LED Report

Light-emitting diodes, emerging as the light source of the future, offer an unlimited array of choices in lumen output, color and form.

The history of lighting is a saga of discovery, upheavals and sometimes unexpected twists and turns as product developers seek to create better products. The first lighting system for buildings and streets burned natural gas in a ventilated glass enclosure. When Thomas Edison produced the first practical incandescent lamp, improvements in gas lighting followed, with the use of mantles soaked with the rare earth compound thorium oxide, which converted the gas flame's heat energy and ultraviolet radiation into visible radiation. However, these improvements were not sufficient to make a flaming wick the lighting technology of choice.

Incandescent lighting eventually won out over gas lighting because of the obvious advantages of convenience in the installation of electrical power wiring and the use of a switch to control operation. In the 1940s, fluorescent systems and high-intensity discharge (HID) lamps, in many sizes and wattages, began offering longer life and lower power consumption than incandescent sources.

Light-emitting diodes (LEDs) and a sister technology, organic light-emitting diodes (OLEDs), are now on the cutting edge of new product development in the lighting market. These technologies make up the field of solid-state lighting (SSL). LEDs and OLEDs are poised 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.

Incandescent and arc-discharge light sources have filaments or cathodes that break or burn out, and they have numerous other characteristics that inherently limit their lumen-per-watt efficacy (efficiency of a light source as measured by lumens per watt). However, the efficacy of LEDs is not limited by the same fundamental factors as traditional light sources, but instead seems to be almost limitless, considering the constant new discoveries and improvements in the field.

In 1962, when Nick Holonyak Jr., working at General Electric, invented the first practical light-emitting diode operating in the red portion of the visible spectrum, he predicted these semiconductor devices would transform the lighting industry. Now a professor at the University of Illinois in Champaign, Holonyak continues to watch LEDs encroach on, and then dominate, many lighting applications such as indicator lights, traffic signals, exit signs and decorative/architectural lighting. Improvements in lumen output, color stability, longevity and other performance factors are continually unfolding.

What is Solid-State Lighting?

Solid-state lighting refers to devices made from semiconductor materials that convert electrical energy into visible and near-UV wavelengths and some heat. These devices are assembled in a package called a “die.” The semiconductor materials are crystals that usually combine 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) that exist at different energy levels, separated by a “band-gap.”

These devices, generally called “dies” or “chips,” can vary in size from tenths of a millimeter to greater than a square millimeter. Technically speaking, the LED is essentially a diode that permits current to flow in only one direction. The diode is formed by bringing together two slightly different semiconductor materials, called layers. The n-type layer 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, several 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 bandgap of the semiconductor, where the recombination occurs. The best aluminum indium gallium (AlInGa) LEDs (red and amber) offer 40 percent to 50 percent electrical-to-optical efficiencies. The best indium gallium nitrides (InGaN) LEDs (ultraviolet, blue, green and white) achieve 30 percent to 35 percent conversion efficiencies.

Because the design and construction of the die and its packaging are continually improving, the useful light output of a typical LED is increasing. In addition to increased size, LED construction has also changed to make these devices more efficient. The crystals forming early LED junctions were grown on light-absorbing substrate materials. Now, by using transparent substrates and optimizing the shape of the semiconducting element, the amount of light able to escape the die is increased. Other improvements include nanoimprint lithography equipment for making LEDs with photonic crystal structures. These are two-dimensional diffraction gratings that enhance the extraction of light from the LED surface.

Early LEDs, such as those used as indicator lights on electronic equipment, created very narrowband, but not quite monochromatic, light ranging in color from yellow-green to red. But with the development of aluminum gallium indium phosphide (AlGaInP) and InGaN, LEDs were developed that had much higher light output than the early indicator lamps. For the first time, these materials created LEDs with peak wavelengths at any part of the visible spectrum.

Although LEDs can provide every color in the rainbow for attention-getting designs in decorative, architectural highlighting and entertainment venues, the biggest goal now is the development of high-brightness LEDs (HBLED), especially those that produce visible white light. If LEDs can produce high-quality, long-lasting white light at a cost competitive to conventional lamps, they may very well replace conventional lamps in many applications. LEDs in the “power” segment, from 350mA upward, will become the products that can replace fluorescent and HID luminaires in the future.

White light is achieved in three different ways. The first method is color mixing, using individual LED colors: blue and yellow to create a dichromatic white source; red, blue and green LEDs to create a trichromatic white source and blue cyan, green and red LEDs create a tetrachromatic white source.

A second method is binary complementary wavelength conversion. A blue LED is complemented with a yellow phosphor to create 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 achieved.

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.

