Led lighting outdoor design challenge dec2013


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Led lighting outdoor design challenge dec2013
LED Magazine Editorial Digest

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Led lighting outdoor design challenge dec2013

  1. 1. SPONSORED BY: 2 Five rules for designing roadway lighting 9 LED modules bring energy savings to high-mast outdoor lighting 15 Advanced thermal characterization improves LED street- light design EDITORIAL DIGEST Outdoor lighting challenges SSL component and system designers Although outdoor lighting can benefit from the advantages of solid-state lighting — such as lifetime, reliability, and energy efficiency — the application presents particular challenges: the high lumen output required by high- mast lighting, environmental conditions, and avoiding inefficient distribution of light, to name a few. This digest will address how outdoor lighting system designers can apply thermal management techniques and materials to improve resistance to ambient conditions, and integrate LED sources that meet the mechanical design and luminous efficacy requirements of a changing outdoor environment.
  2. 2. LEDs Magazine :: EDITORIAL DIGEST 2 * This article was published in the April 2013 issue of LEDs Magazine. Five rules for designing roadway lighting Effective use of LED sources and emerging knowledge of human visual systems guide best practices for SSL roadway lighting. O F THE MANY design challenges facing LED-based solid-state lighting (SSL) applications, perhaps there is none greater than that of expectations. There are expectations around the application. There are expectations around the incumbent technology. There are expectations around the way it has always been done, and, as a result, there are expectations around the way it should be done going forward. What if we were able, however, to design with a clean sheet of paper? Take roadway lighting as an example. If we were to take that application, deconstruct it, and come at it from a different angle, what might we do differently, and how are LEDs specifically suitable tools in this redesign? When we think about the job of lighting a roadway, we are conditioned to think about what is happening right in front of us. We think about targets in the road and response time in identification. In fact, the entire series of metrics for roadway lighting is modeled around these requirements. From this Fig. 1. A driver’s view of a simulated roadway scene, illustrating conventional forward auto lighting combined with traditional roadway lighting.
  3. 3. Five rules for designing roadway lighting 3 LEDs Magazine :: EDITORIAL DIGEST standpoint, our examination of roadway lighting is fundamentally no different than our examination of office lighting. The conditions and demands of the tasks, however, couldn’t be more different. Rule #1: Zoom out and consider the bigger picture. Once we step back, one of the things we can appreciate regarding roadway lighting is that we are invariably talking about night-time situations. While the human visual system has an amazing ability to tolerate a wide range of conditions, the mechanisms that allow for those ranges vary for different lighting levels — night-time environments especially. To better appreciate how those mechanisms come into play, we need to consider the retina and its component parts. The retina is incredibly complex, but its basic role can be summarized by two types of photoreceptors: cones and rods. Cones are located predominantly in the center of the retina in the fovea. Rods, which greatly outnumber cones, surround the fovea and encompass the periphery of the retina. The retina is in simplest terms a camera. It produces images for the central nervous system (CNS) to interpret. The CNS-to-photoreceptor pathways best define the photoreceptor’s role in vision. Each cone, in effect, has its own direct path to the CNS. A quanta of information is personally escorted to the brain for processing. This one-to-one relationship defines its role in higher order perception such as fine detail discrimination and color analysis. The peripheral vision pathways to the CNS are shared by large groups of neighboring rods. Light that grazes one edge of the group triggers a response on the far edge. Through this mechanism, rods preform their basic role of gross peripheral motion detection. Fig. 2. A driver’s view of a simulated roadway scene, illustrating asymmetrical forward lighting for objects on the roadway combined with peripheral roadway lighting for detecting objects near the road.
