Researchers, manufacturers, industry insiders, and observers gathered in Nashville, TN, January 29–31, 2018, to participate in DOE's 15th annual Solid-State Lighting R&D Workshop. DOE SSL Program Manager James Brodrick emphasized that there still remains plenty of technology-development headroom for solid-state lighting. For example, he noted, today’s highest-performing LED devices can produce 160 to 170 lm/W, whereas DOE projects that 255 lm/W is achievable using phosphor-converted LED architectures. Brodrick said that with greater breakthroughs in LED performance, the phosphor can be removed from the system, enabling efficacies of 350 lm/W, with luminaire efficacies of 280 lm/W. He observed that for OLEDs, enabling technologies involving materials, light extraction, anodes, and encapsulation have been demonstrated, but they need to be integrated into low-cost manufacturing processes. And Brodrick pointed out that SSL’s remaining challenges are harder than the earlier challenges.
LED INNOVATIONS AND ADVANCEMENTS
Matthias Sabathil of OSRAM talked about innovations in LED lighting, focusing primarily on those innovations that will take us beyond the “lumen race.” Urging attendees to “look beyond the lumen,” he emphasized that photons can do much more than just illuminate. Sabathil discussed the way in which technologies in general evolve, pointing out that the first automobiles looked very much like horse-drawn carriages, but that car designs eventually broke free of those constraints – which will happen with lighting when we get past a legacy-bound mindset. To drive LED innovation in the future, he said, we should continue to push the technology while preparing for the day when it enters a “saturation” phase of marginal integral improvements, and should also enable novel applications by delivering smart building blocks for a combinatorial world, and provide seamless integration of digital lighting via smart components and platforms.
Werner Goetz of Lumileds discussed LED science and technology advances. He highlighted key focus areas for LED efficacy improvement, ranging from epitaxy and die development, to narrowband phosphors and quantum dots, to droop reduction, to converter saturation, to improvements to InGaN green and AlInGaP amber and red. Goetz reviewed major technology advancements made to date, noting that mid-power LEDs achieve ~200lm/W and high-power LEDs achieve 150 lm/W; that high-luminance, compact sources that deliver up to 100 meganits with controlled forward-directed beams are becoming available; that narrowband red phosphors and quantum dots enable LEDs with CRIs >90 to have efficacies close to those of LEDs with CRIs of 80; and that quantum efficiency of direct-emitting green and amber LEDs continues to improve, with green InGaN LEDs with EQE >35% now commercially available and having the potential for significant further advancement.
THE FUTURE OF LIGHTING EFFICACY
A panel moderated by Jeff Tsao of Sandia National Laboratories considered the future of lighting efficacy from the perspectives of lighting design, computational light modeling, and the human physiology of vision. Tsao set the stage for the discussion by breaking down lighting efficacy into several terms. One of them was source efficacy, which he noted has been industry’s main focus up until now, with the emphasis being on the power efficiency of SSL sources at wavelengths that fit within the human eye’s response and that therefore can be translated into lumens. But Tsao said that we need to account for another kind of efficiency, which could be called “space + time + spectral-distribution efficiency.” And he also noted two other kinds of efficacy: use efficacy and application efficacy. “There’s a lot to be done in reconceptualizing lighting efficacy, as we enter this new world of engineered solid-state lighting, combined with a new world of computational light simulations,” Tsao said.
Darcie Chinnis of HLB Lighting Design offered a lighting designer’s perspective on efficacy. She noted that a designer’s goals involve function, beauty, mood, code-compliance, adherence to a budget – and efficacy, if it’s motivated by such things as certification or utility rebates. When efficacy matters, Chinnis said, she must consider who occupies the space (e.g., their age and expectation of brightness), what the task is (and whether it moves or changes), how the space is configured, what the influence of time is, and what the optimal spectrum is. “It’s not just about the source efficacy,” she said, “and it can’t – or shouldn’t be – solved by algorithms alone.” Chinnis advocated reframing the discussion of efficacy to acknowledge that there are realistic limits to source efficacy and to “optimizing” applications, realistic definitions of visual performance, realistic reliance on lighting controls, and the important impact of time. She advocated that designers layer in new and exciting tools that involve such things as virtual reality, luminance-based design, and tunable sources.
Kevin Houser of Penn State University discussed efficacy in applied lighting. He distinguished between efficacy (a ratio of benefit to cost) and efficiency (a product of ratios of like quantities). Observing that today, benefit per cost in applied lighting is largely defined as lm/W, Houser called the lumen “a poor proxy for benefit,” noting that there are many other ways to characterize optical radiation. He said that minimally, the benefits of lighting to people are visibility, color quality, psychological reinforcement, and circadian stimulation; but he noted that lighting also has prerequisites, especially related to health (e.g., minimized flicker). Houser also said that using the watt as a proxy for cost is imperfect. He emphasized that when lighting for people, the benefit should be defined by human needs. “It makes good sense for the environment and people to create products with the greatest human benefit, with the smallest energy, environmental, and financial costs,” Houser concluded.
Pradeep Sen of the University of California, Santa Barbara talked about efficacy from the point of view of computation – specifically, focusing on the idea of controlling the light so that as little of it is wasted as possible. He defined “wasted light” in this context as light that doesn’t hit the human eye. Sen estimated that the amount of light that’s wasted is on the order of magnitude of 107, which leaves so much room to play with that increasing present levels of efficacy by a factor of 100 is just a drop in the bucket and thus might be feasible. The idea, he said, is to make lighting human-centered, so that it’s directed only where and when we need it. Sen envisioned light shining, as much as possible, only in directions where a person is looking at the time, and explored a number of mechanisms – beyond just occupancy sensors, to include such things as Wi-Fi, eye-tracking, and sophisticated algorithms – that could make this possible. He invited the audience to collaborate and participate in such an effort, saying “We all need to be communicating and working together.”
