Led lighting outdoor design challenge dec2013

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Led lighting outdoor design challenge dec2013 LED Magazine Editorial Digest


<ul><li>1.SPONSORED BY: 2Five rules for designing roadway lighting 9LED 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. </li></ul><p>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 drivers view of a simulated roadway scene, illustrating conventional forward auto lighting combined with traditional roadway lighting. 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, couldnt 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 photoreceptors 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 drivers 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. 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. 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, dont 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. 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. 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. 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 LEDs high-mast retrofit module. 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. 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 pr...</p>