All of the white LED products have deficiencies, but the manufacturers are constantly working on these shortcomings. For example, all “white” LEDs suffer from a color shift over time. Also, color mixing using various color LEDs to create a combination that emits a white light has quality issues over time, because different materials degrade at different rates. In addition, the manufacturing methods used may also influence depreciation in the same basic color.

Manufacturers work to “bin” their LEDs, which means that they inspect and separate the individual dies into batches of products that have similar initial appearance and lumen maintenance characteristics. This means they will have a consistent appearance in operation.

Typical phosphor-based white LEDs have color rendering index (CRI) values comparable to discharge lamps (fluorescent and HID lamps). Many people mistakenly believe that a high CRI means high color rendering properties. Actually, the CRI is merely an index of how similar a light source makes colors appear in comparison to a reference source such as incandescent. That's why an incandescent lamp has a CRI near 100. Recent studies show that mixed-color “white” LED systems with a CRI in the 20s can result in higher color preferences than systems with a CRI in the 90s. Recognizing the limitation of the current CRI rating, international standard-making groups are now developing new measurements to better characterize the color-rendering properties for all light sources, including LEDs.

The LED source can also be dimmed by reducing the forward current. This is accomplished by rapidly switching the forward current on and off using pulse width modulation. By adjusting the relative duration of the pulse and the time between pulses, the total light output of the LED is reduced without the distraction of flickering.

One of the biggest misconceptions about LEDs is that they are cool light sources, because the LED doesn't generate infrared energy. But, unlike an incandescent bulb, which sheds its heat radiatively, an LED must dispel heat by convection 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, since 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.

LED system efficiency is defined by the luminous flux (lumens) produced by the system power input (Watts) and is expressed in lm/W. While LEDs are considered to be reliable and to offer long life, the lighting industry has no agreement on the definition of an LED source or the useful life of the LED. 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 percent lumen maintenance after 50,000 hours of use under standard operating conditions.

Traditionally, a lighting specification involves the selection of a fixture after comparing the data sheet specification of a number of products. However, rather than being an off-the-shelf item that is catalog ordered, the LED die is only one component of the luminaire.

A typical solid-state lighting module, or fixture, can consist of: a die; a circuit board for mounting the die; a heat sink for thermal management; a power supply or driver; primary and secondary optics; and other physical and mechanical features, such as a mounting frame and a flat lens, or diffuser. This makeup of components may change in the future.

Since an LED product can take on a variety of sizes and shapes, they offer tremendous opportunities for innovative 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 the light 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, they operate on low-voltage DC power, and also directly on AC power. They can start instantly at temperatures as low as -40 degrees Celsius and are easily dimmed and controlled. The absence of infrared and ultraviolet energy radiation make LEDs very desirable in many applications. They are also environmentally friendly, meeting new ecological regulations that ban mercury and lead from the waste disposal stream.

The road ahead

According to the U.S. Department of Energy and manufacturers and researchers in the lighting community, solid-state lighting is here to stay. Companies such as Lumileds (Philips/Agilent Technologies), Gelcore (GE), Osram OptiSemiconductor (Osram/Siemens), Nichia, Cree, Uniroyal Optoelectronics, PerkinElmer Optoelectronics, Toshiba, Seoul Semiconductor and Panasonic are developing LEDs, and are constantly achieving improvements in performance. These firms believe LEDs and organic light-emitting diodes will replace incandescent, halogen, fluorescent and HID sources in just about every application in ten years or less.

Manufacturers, specifiers and end users have a stake in the creation of useful solid-state lighting standards. The characteristics to measure, the methods for measurements and the qualified laboratories for accreditation have yet to be identified and agreed upon. The parameters to be measured include: luminous flux, efficacy, useful life, color rendering index (or an updated replacement) light distribution and binning.

Many organizations are cooperating on developing these standards. For example the Illuminating Engineering Society of North America (IESNA) and the Next Generation Lighting Industry Alliance (NGLIA) have joined forces with the U.S. Department of Energy (DOE) Building Technologies program to achieve a number of goals. One of the goals in the Memorandum of Understanding (MOU) between the DOE and the IESNA is creating guidelines and procedures for photometric measurements of LEDs, laser diodes, organic LEDs and any other semiconductor light sources of the future.

NGLIA and the DOE are working to develop Energy Star criteria for white solid-state lighting products, which are divided into two categories. Category A includes limited-scale applications such as task lighting, recessed downlighting and wall washing. Category B covers general illumination.

The author has written about lighting for Electrical Construction & Maintenance and Electrical Wholesaling magazines for more than 40 years. He also covers the voice-data-video market. You can reach him at (718) 224-4252 or by e-mail at [email protected].

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