UNDERSTANDING THE EFFECTS OF LIGHT ON HUMAN PHYSIOLOGY IN REALISTIC SETTINGS
Bob Davis of Pacific Northwest National Laboratory (PNNL) moderated a panel discussion of recent findings that build on what’s known about the physiological effects of light, and what research is necessary to fill gaps in understanding. Shadab Rahman of Harvard Medical School explored the effects of visible light on human sleep and circadian rhythms. He noted that biological responses to light depend on the characteristics of the light stimulus, including its intensity, duration, timing, pattern, wavelength, and history. Rahman explained that exposure to light before bed can disrupt sleep by interfering with the body’s production of the hormone melatonin, which affects circadian rhythm. Because of the same general mechanism, he said, decreasing the blue content of light in the later hours of the day promotes sleepiness, and using blue-enriched white light in the workplace improves self-reported alertness and performance as well as sleep quality. Rahman reviewed scientific evidence for these effects, and discussed a prototype LED lighting module with variable white-light spectra, which was developed to replace existing fluorescent units on the International Space Station. He emphasized that light exposure affects a wide range of physiologic processes, and that stable light-exposure patterns are essential for health and wellbeing.
Jennifer Veitch of Canada’s National Research Council focused on what research is needed to support standards and recommendations for healthful lighting. She reviewed CIE 218:2016, Research Roadmap for Healthful Interior Lighting Applications, and its 28 detailed questions, and emphasized that we still don’t know how much light is necessary to optimize circadian function and produce other biological effects. She noted that CIE JTC 9, an international standard expected to come out in the spring of 2018, is a system for measuring light response of the intrinsically photosensitive retinal ganglion cells (ipRGCs) that will define spectral sensitivity functions, quantities, and metrics to describe radiation for its ability to stimulate each of the five photoreceptor types. Observing that “not enough scientists study light and lighting,” and that science-funding agencies don’t prioritize such research, Veitch urged the lighting industry to actively build consensus about fundamentals and actively participate in human-factor R&D.
Ron Gibbons of Virginia Tech Transportation Institute talked about a DOE-funded study he’s conducting to measure the impact of different types of roadway lighting on levels of melatonin in drivers, pedestrians, and nearby sleepers. He explained that five different light sources, each one with a different spectral content, will be evaluated in terms of the circadian and neuroendocrine phototransduction of the ipRGC system, using custom-developed software. Gibbons observed that 11 participants will be recruited to participate in each of the study’s three exposure experiments, which will be conducted on the Virginia Smart Road. He noted that melatonin levels will be measured by sampling blood and saliva, and that a linear mixed-model analysis will be used to assess the effects of lighting type. Gibbons said the analysis is expected to provide insights into the overall effect of light exposure on the melatonin levels of drivers on the roadway, pedestrians along the roadway, and homeowners with bedrooms exposed to street lighting.
Gena Glickman of the University of California, San Diego talked about a study she’s conducting on novel lighting strategies for optimizing circadian health and alertness in nightshift workers. She noted that 15 million Americans work outside of a nine-to-five schedule, and that this can cause accidents and injuries as well as have mental and physical health consequences. Glickman explained that the harm is due to three factors – circadian misalignment, sleep deprivation, and light at night – and that lighting can be used to facilitate circadian adjustment or increase alertness and performance, or it can be minimized to reduce any disruptive effects. Her study, she said, will employ two evidence-based lighting interventions to address two different light responses: circadian phase resetting, to maximize input during the day and minimize input close to bedtime; and acute alerting, to make the person more alert. Glickman noted that individualized and dynamic lighting environments have the potential to be particularly effective in populations with significant variability in circadian phase, such as nightshift workers.
BALANCING PERFORMANCE, SAFETY, ENERGY SAVINGS, HEALTH, AND ECOLOGICAL IMPACTS FOR ROADWAY LIGHTING
Paul Lutkevich of WSP discussed roadway lighting from the standpoint of such factors as safety, connectivity, light trespass, and human physiological impacts. Distinguishing between what we know and don’t know, he noted that light at night has a physiological effect, the severity of which depends upon spectral content and dosage, and that we don’t know if future research will show that well-designed street lighting has an impact on health, although current research doesn’t seem to indicate that it does. Lutkevich said we need to have a spectrally weighted limit on dosage from street lighting. He noted that while light at night can affect plants and animals, and different spectral content sources have different impacts, we don’t have a good method of quantifying those impacts in terms of lighting output, direction, and spectral weighting. Lutkevich observed that although roadway lighting not only seems to modify driver behavior but also has an impact on safety, with spectral content impacting visibility, many other factors contribute to crashes.
Day 1 concluded with three concurrent rump sessions: open discussion and Q&A on DOE R&D funding mechanisms, R&D related to light at night, and connected lighting R&D.
