Images courtesy of Cree
The large energy savings potential using light-emitting diodes (LEDs) for general illumination is undeniable, and the adoption of LED lighting is increasing at a rapid pace. A number of LED lighting fixtures manufactured today exceed the efficacy and lifetime of conventional lighting technology in certain applications. While the performance and cost structure of LED lighting technology is evolving swiftly, we are just scratching the surface of innovations in LED fixture design that can improve the value proposition.
ANATOMY OF AN LED An LED is a semiconductor device that generates light. Those used for lighting are made by depositing indium, gallium, and nitrogen on a silicon carbide (SiC) or sapphire wafer to create the active layer or light-generating layer. Metal contacts are added to the semiconductor film and the wafer is then diced into many small LED chips, also called die. When an electrical current is applied to the chip, electrons and holes form and recombine in the active layer to produce light, which is generally blue or green for indium-gallium-nitride LEDs, the type of configuration used for creating white LEDs.
Contrary to an incandescent lamp, the evacuation of heat from the LED chip is of critical importance since heat in the LED chip drops its efficiency.
The LED chip is then mounted into a package where a phosphor is added to convert some of the blue light to other colors such as red and yellow. These new colors combine with the remaining blue light to produce a white light. Finally, the package is topped with a lens to help direct and shape the light for the intended application. The LED package provides protection for the LED chip from the outside environment and acts as a conductive path to carry generated heat away from the chip via a thermal pad.
The LED package is then incorporated into a light fixture, or luminaire, along with a heat sink for thermal management, a driver for power conversion, and secondary optics for tailoring the light distribution. The LED luminaire requires a system design approach that carefully integrates and balances the competing requirements of optics, mechanics, electronics, thermal management, and light-generation elements.
TRADITIONAL LAMPS VS. LED TECHNOLOGY LEDs and traditional light sources have very different features that affect how they perform. Incandescent lamps make light through heat. Electricity is passed through a tungsten filament, and the filament's electrical resistivity creates enough heat to make it glow. LEDs generate light through a process called electroluminescence, in which photons are generated by the recombination of electrons and holes in the semiconductor film. Contrary to an incandescent lamp, the evacuation of heat from the LED chip is of critical importance since heat in the LED chip drops its efficiency. LED lighting has a number of attributes that make it advantageous for many applications. Beyond their much-touted efficiency and longevity, a number of other attributes make LEDs desirable for many applications, including:
Taking advantage of the unique attributes of LEDs will allow new features and performance capabilities not available with traditional lighting. For instance, directionality is what makes LEDs advantageous in numerous applications. They can outperform traditional lamps in a recessed downlight since they focus light down to the target surface. None of the LED light is directed up into the fixture where it is wasted—as is the case with compact fluorescents.
DESIGNING FOR LEDs
Designing LED fixtures requires a choice between a complete luminaire based on LEDs or an LED-based lamp for an existing fixture. A complete luminaire design can best harness the LED characteristics by better balancing the competing requirements of optics, mechanics, electronics, thermal management, and the light engine compared to a retrofit lamp design. Designing for an existing socket form-factor limits the heat dissipation solutions, the space available for driver circuitry, and the optical design choices. Removing these constraints provides degrees of freedom to innovate and create greater value for the end-user through better performance, longevity, and cost.
Fluorescent troffer fixtures are a good example of how focusing on a fixture instead of a replacement lamp can provide performance benefits. According to the Department of Energy's CALiPER testing program, LED replacement products do not match the performance of the benchmark 4-foot linear fluorescent lamps. (See CALiPER Benchmark Report, January 2009: Performance of T12 and T8 Fluorescent Lamps and Troffers and LED Linear Replacement Lamps, bit.ly/nwH4KN.) The CALiPER program did show that 2-foot by 2-foot LED recessed troffers outperformed their fluorescent benchmarks. New 2-foot by 4-foot LED troffer replacement products entering the market also outperform their fluorescent counterparts, while the available LED replacement tubes do not demonstrate the same level of performance. This example illustrates how freedom from traditional form factors can provide improved product performance and added value for the end user.
A smart system design is critical when trying to optimize performance and cost. A smart fixture design should tie in some of the necessary fixture elements, such as the trim in a recessed downlight, to improve the system performance. In the recessed downlight example, the downlight trim can use more of its surface to dissipate heat from the LEDs while maintaining the aesthetics of the fixture. This integration of functionality into the essential fixture elements can keep costs down too.
Some of the features of LEDs also provide the opportunity to improve the function of various luminaires. The compact nature of the LEDs can provide slim, low-profile fixtures for applications such as undercabinet lighting or parking-garage lighting, that reduce the fixture size seen with traditional technology.
Some attributes of LEDs can also provide new opportunities to tailor light output in ways that would not be practical with conventional technology. In addition to the ease of controllability for smart lighting controls—such as occupancy sensing, daylight harvesting, and demand-response—the ability to tune spectral characteristics can provide new functionality. The ability to adjust the color distribution of light or the intensity of a particular part of the spectrum can have impacts in health and productivity. It can also give rise to adaptive lighting, where this tuning is used to create different lighting environments.
To access the full potential of LEDs for general illumination, the unique and superior properties of LEDs must be considered and designed into LED fixtures. LED products that fit existing lighting sockets are not able to use LEDs to their full advantage. Breaking away from these traditional form factors will help unlock the technology's full promise.
Binning: What's All the Fuss?LED manufacturers have to manage the variations in LED properties during mass production to provide device performance that is repeatable. Packaged LEDs are sorted based on key properties such as luminous flux and chromaticity, creating “bins” in which LEDs are sold.
Much of the headache with binning can be eliminated at the LED package level. Manufacturers have sophisticated systems to mix and match LED chips and phosphors to produce multichip packages that result in very tight color control within a two-step MacAdam ellipse. This is comparable with that of incandescent bulbs, which is the highest bulb standard for color consistency. These types of LED packages eliminate the need for chromaticity binning, thus enabling luminaire and lamp manufacturers to deliver consistent color with ease. Some LED luminaire manufacturers choose to do this mixing and matching themselves with individual LED components, but many luminaire makers rely on the LED manufacturer's expertise in producing color-consistent LED packages for ease of luminaire manufacturing. These tightly binned LED components essentially eliminate the issue of chromaticity binning for most of the SSL value chain.
Another type of binning that has gained prominence involves the “hot/cold factor,” which compares the relative light output of the LED package at a high temperature. In addition to the change in light output, the chromaticity also shifts as a function of temperature. Some manufacturers have begun to offer hot binning, where the package performance is measured at high temperature. This is an attempt to help the customer better understand what the expected performance of the package will be at the steady-state operating temperature of the luminaire. The challenge comes if the customer's LED luminaire does not run at the same temperature and operating current that the LED manufacturer uses for hot binning. The luminaire maker still needs to scale the LED package's performance for their fixture design. It is also essential that LED manufacturers publish data on how the LED light output and color point shifts with increases in temperature and drive currents. This way, a luminaire maker can determine the performance of the LED package at their given luminaire operating conditions. Many manufacturers have design tools to help customers estimate LED output at their operating conditions.
Monica Hansen is a research scientist and contracts manager at Cree, where she concentrates on research related to general illumination applications including managing Cree's government contract research on solid-state lighting. During her 12-year tenure at Cree's Santa Barbara Technology Center, she has led an R&D team developing gallium nitride laser diodes and managed the metal organic chemical vapor deposition growth laboratory before moving into her current position. She holds a doctorate in Materials Science from the University of California, Santa Barbara, and a bachelor's in materials science and engineering from the University of Arizona.