The lighting industry has been buzzing with news of solid-state light source advancements for several years. As one light-emitting diode (LED) lighting fixture manufacturer puts it, flat printed circuit boards versus bulbous or tubular linear sources open up a whole new world of design possibilities. Now, the LED and its sister technology, the organic light-emitting diode (OLED) — two light sources that constitute solid-state lighting — have the potential to replace many light sources that either heat a tungsten filament to incandescence or use a pair of filaments within a glass envelope to create an ionized arc stream.
What are LEDs?
Light-emitting diodes are tiny devices made from semiconductor materials that convert electrical energy into visible and near-UV wavelengths — and some heat — when they are assembled in a package and connected to an electrical circuit. Specifically, the semiconductor materials are crystals comprised of combinations of two or three elements, such as gallium phosphide (GaP) or gallium indium nitride (GaInN). These unique combinations of elements have distinctive crystalline structures that can accommodate both electrons (negatively charged) and holes (positively charged electron vacancies), which exist at different energy levels, separated by a “band-gap.”
Generally called a die or chip, which can vary in size from tenths of a millimeter to more than a square millimeter, the LED (diode) permits current to flow in only one direction. This diode is formed by bringing together two slightly different semiconductor materials, called layers — an n-type layer that has an excess of negative charge (electrons) and a p-type layer that has an excess of positive charge carriers (holes), which are locations for the electrons to fall into. Electrodes are placed on each end of this assembly, or structure. The junction or interface of the two layers (called the p-n junction) is where electrons and holes are injected into an active region.
When a forward voltage is applied to this structure (negative to the n-layer and positive to the p-layer), electrons move from the n layer toward the p layer, and holes move toward the n area. Near the junction, an electron and a hole radiatively recombine, emitting a photon (essentially, the electrons move across the p-n interface and fill holes on the p side, falling into a lower energy level). Ideally, the excess energy from each electron's transition results in the spontaneous emission of a photon. In practice, however, a number of things happen to reduce efficiency — and only a fraction of the electrical energy is converted into useful (generally visible) light.
The energy of the photons, and thus the wavelength, is determined primarily by the energy band gap of the semiconductor, where the recombination occurs. The best aluminum indium gallium (AlInGa) LEDs (red and amber) offer 40% to 50% electrical-to-optical efficiencies. The best indium gallium nitride (InGaN) LEDs (UV, blue, green and white) achieve 30% to 35% conversion efficiencies.
Where are they being used?
Over the years, these tiny diodes have improved in color range, lumen output, color stability, lifespan, and other performance factors, allowing LEDs to encroach on, and dominate, many niche lighting markets from indicator lights and traffic signals to exit signs and decorative/ architectural lighting.
Because a single LED is comparatively small, it can be assembled into an array of dozens or even hundreds of dies on a panel or a strip. Therefore, an LED “fixture” can be any size or shape. As a result, solid-state lighting has the potential to reduce complexity in the selection, design, and installation of lighting fixtures. Traditional and newly emerging lighting fixture manufacturers are actively studying applications in which LEDs are suitable and then working backward to develop fixtures that can be easily mounted, effectively maintained, and economically controlled.
For suppliers striving to introduce LED lighting into the general illumination market, the biggest challenge is to provide high lumen output from dies that produce white light. For electrical engineers and installing electrical contractors, the challenge is to learn more about this technology so that they can specify and install high-quality, specification-grade products to the satisfaction of demanding clients.
The standards game
At present, virtually no standards for measuring, reporting, testing, or applying LEDs exist, but overly optimistic or irrelevant claims about these emerging LED products could severely curtail market acceptance. Fortunately, many organizations, believing that LEDs offer tremendous market potential, are cooperating in standards development.
For example, the Illuminating Engineering Society of North America (IESNA) and the Next Generation Lighting Industry Alliance (NGLIA) are working with the U.S. Department of Energy (DOE) Building Technologies Program to achieve a number of goals, including developing procedures for photometric measurements of LEDs, laser diodes, organic LEDs, and any other semiconductor light sources of the future.
In 2007, Underwriters Laboratories Inc. (UL) will publish UL 8750, “Outline of Investigation (OOI) for LED Light Sources for Use in Lighting Products.” When released, this OOI will be applied in any investigation of the LED light sources used in UL-listed lighting products. According to UL, in terms of fire risk, the temperature rise exhibited by LED systems might bring them close to the limit of 90°C — the maximum temperature permitted on surfaces under the building codes within the U.S.
Obstacles to overcome
Before LEDs can enter the mainstream lighting arena, the following issues must be addressed:
- Useful life
This is probably the most misunderstood variable in defining performance. Having a passive end of life, the light output of an LED diminishes over time. Some manufacturers may state a 50,000-hour useful life, with 70% lumen maintenance, while others may boast of a 1,000,000-hour life without stating the lumen maintenance at the end of this time period. Thus, one possible definition of “lifetime” is the number of hours required for the source output to decline to a certain percentage of its initial output. Many specification-grade LEDs achieve 70% lumen maintenance after 50,000 hours of use under standard operating conditions.