WORKING WITH THE DOE SSL PROGRAM
Joel Chaddock of the National Energy Technology Laboratory led an open discussion and Q&A on the DOE SSL funding opportunity announcement (FOA) process, the Small Business Innovation Research/Small Business Technology Transfer (SBIR/STTR) grant process, and the collaborative OLED testing opportunity. He reviewed the DOE competitive R&D process through all five stages, outlined best practices for applicants, talked about milestones and metrics, and went over common mistakes as well as performance monitoring. Chaddock also reviewed SBIR/STTR opportunities and the purposes and benefits of the collaborative R&D OLED testing opportunity.
BRIDGING R&D EFFORTS RELATED TO LIGHT AT NIGHT, SKY GLOW, AND ANIMAL RESPONSES TO LIGHT
Jeff Tsao moderated an open discussion and Q&A that focused on current research efforts to understand and mitigate sky glow and ecological impacts on nocturnal wildlife, and what’s needed moving forward. Each speaker spoke briefly before the floor was opened to audience questions, which led to an energetic and extensive interchange between audience and panelists. Tsao set the stage by describing a new satellite tool that’s become available, the Visible Infrared Imaging Radiometer Suite (VIIRS), which is a 22-band imaging radiometer that’s absolutely calibrated against the sun once every orbit around the Earth. He explained that the VIIRS was used in a four-year study conducted by German researchers, who found that in general, the Earth is becoming brighter at night. But to stimulate discussion, Tsao offered a number of possible conclusions one might draw from that (e.g., that we’re producing more artificial light, that sky glow is getting worse) and brought up a number of caveats (e.g., VIIRS can’t detect wavelengths shorter than 500 nm that contribute to white LED or HPS-induced sky glow) as well as possible calls to action (e.g., SSL is well-suited for controlling light distributions in space, time, and spectral content, and thereby for minimizing the wasted light that sky glow represents).
Bob Davis discussed sky glow, focusing on a DOE study of the expected contributions to sky glow from converting high-pressure sodium (HPS) street lighting to broader-spectrum (i.e., white-light) sources, with specific focus on LEDs. The study found that all of the LED product conversions reduce sky glow relative to an HPS baseline when the results are expressed as unweighted radiant power, for both near and distant observers; and when the results are scotopically weighted to evaluate the effects on human vision, some LED products reduce sky glow for the near observer compared to the baseline, and others increase it, depending on their relative content of shorter wavelengths. Davis noted that three main variables can affect sky glow: spectral power distribution (SPD), light output, and how much light is coming above the horizontal. He pointed out that well-designed LED luminaires have no uplight and emit fewer lumens because they have better uniformity, so it’s probable that they contribute less to sky glow even independent of SPD, although the CCT (correlated color temperature) of LED streetlights has been coming down.
Dianne Ingram of the U.S. Fish and Wildlife Service (FWS) talked about the effects of artificial lighting on coastal species, from an in-the-field perspective. She focused on three kinds of wildlife to illustrate these effects, explaining how artificial lighting can cause reduced foraging in beach mice, disorient sea turtles, and cause birds to fly at night, resulting in fatal bird collisions. Ingram noted that the FWS has already invested $8 million in lighting that’s friendlier to sea turtles, mostly in Florida, in a project that involved lighting assessments, retrofit plans, coordination with site managers, and the actual retrofitting of the lighting. She cited a number of opportunities for collaboration with industry, including developing procedures for rapid lighting assessments, retrofit replacements, and incentives; updating the intensity thresholds and photometric specs of amber LEDs; and establishing a lighting certification lab with expertise to certify wildlife-friendly lighting products.
RESEARCH FOR CONNECTED LIGHTING SYSTEMS (CLS)
Monica Hansen moderated an open discussion and Q&A on opportunities to speed the development of connected lighting systems. Each speaker spoke briefly before the floor was opened to audience questions, which led to an energetic and extensive interchange between audience and panelists. Mark Hand of Acuity Brands focused on industry standards. He said connected lighting often amounts to 10% of Acuity’s sales, yet it accounts for a much bigger part of the development work. Hand considered what would increase adoption of controls and move the market. Observing that in the past, the only places with significant adoption have been where legislation was enacted that required it, he pointed out that a major hindrance is the lack of standards between companies. Hand noted that no industry standards exist for interoperability between platforms, and that it’s a highly volatile field. He said Acuity hasn’t “joined that train” yet, because there aren’t legitimate standards in place for interconnected systems; it appears to be too soon to see adoption on a large scale until a stagnant system is in place. Hand said that Bluetooth as a communication standard is similar to what Acuity is looking for, and that the current contenders, such as Zhaga and Zigbee, aren’t yet sufficiently solidified and versatile.
Michael Poplawski of PNNL discussed interoperability and application program interfaces (APIs). He said that the degree to which connected lighting moves lighting out of its “historical silo” – and thus increases its value – will primarily be a function of “if, when, and to what degree” interoperability is facilitated, and that currently APIs are the main facilitators of interoperability. Poplawski reviewed a DOE study of APIs, which found that their usability and value are a function of their availability, completeness, quality, and stability, each of which varies significantly across today’s available CLS APIs; that their maturity needs to be advanced; and that they need to be treated like a feature and capability that companies are competing on. He said APIs should be characterized by their maturity and benchmarked against competition and alternatives. One way to gauge the development progress of an API, Poplawski noted, is by openness and accessibility, environments and documentation, quality and ease of use, community and support, and sales and marketing. He said CLS vendors should not treat APIs as an afterthought, but rather should treat them as a key, high-value, competitive feature.