  4. 4. Five rules for designing roadway lighting 5 LEDs Magazine :: EDITORIAL DIGEST Using night-time driving as an example of the mechanism, our eyes are directed for the majority of time at the roadway, where the cones are aiding in the analysis of detail. When something appears in the periphery, say a deer approaching the shoulder of the road, this sight registers across many groups of rods, signaling movement to the CNS. At this point, the eyes move and perhaps the head pivots, so that the cones can be engaged for better detail analysis and subsequent reaction. Rule #2: Appreciate the importance of peripheral detection in night-time driving. Our current metrics are concerned with foveal vision exclusively, yet the fovea takes up a tiny percentage of the visual field. We essentially light the road as depicted in Fig. 1. Mark Rea, director of the Lighting Research Center and professor at Rensselaer Polytechnic Institute, has written extensively on the subject. Rea has said that considering just the fovea in driving is akin to driving while looking down a long, narrow tube. Given the choice, would we choose the field of vision on the inside of the tube or the outside in order to drive? While what is inside the tube is important, this example illustrates that the outside of the tube — our peripheral vision, at the very least, deserves some consideration. While rods work in groups, they are individually much more sensitive to light than cones. Able to absorb and register even a single photon, one immediately sees their advantage in night-time conditions. Indeed, as light levels drop, the rod-to-cone activation ratio increases until rod sensitivities are at a peak level in night-time conditions. Rule #3: Consider the different sensitivities of the photoreceptors. Where the spectrum of light is concerned, the rods and cones respond similarly to higher wavelengths. Rods are, however, much more sensitive than cones to lower wavelengths, especially after they have time to adapt to night-time conditions. If one of our goals is to optimize the lighting to better aid in peripheral target detection, we should be working with a spectrum that is optimized to that task and optimized to the photoreceptors (rods) engaged in that task. Rule #4: Eliminate double work. Regardless of the importance of peripheral vision, we still need cones for sign identification/reading and analysis of detail in the roadway. The metric that
  5. 5. Five rules for designing roadway lighting 6 LEDs Magazine :: EDITORIAL DIGEST matters, just as in office lighting, for example, is contrast. How do we present the task in proper relief? Strong forward lighting (such as provided by car head lamps) with narrow optics will optimally illuminate the vertical plane and present a snappy, sharp shadow with an excellent dichotomy between light and dark. Current roadway metrics, mostly concerned (again, like office lighting) with horizontal illumination, don’t even consider the vertical plane. As written, the application requirements only consider overhead lighting, which can have a deleterious effect on contrast when combined with forward lighting on cars. Roadway lighting needs to complement forward lighting on automobiles and aid in the creation of contrast and clear, decipherable indicators to which our CNS can respond. Rule #5: Light the edges. More importantly, however, is the ability to identify hazards prior to them being in the roadway. Rea has suggested, only partially in jest, that better viewing conditions may be gained by simply pivoting roadway lighting 180o in order to light the shoulder (Fig. 2). The job of lighting the roadway is then left to headlights. The optimal solution is most likely a combination of that approach and current practices, but the clues are there. The issue with incumbent technology in roadway applications is the one-size- fits-all limitations. We start with a high flux, high wattage, omnidirectional light source, and we attempt to corral the beam to meet the application. The approach is inherently inefficient from an optical perspective. There is no opportunity for nuance or spectral shaping. Fig. 3. Cree XSP street lights installed in Hollywood, CA, focus light on the roadway, limiting back light.
  6. 6. Five rules for designing roadway lighting 7 LEDs Magazine :: EDITORIAL DIGEST SSL in roadway lighting With LED point sources, we build a fixture piece-wise until we have the perfect distribution — no more; no less. As Fig. 3 shows, SSL fixtures can be designed to produce almost no light behind the poles. Through proper binning, we are able to spectrally shape the output in order to best match the visual needs. In the example we have been using for roadway lighting, we can imagine many different designs or a combination of attributes in one package. We could have a component of the beam that lights the shoulder and surrounding areas of the roadway for the optimal spectrum of the rods. We could concurrently light the roadway with another spectrum ideal for foveal vision and contrast. We could have peripheral lighting that stays on constantly in rural settings or in areas of high deer traffic. Conversely, thanks to SSL instant start capabilities, we could have peripheral lighting that comes on as a function of peripheral motion. The fact is that a conversion of roadway lighting to SSL is happening at a rapid pace, driven in many cases by energy efficiency and low maintenance. The city of Los Angeles has retrofitted more than 115,000 street lights with LED fixtures (see Fig. 4). However, SSL can go beyond saving energy by providing significant enhancements to roadway safety. Fig. 4. The City of Los Angeles has replaced more than 115,000 street lights with energy-efficient LED fixtures.
