
Cities across the U.S. have embraced LED-based traffic signals, pedestrian signals, and other signage. The case for LED lighting in these applications is compelling. In New York City, for example, over 10,000 new signalized intersections use pedestrian signals powered by LEDs. Not only are the signs brighter, less expensive than incandescent, and more pedestrian-friendly, but according to the Mayor's Office, they will also save the city over $6 million in energy costs and last nearly 10 years longer than their traditional counterparts.
Similar projects are being considered or implemented in cities throughout the country. "In addition to the obvious benefits, the municipalities have reduced exposure to liability since the frequency of signal outage is lower," says Bill Ryan of Philips Lighting. "When a signal is out, it is dangerous for the public as well as for the crew sent to repair it."
>>More info: www.nyc.gov |
The lower luminosity requirement in many task-lighting applications has enabled LED sources to make some impact already, even though they are expensive by comparison to alternatives. For example, light bars containing multiple LEDs are replacing fluorescent tubes in many retail lighting settings. Display lighting with LEDs gives the merchandiser the ability to create any color temperature and color rendering (the effect light has on illuminated colors) to favorably display merchandise. Additionally, since each LED could be controlled independently, a single fixture could be used to produce different light-ing zones.
As an example, a jeweler may want to "tune" lighting slightly toward a particular tint in order to help make their diamonds "sparkle," while the pearls next to them in the display may show better using a slightly different illumination. Similarly, warmer or colder tones help make items such as clothing, meats or produce, look more attractive to the consumer. "The ability to 'dial-a-mood' can have a very real impact on merchandizing," adds Russello. Examples of controlled mood lighting can already be found in "archi-tainment" applications, such as in restaurants and dance clubs.
Behold the futureAs LED sources continue to carve out space in the decorative and informational areas, ongoing work to improve total luminosity continues. Higher power output LEDs for illumination applications are also on the horizon. It will be exciting to witness the transformation in lighting that will result as the flexibility of LED packaging unlocks the creativity of lighting designers.
James WalkerFounder and President of JayWalker Consultingwww.jaywalkerconsulting.com