Dorene Maniccia of Philips talked about the research needed to accelerate connected lighting advances and adoption. She said that currently, the “scales are tipped,” in that there are a lot of people working in the Internet of Things (IoT) domain and a lot of people working in the human-centric lighting design domain, but there’s an imbalance between the quality of the visual environment and the IoT. Maniccia said that as she sees it, case studies for the IoT are largely focused on such things as space optimization and decreased energy consumption but often ignore the quality of the visual environment. She said there’s a need to emphasize people as a part of these development processes, and that the IoT groups should emphasize such human-centric concerns as visibility, visual comfort, well-being, and circadian rhythms in order to optimize the user experience.
USING MORE DATA FOR LIGHTING SCIENCE
Shadab Rahman kicked off Day 2 by reviewing some recent large-scale epidemiological work related to the effects of light on health, discussing the potential benefits of gathering larger sets of evidence, reviewing other large-scale data approaches, and suggesting new technological approaches that can enable low-cost collection of larger sets of lighting data. He reviewed a number of studies that showed a relationship between light exposure and circadian behavior, but pointed out that the small sample size makes these studies less generalizable and reproducible. That’s why, Rahman said, larger studies are also needed. He cited several sources of “big data” for larger studies, such as electronic health records, smartphones, and social media, and described a study he and his colleagues are conducting in collaboration with the app f.lux. “Big data is the way forward,” Rahman said.
R&D DIRECTIONS OF COLOR-TUNABLE LIGHTING
Monica Hansen of LED Lighting Advisors moderated a panel that explored the technology challenges associated with tunable lighting, including methods of color mixing, multi-channel driver development, novel color sensors, and the testing required to characterize a color-tunable product. Steve Paolini of Telelumen reviewed a number of challenges to tunable lighting (e.g., multi-channel current sources and sensors, a simple interface, and missing or inefficient wavelengths). He pointed out that lumens are a weighted measure of optical watts, with a peak at 555 nm (green) and falling off toward red and blue. Paolini noted that spectrum is important for fidelity and preference; natural light sources have broad, variable spectrum; daylight is a good model for tunable systems; and broader wavelength range and less dropout in SPD increases color quality and impact. He called for revisiting the lumen to consider how LED sources (both monochromatic and polychromatic) behave across the spectrum and how we perceive light across our entire visual field. “As we move to using more of the spectrum beyond optimizing the central cone response and for tunable spectrum systems, efficiency may be a more useful metric than efficacy,” Paolini said.
Matt Weaver of Lumenetix talked about how and why his company developed its line of color-tunable LED light engines that he said emulate the quality and range of the color of sunlight. He distinguished between the three forms of color-tunable lighting: two-channel white, color point (red-green-blue), and spectral (a combination of four or more variable light sources). Weaver used the analogy of the now-defunct Yugo automobile, which was low-cost but very poorly designed and thus had many defects. He observed that markets escalate, and that today the cars everyone is driving are all technical marvels that would once have been considered racecars. With lighting, Weaver said, “the driver may not be status, but rather perceptions of quality and health.” He predicted that spectral tuning may someday be accepted as a very common-sense feature in lighting.
Payman Zarkesh-Ha of the University of New Mexico discussed the work he and his colleagues are doing to develop a plenoptic sensor for color-tunable lighting. He explained that their approach involves using a guided-mode resonance filter. Zarkesh-Ha noted that sensors are essential components in future LED systems that require feedback control, but that existing off-the-shelf color sensors aren’t suitable for advanced multicolor LED lighting systems that can deliver high-quality lighting to improve human health and productivity. He observed that plenoptic sensors can deliver light-field information (angle and wavelength) with the high resolution needed for high-quality LED lighting. Zarkesh-Ha added that with such sensors, angular selectivity can be used instead of wavelength selectivity, and the sensor can be combined with a time-of-flight system to determine the angle of the reflected light. He also noted that plenoptic sensors can be used for occupant counting and tracking.
Chad Stalker of Acuity Brands focused on the market side of tunable-white lighting products. He observed that as a result of discussions at the 2017 DOE SSL R&D Workshop, the DesignLights Consortium® began to look at color-tunable products and created specifications for them. Stalker commented on these in the context of challenges that have to be faced. He identified driver architecture as one of the next challenges, and noted that it’s also important to push pricing down to make the products more mainstream, and that in addition, the products should be plug-and-play. Stalker observed that at a recent International Association of Lighting Designers roundtable, many of the attendees professed to be intimidated by tunable-white systems, which means that’s another barrier that needs to be overcome.
The evening of Day 2, researchers representing 50 projects presented their work at a poster session that provided attendees with additional opportunities for discussion, information exchange, and potential partnering. The projects were funded not only by the DOE SSL program, but also through DOE under the Small Business Innovation Research (SBIR) grants, the Advanced Research Projects Agency–Energy (ARPA–E), and the National Science Foundation (NSF). Four posters representing the SSL student competition included winners from the University of California, Los Angeles, the University of Central Florida, Penn State University, and the University of New Mexico. In addition, six R&D participants demonstrated products and prototypes, ranging from hybrid downconverting structures to an ultrathin flexible LED lighting panel to building controls integrated using a power electronic transformer (PET) power distribution system.