  7. 7. Five rules for designing roadway lighting 8 LEDs Magazine :: EDITORIAL DIGEST The options are open-ended. What is clear is that new technology allows designers the opportunity to not only work with new tools but also return to the applications themselves and rethink the way things are done. When we do that, the value of lighting is optimized in its abilities to help people. We escape the morass of expectations, and we evolve as an industry. DON PEIFER is a senior product portfolio manager at Cree.
  8. 8. 9 * This article was published in the June 2012 issue of LEDs Magazine. LEDs Magazine :: EDITORIAL DIGEST LED modules bring energy savings to high- mast outdoor lighting While LEDs have pervaded a variety of street- and roadway-lighting applications, most owners of high-mast lights have stayed with HID lamps. A Maine case study indicates significant potential for SSL in the higher-power lights used in places such as freeway interchanges. W E ROUTINELY COVER case studies of LEDs used in outdoor, street- and area-lighting applications where solid-state lighting (SSL) is delivering significant savings in both energy and maintenance costs. But repeatedly at conferences the prevailing wisdom among speakers has been that the high lumen output required in high-mast applications would require SSL fixtures that cost far more than metal-halide (MH) or high-pressure sodium (HPS) sources – an even greater cost differential than is the case with normal street lights. Presumably the high cost can stretch the payback beyond what municipalities or transportation departments are comfortable with. The Maine Department of Transportation (MaineDOT), however, is testing LED-based lights in a high-mast retrofit and the results are promising. Fig. 1. Global Tech LED’s high-mast retrofit module.
  9. 9. LED modules bring energy savings to high-mast outdoor lighting 10 LEDs Magazine :: EDITORIAL DIGEST High-mast lights are quite different in nature from more typical street or roadway lights. High-mast fixtures are regularly mounted at 60 ft to more than 100 ft above ground level and occasionally as high as 250 ft. Normal street lights are typically mounted at heights lower than 60 ft, and many are in the 30-ft range. The applications for high-mast lights include installations at transportation terminals, other large, outdoor maintenance or storage yards, and specialty roadway applications. The aforementioned freeway interchange installations are probably the most common roadway application, although you will find some high-mast lights within municipalities in busy areas. In street-light installations, the lighting designer normally specifies a rectangular beam distribution or pattern that directs the lumens precisely and eliminates light spill. The pattern is designed to evenly illuminate the roadway with maximum spacing between poles. High-mast applications rely on more of a circular or square pattern and are designed to distribute light evenly over a maximum-sized radius or area. If you look at legacy lights installed in North America, you can generalize about the two disparate applications in terms of energy usage. Municipalities typically install 250-400W HPS lights individually on a pole in street-light applications. High-mast installations regularly gang 2, 4, 6 or 8 1000W HPS fixtures spaced evenly around a single pole. Potential savings Clearly there is potential for savings in such high-mast applications. Including Fig. 2. Workers retrofit a lowered high-mast fixture.
  10. 10. LED modules bring energy savings to high-mast outdoor lighting 11 LEDs Magazine :: EDITORIAL DIGEST the ballast, a 1000W HPS light actually consumes as much as 1200W. LEDs could certainly cut that energy usage. Plus consider the potential maintenance savings. About high-mast light owners, Jeffrey Newman, president of Global Tech LED, said “They have been replacing lamps once per year.” Global Tech manufactures LED modules designed for use in high-mast retrofit applications. The modules include six clusters of seven LEDs for a total of 42 Philips Lumileds LEDs per module (Fig. 1). Global Tech has developed customized lenses that cover each LED cluster to control the beam pattern. Depending on the application, as many as four of the Global Tech modules might be used to replace a single high-output HID lamp. Newman is quick to attack the question of affordability of LEDs in the high- mast application. He laid out a theoretical comparison where the LED alternative dissipates 600W while the incumbent lamp is the 1200W HPS lamp and ballast. According to Newman the 600W LED reference case is a very conservative example, because most likely you would use a lower-power LED configuration. The LED approach saves 600W. Based on a burn time of 12 hours per night, the savings amount to 2628 kWh per year. At a rate of $0.12 per kWh, that electricity saving equates to around $315 per year. The price a municipality would pay for the retrofit would depend on distributor pricing, but Newman said that MaineDOT is paying in the range of $1200 to $1300 per kit including credits supplied by the state. So the payback is in the four-year range before you consider maintenance costs, and perhaps a lower-power LED implementation. Fig. 3. An LED-based high-mast pole in Waterville, Maine.