- Lumen output
Generally, standard lamps used in the lighting industry have lumen ratings for initial and mean lumens that are consistent within a manufacturing run of the product. However, LED lumen output can vary by as much as 20% — from one die to another — in a typical manufacturing run. Additionally, the lumen output of a group of LEDs when assembled into a fixture can have a reduction in lumen output because the fixture construction design restricts heat dissipation, and the LED operates at a junction temperature higher than optimum. Unlike an incandescent bulb, which sheds its heat radiatively, an LED has to displace heat by convention and conduction. Therefore, if the LED die junction does not operate below its maximum rated temperature during operation, both light output and life are reduced.
Although LEDs can produce every color in the rainbow, the most desirable color for general illumination is white, which is achieved in two ways. The first method is called color mixing, using a combination of RGB (red/ green/blue) LEDs, which produces a white light with little variance in Kelvin color temperature. However, because of the high cost of this technique, it is usually restricted to entertainment and special architectural highlighting applications. A second method is called phosphor converted blue (PCB). A blue LED is complemented with a yellow phosphor to create a cool white light with a typical color temperature of 5,500K. By adding a secondary red phosphor, a warm white color with a color temperature of 3,200K is gained. The third method is ultraviolet wavelength conversion, in which a single ultraviolet LED is used to excite a tri-color phosphor coating, which, in turn, produces visible “white” light.
Although all of the white LED products have deficiencies, manufacturers are constantly working on these shortcomings. For example, all “white” LEDs suffer from a color shift over time, and color mixing using various color LEDs in combination has problems because different materials degrade at different rates. In addition, manufacturing methods may also influence depreciation in the same basic color. One way manufacturers work to overcome this problem is to “bin” their LEDs, which means they inspect and separate the individual dies into batches of products that have similar initial appearance and lumen maintenance characteristics, so they will have a consistent appearance in operation.
- Heat dissipation
One of the biggest misconceptions about LEDs is that they are cool light sources, perhaps because the LED doesn't generate infrared energy. However, unlike an incandescent bulb, which sheds its heat radiatively, an LED has to dispel heat by convention and conduction. If the LED die junction does not operate below its maximum rated temperature during operation, both light output and life are reduced. Additionally, because the light output of an LED for a constant current varies as a function of the junction temperature, some system manufacturers have a compensation circuit that adjusts the current through the die to keep a constant lumen output at various ambient temperatures. Specification-grade LEDs use a heat sink with metal fins, or wings, for proper heat transfer, which is an important factor in fixture design.
- Photometry measurement and reporting
Currently, each manufacturer creates its own photometry guidelines. The data presented can be the result of a conservative judgment on the lumen output of an LED fixture, or the data can be effectively overstated. As part of its Commercial Product Testing Program (CPTP), the DOE's Solid-State Lighting initiative recently had various laboratories test four LED luminaires, selected to represent a range of designs and manufacturers. The results indicate that the efficacy of recently made LED luminaires was significantly lower than the efficacy values for the LEDs used. As a result, the report expresses concern that, in these early stages, product performance data could be misleading.
The road ahead
Because LED products can take on a variety of sizes and shapes, they offer tremendous opportunities for innovative lighting solutions in both interior and exterior lighting design. Rather than distributing light from a single, bright source within a fixture, LEDs can place light across a surface or deliver it in multiple planes. They can be integrated into architectural materials, such as concrete, and they can edge light glass or plastic panels.
Rugged and void of catastrophic failure, solid-state lighting devices can operate on low-voltage DC power or directly on AC power. They can start instantly at temperatures as low as -40°C, and are easily dimmed and controlled without any distracting flicker. The absence of infrared and ultraviolet energy radiation makes LEDs attractive in many applications. Moreover, they are environmentally friendly, meeting new ecological regulations that ban mercury and lead from the waste disposal stream.
In an effort to boost solid-state lighting use, the DOE is extending its Energy Star program to LED-based luminaires. Energy Star is a voluntary labeling program designed to identify and promote energy-efficient products. This draft document defines eight designated correlated color temperatures (CCT) values. The LEDs must have a useful life of 35,000 hours based on average rated lumen maintenance of at least 70% of initial device lumens. Also included are definitions of color spatial uniformity and color maintenance over lifetime, as well as driver, packaging, and warranty requirements for luminaires.
Outdoor lighting may be the first general illumination application to gain a foothold — and for specific economic reasons. One light source manufacturer has developed an LED street light with a “light engine” having an output of 80 lumens per watt. Considering the expense of relamping a typical streetlight using an HID lamp with a typical life of 15,000 to 25,000 hours, the mean time to lamp replacement could be greater than three times that life. Thus, the added initial cost of the LED luminaire is recovered over time in a reduction of lamp maintenance expenses.
As government regulations continue to restrict the use of less efficient light sources, LED development will continue to thrive.