LED TRACK SESSIONS
A panel moderated by Jeff Tsao reviewed the state of the art and the challenges associated with downconversion materials, and discussed potential research opportunities – including novel approaches to mitigating efficiency droop, to efficient direct emission in the green-yellow-red, to narrow-line-width downconversion in the red, and to downconversion at high light fluxes. James Murphy of GE Global Research focused on narrowband emitting phosphors. He noted that by switching to narrowband phosphors, near-infrared spillover is no longer an issue, thus promoting higher efficacy. Murphy said his approach is to create a host phosphor that’s transparent and dope it, which produces luminescence when a blue light hits it. He stated that there needs to be a very precise balance of activators, phosphors, and other materials. Murphy noted that GE PFS phosphor material is now commercialized in medium-power packages, and the display industry is currently leading the way in terms of what DOE is looking for. He considered whether some of the display optimizations can be transferred to SSL. Murphy observed that the general market trend is a shift toward mid-power packages, which may have potential for decreasing droop, because it’s possible to increase the number of packages and decrease the current they’re run on. He noted that as industry moves toward narrowband materials, reliability tests become increasingly important.
Peter Palomaki of Palomaki Consulting talked about quantum dots (QDs), which he explained are tiny semiconductors that typically have a nonorganic core and shell covered by organic material. He observed that the QD industry is currently driven by displays, noting that QDs degrade very quickly when exposed to oxygen, which becomes a difficulty in product design and manufacturing. Palomaki said that for displays, it all comes down to the color gamut they offer; whereas for lighting, it comes down to the narrow full width half maximum (FWHM). He noted that QDs also allow tunability across the entire visible spectrum, which decreases the tradeoff between efficiency and CRI. Palomaki said that phosphors beat QDs in thermal quenching, but that displays don’t need to operate at the higher temperatures encountered in lighting. He cited opportunities for QDs – including finding alternatives to CdSe, improving efficiency, reducing FWHM, and increasing stability – and reviewed major challenges, such as loss at high temperatures and the need for nano- or micro-encapsulation.
Isaac Wildeson of Lumileds talked about III-nitride blue, green, and longer-wavelength direct emitters. He observed that there’s been a threefold improvement in white-LED efficacy over the last decade. Wildeson noted that internal quantum efficiency (IQE) is the area of efficiency with the greatest room for improvement, and that controlling carrier density within each well allows for a decrease in droop, which is an area that’s currently being advanced. He said materials that improve carrier spreading typically degrade material quality, which is a problem that has potential for improvement. Wildeson noted that peak efficiency occurs at more than an order of magnitude of difference in current between green and blue, which creates an inevitable droop with one color; but putting AlGaN interlayers on top of quantum wells results in improved peak efficiency at longer wavelengths, although the benefits of applying this at green wavelengths are a bit less because it increases development difficulties, so novel approaches will be required for III-nitride materials to go beyond green.
Manos Kioupakis of the University of Michigan focused on understanding and mitigating droop in nitride LEDs. He recounted the challenges with such LEDs, including droop (lower efficiency at higher power), the green gap (lower efficiency for longer wavelengths), polarization fields separating electrons and holes, and composition fluctuations localizing carriers. Kioupakis considered predictive modeling of materials, focusing on the Density Functional Theory. With regard to the green-gap problem, he noted that efficiency droop increases with increasing polarization fields, with lower efficiency for LEDs at longer wavelengths. Kioupakis considered various ways to improve efficiency, including the use of zinc-blende InGaN, which has no polarization fields but requires new substrates; growing more quantum wells, which reduces carrier density but in so doing induces poor carrier transport; growing a single thick quantum well, which reduces carrier density but the InGaN is mismatched to the GaN; making the quantum wells thinner; and adding boron (up to 10%) into the InGaN.
A panel moderated by Monica Hansen discussed the impact of new LED source and optical control methods to enable new products or more-efficient design in areas from headlights to white-tunable luminaires. Brian Cumpston of Bridgelux talked about the opportunities and challenges of dim-to-warm LEDs. He noted that the primary objective of these LED products is often to mimic incandescent/halogen dimming, and that market adoption has been much faster than with multichannel devices, largely because no additional control systems are necessary and prices are very similar. With regard to color quality, Cumpston said that high Ra and R9 are important attributes that should be given high priority in product development, and that it’s important to keep those metrics high across the whole dimming range. He emphasized that dimming saturation should be considered. As for dimming control, Cumpston noted that it’s typically achieved with passive devices such as resistors, but that adding things like resistors isn’t great for power consumption; that for most solutions, pulse-width modulation dimming can’t be used because it will dim but stay at a high CCT; and that it’s possible to use a combination of modulation schemes. He also reviewed the optic requirements.
Paul Fini of Cree talked about LED component and system optics. He noted that high-power LED packages typically have a dome design in order to maximize “first pass extraction,” which, as a tradeoff, creates a magnification effect of sorts. Fini observed that package optics can also be used for diffuse emission, and that cubic or rectangular lenses are currently being considered because such systems provide a broadened far-field emission over angle (which is well-suited for omnidirectional products) and are good at avoiding the “pixelation” that tends to occur through the diffuser, thus allowing for wider spacing of LEDs within the system. He said it’s possible to maximize luminaire efficacy at a manageable package optical efficiency hit, and that since directionality is important, he and his colleagues were able to create a higher “luminance per lumen” – essentially decreasing directionality within omnidirectional products – and that this new package was also able to increase center-beam candlepower over a halogen PAR38. Fini noted that lightguides can create new opportunities because they can be used to minimize the size and weight and maximize the optical efficiency, but they must be thin in order to keep high efficiency. He concluded that there’s a real synergy between package optics and system optics.