  11. 11. LED modules bring energy savings to high-mast outdoor lighting 12 LEDs Magazine :: EDITORIAL DIGEST According to Newman, the LED project in Maine is about more than savings and payback and is focused on keeping the lights on. He said, “They were shutting the lights off at 11 pm at night because of the expense.” The LED retrofit will allow the lights to burn all night, although the long-term plan may also entail dimming the lights late at night. Maine Interstate 295 Ron Cote with MaineDOT said that he was doubtful that an LED- based product could serve in the high-mast application when Global Tech first approached the state. But after seeing the modular approach, MaineDOT retrofitted one high- mast pole with the Global Tech modules eight months ago. Cote reports that the retrofitted fixtures on the pole have been problem free. The LED kits replaced 1000W HPS lamps. The project used four of the Global Tech modules in place of the 1000W lamps. Each module dissipates 98W for a total of 392W per fixture. The retrofit relies on a metal mounting plate with four holes for the modules, and the plate is attached to the reflector of the existing fixture. Once installed Cote said that the LEDs provide 1 fc at ground level out to a distance of 200-300 ft. He said, “Up until now, there hasn’t been an LED fixture that could touch the light distribution of HPS.” But Cote said that the installed LEDs are providing comparable performance. After testing the one pole, MaineDOT is retrofitting eight additional poles at two freeway interchanges. Cote reports that the retrofit process is relatively simple. Fig. 4. LED high-mast lighting (top) compared with HPS high-mast lighting (bottom).
  12. 12. LED modules bring energy savings to high-mast outdoor lighting 13 LEDs Magazine :: EDITORIAL DIGEST Typically high-mast lights are mounted in such a way that cables can be used to lower the fixtures to ground level as opposed to requiring a bucket truck for service (Fig. 2). Cote said it typically takes about 20 minutes to lower a set of lights and another 20 minutes to raise the fixtures back up the pole once service is finished. He said it also takes workers about 20 minutes per fixture to install the retrofit. MaineDOT is able to afford to burn the LED lights all night. Cote said that the SSL retrofit is delivering about 66% in energy savings. The energy cost per freeway interchange has dropped from $800 to $266 per month. The lights are also superior in terms of quality. Fig. 3. shows one of the Maine LED high-mast lights. Cote said the broad-spectrum light and 5000K CCT provide better visibility. Before and after photos weren’t available for the Maine installation. But Fig. 4 shows LED and HPS high-mast lights from a Global Tech project at a Florida shipping container terminal. The energy-conservation-oriented Efficiency Maine organization also commented on the quality of the SSL retrofit. “I went by the Waterville exits this morning on the way in,” said Michael Watson, project engineer at Efficiency Maine. “The Kennedy Memorial Drive exit is done, all four towers have the LED fixtures and it looks great. They also had one done at the Main Street exit and what a difference it makes compared to the HPS fixtures. The LEDs really light it up nice.” Controls and dimming Looking forward, Cote said that MaineDOT is contemplating a retrofit of 108 additional poles – the entire high-mast inventory along I-295. Moreover the department may consider dimming the lights for five to six hours each night to further reduce energy consumption. Newman estimates that with dimming the energy savings could stretch to 80%. Global Tech uses a combination of a custom microcontroller (MCU)-based control circuit on each module along with a modular Philips Lighting driver. The MCU can dim the lights to any level required. The MCU bases the dimming operation on the photocell that is already used on each pole to turn the lights on. Newman said a typical scenario is what he calls 561. Five hours after the lights come on,
  13. 13. LED modules bring energy savings to high-mast outdoor lighting 14 LEDs Magazine :: EDITORIAL DIGEST the MCU dims the lights. The lights remain dimmed for six hours and are brought back to full brightness for one hour. Newman said Global Tech has also developed a wireless control network that can optionally be installed in the retrofit modules. For now, MaineDOT is not installing modules with wireless support. Cote said that MaineDOT will likely test dimming at a single interchange in the next phase of the project. The department will then seek input from the public and other interested parties on the light levels. The savings potential of LEDs on the Maine interstate system is significant. Cote said that the state spends $750,000 annually on interstate highway lighting. Not all of the lighting is high-mast. But Cote thinks the state could definitely save a third of the total just through a move to LEDs on high-mast poles. MaineDOT also expects to realize significant maintenance savings, although they haven’t projected a figure. Cote said, however, that they were expecting 50,000 hours of life from the LEDs. That would certainly curtail the maintenance cycles for replacing HPS lamps. MAURY WRIGHT is the Editor of LEDs Magazine.