Benno Spinger of Lumileds discussed automotive headlamp matrix systems with improved efficiency. He said the goal is an adaptive driving beam that also accounts for the driving situation and has a glare-free high beam that would selectively block light in the direction of oncoming cars; however, he pointed out, the bulb must be able to withstand very high heat. Spinger said such a system should use a simple aspherical lens or a reflector in a cost-efficient system optic. But he noted that as an alternative, indirect projections could be used, in which case the difficulty of increasing and decreasing collimation as the approaching car gets closer and farther away could perhaps be solved by gluing the collimator directly to the LED surface, which would require that the indices of collimator and converter matched. Spinger concluded that automotive headlamp matrix systems can optimize functionality in this application and also transform the aesthetic on the front of a car by offering nontraditional form factors.
Omar Rivera of MVP Consulting Group discussed luminaire efficacy, focusing on the state of the technology. He noted that the main design challenges involve original equipment manufacturer (OEM) chip selection (because the chip will only improve in small increments) and thermal design. Rivera observed that the best performance characteristics tend to be found at 70 CRI and 5000K. He said synergizing all the components is the key to optimizing efficiency. Noting that aluminum is a better conductor of heat, Rivera pointed out that it has its limitations and is not perfect, which leaves room for improvement. He said the LED chip is key but it is “not the silver-bullet” solution to a high-efficacy final product. New materials need to be explored to reach the next level of heat dissipation, which Rivera said is the largest area for potential improvements in efficacy. Considering industry concerns, he stated that if high-performance achievement stalls, products will lose value over time as the prices of standard products decrease, and noted that high-efficiency products fund the industry.
Morgan Pattison of SSLS, Inc., moderated a panel that focused on advancements in metal-organic chemical vapor deposition (MOCVD) technology, phosphor synthesis, and flexible luminaire manufacturing, as well as critical manufacturing R&D that can reduce cost-consistency-performance tradeoffs and have the added benefit of promoting domestic manufacturing. Chris Morath of Veeco talked about epitaxy requirements for micro-LED display. Stating that the goal is to scale down commercial LED signage by a factor of 104 for consumer display applications (to 10x10 μm μLED), he explained that the trouble is that manufacturing processes for standard LEDs aren’t applicable at a scale that small; the LEDs need to be as small as 3x3 μm2 for smartphones and 10x10 μm2 for TVs. Morath reviewed the advantages of micro-LED displays over LCD and OLED, including power efficiency, brightness, and color. But he predicted that micro-LED displays probably won’t hit the main display industry markets until 2020–2021. Morath said the biggest driver of low prices is the size of the μLED. He explained that when simply scaling current LED processes, you start to run into issues below about 50 microns, when sidewall recombination becomes a problem, and that the new methodology requires many changes; but he and his colleagues are focusing on two mass-transfer approaches, which apply to the low-mid range and involve “mass transfer with interposer.” Morath said Veeco’s manufacturing process can achieve 80–100% wavelength uniformity yield.
Kamal Hadidi of Amastan Technologies talked about his company’s UniMelt technology platform – which involves high-temperature processing of borides, carbides, oxides, nitrides, metals, and alloys – and reviewed major challenges in common processing methods. He said that current issues in phosphor production involve solid-state process issues, adding that most issues arise with defects or non-uniformity in materials, which come from the processing method. Hadidi reviewed the advantages of the UniMelt process, which he described in step-by-step fashion. He explained that the objective is to create materials with significant controllability of attributes, all in one process, even for materials with complex stoichiometry. Hadidi stated that in theory, the process can also be used for QDs, because plasmas have been used to make QDs and are thought to be the most efficient and economical way.
Stan Weaver of GE considered modular manufacturing in LED lighting, focusing on the system level. He noted that the display and LED lighting industries are changing very quickly in many different regards, and that a modular system allows these changes to be made more easily. Weaver said it’s necessary to be able to add or remove these modules quickly for rapid product changeover, which allows for reduced labor, higher throughput, and onsite assembly. His vision of modular assembly involves four units: light engine, assembly and casting, wiring and controls, and driver. With regard to the light engine, Weaver considered whether we can have plug-and-play optics, and whether we can reconfigure the light output of one engine. With regard to assembly and casting, the goal is to maximize space utilization and reduce enclosure weight. As for wiring and controls, the approach Weaver advocated was socketed connect, with wireless sensing. In considering the driver, he brought up the questions of integrated surge protection and wide input and output range, noting that the driver is one of the most difficult components to automate.
Bob Davis moderated a panel that looked at some of the latest research and industry committee activities related to color science, temporal light artifacts (e.g., flicker), and glare, and also considered priorities for future research in these and other lighting-science topic areas. Gilles Abrahamse of eldoLED talked about temporal lighting artifacts (TLAs), which include flicker, stroboscopic effects, and the phantom-array effect. He noted that human impairments from these phenomena include such things as distraction, reduced task performance, headaches, fatigue, and eyestrain. Abrahamse pointed out that almost every LED is driven with some sort of modulation or duty-cycled power supply. He reviewed the various flicker measures and recommended that they be consolidated, because right now there are too many metrics, which makes for a system with difficult comparisons and insufficient information. Abrahamse said there needs to be standardized measurement equipment, and the people who design and specify on these risks, solutions, and metrics need to be educated. He also called for continuing studies on TLAs and how they affect health, comfort, and productivity.
Yoshi Ohno of the National Institute of Standards and Technology focused on color-quality metrics and spectral design of SSL sources. He noted that because it’s a regional standard, TM-30 can’t replace CRI, which is internationally recognized and accepted, but that the difference between the two is that TM-30 represents “all kinds of object colors,” whereas CRI represents “typical daily-life objects.” Ohno observed that the color fidelity index Rf is not an accepted replacement for CRI, that there are perception effects beyond fidelity, and that consumers tend to prefer a slight oversaturation of colors. He noted that the biggest problem with CRI is that it weights under-saturation wavelengths over the oversaturation wavelengths, which is the opposite of what people typically prefer. Ohno said we need an evaluation of overall color quality, not just color fidelity, and also need another descriptor for “color perception/preference.” He said even gamut area isn’t sufficient, because different gamut shapes with the same gamut area can portray colors differently. Ohno called for more research to develop preference/perception metrics suitable for lighting products.
Brad Schlesselman of Musco Lighting discussed discomfort glare. He noted that for nearly a century, the lighting community has been trying to put standards and metrics together to describe glare, the understanding of which is more the responsibility of vision science than lighting science because it needs to be quantified before we can determine how to fix it. Schlesselman explained that glare has been so difficult to quantify because it’s affected by many factors, including source size, position, eye adaptation, contrast ratio (source luminance vs. background luminance), and aging-eye attributes, and that metrics to date – including the DeBoer scale, which is currently the one that’s most used – have been based on subjectivity. He noted, however, that several more-objective physiologically based methods have potential, including pupillometry, electromyography, and facial-feature tracking. Schlesselman reviewed past glare standards and noted that the IES committee that he chairs was put together to design objective measures and metrics to quantify discomfort glare.
OLED TRACK SESSIONS
A panel moderated by Lisa Pattison of SSLS, Inc., examined options for advances in organic stacks, including improvements in the traditional vapor-deposited emitters, transport materials, and solution-processed materials, and also discussed metrics and status for efficacy, color gamut, and reliability. Steve Forrest of the University of Michigan discussed how to make bright, long-lived, and efficient OLED devices for lighting. Noting that the important question for OLEDs is how to maximize blue and how to “keep it living,” he explained that he and his colleagues have long been investigating why blues are failing, and have been doing a lot of spectroscopy to detect defects. Forrest said they’ve confirmed that the emissive layer, and not the barrier layer, is the problem, and he reviewed the progress they’ve made in getting to the bottom of things. He noted that reducing exciton density by using stacked devices and graded recombination regions has shown a tenfold improvement in blue lifetime in his lab. This method works by reducing the collision of excited states in the organic layers, as such collision can result in molecular degradation. Another approach to mitigate degradation is to introduce an excited-state manager into the host, so that hot states are transferred to the manager, where the excess energy is eliminated. Forrest then introduced a concept for long-lived white OLEDs that uses both fluorescent and phosphorescent emitters in a hybrid device. As to future directions, he said the need is to find better manager molecules, to lower frontier orbitals, and to understand the degradation process.
Larissa Bergmann of Cynora talked about blue high-efficiency thermally activated delayed fluorescence (TADF) emitters for OLED lighting. She explained that the OLED industry is forced to use inefficient blue material because high-efficiency blue material isn’t available. Bergmann pointed out that several companies have shown that OLED lighting can exceed efficacies of 100 lm/W, but that so far, only phosphorescent sky blue could be used for high-efficiency white OLEDs, due to the short lifetime of deep-blue phosphorescent emitters. She emphasized that TADF can deliver high-efficiency blue with long lifetime, and said that using the TADF approach, Cynora is focusing on deep blue for displays and has increased the lifetime 1,000x in the past two years, going from just minutes in 2015 to 1,500 hours by the end of 2017. Bergmann stated that her company anticipates having blue emitters later this year, green emitters in 2019, and red emitters in 2020.
Michele Ricks of EMD Performance Materials focused on charge balance. She noted that OLED stacks are complex, and that to develop new materials, we need to understand the interaction between all the layers, and to tune the charge balance to get maximum performance. Ricks explained that her R&D team uses a systematic approach to develop next-generation materials and devices, considering the interactions among layers to develop targeted materials combinations. She noted that in 2010, she and her colleagues showed that charge balance strongly influences blue OLED performance. Ricks explained that efficiency and lifetime have their maximum at different charge balances, and that changing the hole transporting layer or the electron blocking layer unavoidably changes the charge balance, which needs to be accounted for when interpreting performance data. She said that by using oriented emitters, she and her colleagues improved the external quantum efficiency (EQE) of yellow emitters by 30%.
Chris Giebink of Penn State University focused on catastrophic OLED failure and how to address it. He explained that catastrophic failure is due to shorts, but that we don’t really know why they form. Giebink noted that when you’re looking for the origin of shorts, you need to find them early, before they become catastrophic, and that this involves using electroluminescence microscopy. He said it’s important to model their evolution and growth toward catastrophe, predict failure, and develop mitigation strategies. Giebink explained the difference between bright spots and hot spots, both of which form on OLEDs. He noted that hot spots are associated with catastrophic failure and are like volcanoes, with a dim spot in the center that’s surrounded by a bright area. The actual shunt current, Giebink said, is passing through the center. He also discussed achieving thermal stability using Teflon, which improves charge injection and bulk transport.