  14. 14. LEDs Magazine :: EDITORIAL DIGEST 15 Advanced thermal characterization improves LED street-light design A street light is hot-lumens tested in compliance with JEDEC standards S OLID-STATE LIGHTING (SSL) designers who consider thermal properties in their LED based design are more likely to produce luminaires with long- term consistent light output and longer lifetime. In addition, in the case of LED street lights, illumination often needs to be consistent over a range of ambient conditions, which can be assured using the appropriate simulation and thermal testing techniques. This article demonstrates how thermal simulation using computational flow dynamics (CFD), and thermal testing to the latest Joint Electron Devices Engineering Council (JEDEC) standards, can provide the luminous flux of a street-light luminaire under various conditions. The test methods shown can be used in prototype development, product testing or failure analysis of luminaires. What constitutes good thermal design? LEDs, as one of the most efficient light sources available today, are * This article was published in the July/August 2012 issue of LEDs Magazine. Fig. 1. Mentor Graphics’ LED characterization flow.
  15. 15. Advanced thermal characterization improves LED street-light design 16 LEDs Magazine :: EDITORIAL DIGEST becoming more widely used in indoor lighting, outdoor lighting and automotive lighting. Good thermal design based on the application is essential to ensuring the longevity of the LED luminaire because both LED lifetime and light output are closely related to the LED’s junction temperature. When an LED’s pn-junction temperature is hotter, the performance of the LED is impacted in terms of shorter lifetime and decreased light output. In applications such as headlights of cars or street lighting where lives might be at stake, lighting standards are very strict. In addition to the prescribed spatial distribution patterns that are required, illumination levels also need to be provided consistently; for example, even on hot summer nights, luminous flux of LED- based luminaires must meet the lighting standards. This necessitates having the appropriate knowledge about the thermal and light- output properties of LEDs. As of today, diligent lighting design with LEDs cannot be based solely on a manufacturer’s data-sheet values. Information needs to be gathered experimentally by physical testing of LEDs, and the gathered LED characteristics need to be provided for thermal simulation using, for instance, CFD. Thermal characterization of LEDs From a semiconductor standpoint, LEDs are simple pn-junctions, thus it seems that they should be easier to measure, when in actuality they are not. LEDs present a number of thermal characterization challenges. They are often very small, and measuring them un-mounted is difficult. Fortunately, parts can be mounted on Fig. 2. Close-up view of the top cover of the housing (heat sink) of the HungaroLux LED based street- lighting luminaire.
  16. 16. Advanced thermal characterization improves LED street-light design 17 LEDs Magazine :: EDITORIAL DIGEST Fig. 3. CFD thermal simulation results of LED- based street-lighting luminaire where the applied LEDs were all represented by their compact thermal models obtained from T3Ster TeraLED results. two different substrates and the dual- interface measurement principle can be applied for obtaining their junction- to-case thermal resistance. A greater challenge comes from the fact that LEDs, unlike other semiconductors, emit light. Light emission must be considered when measuring the LED’s thermal resistance. For the majority of semiconductor devices, thermal resistance can be calculated by simply dividing the temperature rise by the electrical power applied to the package. This is because all of the supplied electrical power is converted to heat. However, this is not the case for LEDs because a significant proportion of the supplied energy is converted into and emitted as light, making it an efficient light source. Depending on the LED, energy conversion efficiency can be as high as 30-40%. Based on these efficiency figures, if the supplied electrical power rather than the correct (heating) power is used to calculate the package’s thermal resistance, the thermal resistance value would be significantly lower, suggesting that the package (of a less efficient LED) would be far better at dissipating the heat generated in the LED than it actually is. The emitted optical power can be precisely measured to account for the calculation of the real thermal resistance if thermal testing of the LED in question is performed in a CIE 127-2007-compliant total-flux measurement environment, such as a TeraLED system from Mentor Graphics. In this system, the temperature of the LED under test can be precisely set to a desired value by a temperature-controlled cold plate. Such a measurement setup is also suggested by one of the most recent LED thermal testing standards, JESD51-52, which provide guidelines on methods to measure LED light output in connection
  17. 17. Advanced thermal characterization improves LED street-light design 18 LEDs Magazine :: EDITORIAL DIGEST Fig. 4. Luminaire surface temperature map as shown by infrared imaging at an ambient temperature of 20°C. with LED thermal measurements. This standard is one of a group of four new international thermal test standards for LEDs that were published in May 2012. As for the thermal characteristics of LED components, the junction-to- case resistance is the most appropriate metric for packaged LEDs because it characterizes the heat flow path from the point of heat generation at the pn- junction down to the bottom of the case – exactly how LED packages are designed to be cooled. A relatively new standard, JEDEC JESD51-14, for junction-to-case thermal resistance measurement, is based on the latest thermal-transient measurement techniques. This method uses a dual-interface approach in which the thermal resistance of the part is measured against a cold plate with and without thermal grease. The junction-to-case resistance is determined by examining where the two measurements differ. Very high measurement repeatability is required because the thermal impedance curves for the two measurements must be identical up to the point where the heat starts to leave the package and enter the thermal interface between the package and the cold plate. This ensures that the point where the curves deviate is clear. It compares to LEDs mounted on a cold plate attached to an integrating sphere (as the JESD51-52 standard recommends). This method provides the real junction- to-case thermal resistance metric for LED packages if during the two subsequent measurements the cold plate with LED under test is attached to an integrating sphere (as the new JESD51-52 standard recommends).
  18. 18. Advanced thermal characterization improves LED street-light design 19 LEDs Magazine :: EDITORIAL DIGEST Solutions for LED thermal characterization The Mentor Graphics T3Ster thermal transient tester uses a smart implementation of the static test version of the JEDEC JESD51-1 electrical test method that allows for continuous measurement during a heating or cooling transient, which also forms the basis of the JESD51-14 test method for the junction-to-case thermal resistance measurements. This is also the preferred test method in the LED-specific thermal measurement guidelines that are provided in the JESD51-51 standard. The combination of Mentor Graphics’ T3Ster and TeraLED products provide a comprehensive solution for LED testing which meets the requirements of all the mentioned standards (Fig. 1). In high-throughput bulk-testing applications (e.g., in large scale reliability analysis), a multi-channel T3Ster system can characterize many thousands of LEDs in an hour. T3Ster’s accurate measurements capture transient responses of LEDs just 1 microsecond after switching the power off with a temperature resolution of 0.01°C. This means that the earliest possible part of the LED’s thermal response is captured; thus, you can see the influence of key constructional features close to the heat source within the LED package, such as the thermal resistance of the die attach, after a short time. The T3Ster Master post-processing software fully supports the JESD51-14 standard for junction-to-case thermal resistance measurement, allowing the temperature versus time curve obtained directly from the measurement to be re-cast as “structure functions” (described in JESD51-14 Annex A), and then automatically determine the junction-to-case thermal resistance value. Structure functions are also widely used in failure analysis as part of reliability studies mentioned earlier. This combined with LM-80-compliant lifetime tests of LEDs helps establish correlation between LED lifetime and degradation of different thermal interfaces in the junction-to-ambient heat-flow path of LED components (see mycite.omikk. bme.hu/doc/102602.pdf). Because the JESD51-14 methodology yields the junction-to-case thermal resistance as a side product, the step-wise approximation of the structure function up to this thermal resistance value provides the dynamic compact thermal model of the LED package automatically. The identified junction-to-case thermal resistance values may be published on the product datasheet, and the automatically
  19. 19. Advanced thermal characterization improves LED street-light design 20 LEDs Magazine :: EDITORIAL DIGEST generated dynamic compact thermal model of the LED package can be applied directly in CFD analysis software such as Mentor Graphics FloTHERM. The challenge of correct LED thermal characterization is compounded because an LED’s efficiency is adversely affected by the junction temperature. This presents a challenge for both LED vendors and SSL designers. The LED’s light output, junction temperature, and power draw need to stabilize before measurements can be taken. Consequently, the static measurement method used to capture the cooling curve is the only correct approach to characterize LEDs. The combination of the light output measurement (performed with equipment such as TeraLED), and thermal transient testing allows measurement of the light-output characteristics as a function of the temperature. Providing these data as a function of the reference temperature of the cold plate is useful information for SSL designers. But the same data is also available as a function of the LEDs’ junction temperature, which is required for the correct physical modeling of the light output of LEDs, in other words, the input data for hot lumen calculations. Such a combined thermal and radiometric/photometric test setup is recommended by the most recently published JESD51-5x series of LED thermal testing standards. Street-light luminaires Hungary, was to develop street-lighting luminaires with the minimal number of LEDs per luminaire such that all requirements of the rather strict European street-lighting standards could be met for a wide range of road categories. The two principal goals to reach were to obtain the required spatial light distribution pattern (batwing pattern) and to reach the required level of luminance on the road surface under all possible environmental conditions. The first goal required careful optical design for which LED vendors typically publish their LEDs’ so-called trace files. Careful thermal design is required to achieve the second goal because the required level of light output must also be ensured on a hot summer evening. For this, reliable thermal simulations are needed that properly predict the junction temperatures of LEDs assembled into the luminaire. Unfortunately, luminaire
  20. 20. Advanced thermal characterization improves LED street-light design 21 LEDs Magazine :: EDITORIAL DIGEST vendors have not yet published LED thermal models. Thermal data on their data sheet are sometimes questionable because, so far, no testing standard has been explicit about the combined thermal and radiometric/photometric testing of LEDs to be able to yield the real thermal metrics of LEDs. The solution to the thermal design problem of HungaroLux was provided by the combination of Mentor Graphics thermal testing and CFD analysis tools. As described in the previous section, thermal testing of LEDs can yield compact thermal models of their packages that are directly applicable in CFD simulation tools. The CAD file of the HungaroLux street-lighting luminaire (Fig. 2) was also directly used to build the final, detailed system-level thermal model. All 48 LEDs were replaced by their compact models along with a compact thermal model of the LED driver circuitry. From the dissipation of the individual LEDs driven by the nominal forward current (350 mA, 700 mA, 1500 mA), the driver’s dissipation is also calculated. In this way, the luminaire-level CFD analysis is performed with real data that represent the LEDs’ junction temperatures (Fig. 3). The CFD thermal simulation results have been verified by measuring the surface temperature of the luminaire (Fig. 4). Because the temperature dependence of the light output characteristics of the LEDs was known from the same measurements that formed the basis of the LEDs’ compact thermal models, the total luminous flux output of the luminaire also could be calculated. Using this method, the luminaire could be properly sized in terms of the number of LEDs needed to provide the required road luminance level and for the LEDs’ junction temperature. Conclusions Recently published LED thermal testing standards and their commercial implementations provide tools for comprehensive physical testing of power LED components. Measurement results can be easily turned into LED compact models that are directly applicable in CFD-based thermal analysis on the luminaire level. The system-level CFD simulation results also allow the calculation of the hot lumens of the entire luminaire because the combined thermal and radiometric/ photometric test setup used in the physical characterization of LEDs yields data regarding the temperature dependence of the total luminous flux of LEDs. With
  21. 21. Advanced thermal characterization improves LED street-light design 22 LEDs Magazine :: EDITORIAL DIGEST such a diligent and comprehensive characterization method, SSL designers can be assured that their final LED-based products will meet the applicable lighting standards and will provide the expected long lifetime. ANDRÁS POPPE is a marketing manager at Mentor Graphics and an associate professor at the Budapest University of Technology. ANDRÁS SZALAI is the chief financial officer of HungaroLux Light. JOHN PARRY is a research manager at Mentor Graphics Mechanical Analysis Division.