Norman Bardsley of Bardsley Consulting moderated a panel that considered new approaches needed to facilitate the production of flexible OLED panels of arbitrary shape, decrease processing times, increase yields, and reduce waste. John Hamer of OLEDWorks discussed the challenge of roll-to-roll (R2R) manufacturing of OLED lighting for the general lighting market. He said the market introduction of bendable OLED lighting products will start this year. Hamer observed that general lighting is a commodity business and requires commodity pricing, and predicted that OLED lighting will be made by an R2R process within 10 years. He noted that commodity pricing requires mature world-class mass manufacturing at the lowest possible cost, but that there’s a “chicken versus egg” problem, in the sense that dropping the cost will ramp up the volume, while ramping up the volume will drop the cost because of economies of scale. Hamer said that OLED lighting can achieve commodity pricing goals through R2R process development and production, and advocated keeping OLED lighting production in the U.S. “If we don’t do anything, China is going to dominate the OLED industry,” he said. “OLED is ours to lose.”
Neetu Chopra of Kateeva talked about enabling the mass production of flexible and large-sized OLED panels by using advanced inkjet printing. She said that Kateeva’s YIELDjet™ platform addresses the challenges of OLED manufacturing, and has been demonstrated in mass-production for two years for thin-film encapsulation. Chopra noted that many technological advances in inkjet printing were made to overcome historical barriers to mass-production, and that improvements in inkjet-printing equipment are making thin-film encapsulation extendible to foldable OLEDs. She said her company believes its YIELDjet platform can address red-green-blue (RGB) patterning for OLED TVs, and noted that YIELDjet EXPLORE R&D systems have been deployed for field qualification. Chopra stated that precision deposition by inkjet printing has the potential to enable many other technologies that require low-cost patterned films.
Bardsley discussed global R&D in R2R manufacturing of OLED lighting. He noted that there are institutes developing R2R infrastructure, but that very little of this has been applied to lighting in the U.S. Bardsley gave an overview of R2R efforts, including those by Konica Minolta, Fraunhofer, and KIMM. He noted that a lot of the infrastructure for R2R is being developed, and emphasized that patterning is essential. Bardsley observed that laser patterning equipment is available but has not yet been qualified for OLEDs. He reviewed the many benefits of R2R OLED manufacturing, including an estimated cost reduction of 20–30% in comparison to sheet processing, a higher amount of devices because of higher throughput, the possibility of long OLED stripes, and lower clean-room requirements than with sheet-to-sheet.
Bardsley moderated a panel that explored the addition of connectors, drivers, and mechanical supports to create OLED luminaires or to integrate panels into other structures, such as furniture or vehicles. The value of modules and light engines was also considered. Jay Eissner of Visa Lighting offered a fixture manufacturer’s perspective on OLED lighting. He reviewed OLED lighting’s marketing upsides (thin, unique designs; soft, even light; attractive appearance even in the off state) and challenges (high cost, low lifetime and durability, low efficacy, low lumen output, small panels, limited replaceability, limited CCT). Eissner also considered OLEDs’ engineering positives (easy heat management, no circuit board needed), and then looked at the challenges, such as a lack of robustness (up to 4% breakage during assembly), high minimum purchase quantities (though he said this is improving), power conversion components that are bulky compared to the OLED panel itself, and the fact that fluctuating voltage makes it difficult to combine multiple panels on one driver.
Melanie Kimsey-Lin of Boeing discussed the integration of OLED lighting in aircraft. She observed that aircraft lighting covers a broad range, from interior to exterior, and that performance requirements are driven by strict guidelines as well as by customer-defined needs. Kimsey-Lin noted that current airplane programs offer color-tunable lighting systems, which airlines utilize to create custom lighting scenes, and that airlines may further customize their interior through feature architecture and accent lighting. She added that as a critical component of airline branding, cabin lighting systems are subject to intense scrutiny, resulting in strict color and luminance requirements. Kimsey-Lin pointed out that stability and reliability are required of all aircraft components, and that any replacement lighting system must offer the same performance as current systems. She said OLEDs have many perceived advantages, but more investigation is required to ensure that they meet aerospace needs.
Mike Fusco of LED Specialists talked about OLED integration and enabling technologies. He reviewed his company’s work over the past year and half, developing OLED application-enabling technologies to accelerate OLED adoption, and described how he and his colleagues are working in close collaboration with OLEDWorks to develop products. Fusco explained that extensive market research conducted with a range of stakeholders found, among other things, that a low-voltage (Class 2) power distribution scheme will work for most applications; that there’s a need for small, thin, compact low-voltage drivers to be mounted on or in close proximity to OLED panels; that there’s a need for an OLED “module” containing the OLED, driver, mounting frame, and connection feature; that accessories must complement “thin,” lightweight OLED characteristics; and that drivers must be dimmable. He looked at OLED electrical system configurations, dimming, drivers, mounting accessories, design considerations, installation, and further R&D needed.
A panel moderated by Lisa Pattison explored the supporting components of the OLED stack and identified metrics to compare various approaches. Greg Cooper of Pixelligent described his company’s work with nanocrystal dispersion to enable innovative devices with nanocomposites. He explained that Pixelligent uses a high refractive index (HRI) and index gradient to get the light out, and said that OLED lighting will need internal HRI light extraction, which is the most efficient but has two issues that need to be addressed: simplifying the device with less layers and having fewer deposition steps, and increasing manufacturing yield and product lifetime. Cooper cited the issue of color uniformity, noting that too little or too much scattering is bad in that regard. He also considered several other approaches, including silver nanowires, directed emitters, and corrugated substrates. Cooper emphasized that for OLED lighting, solution-processable internal HRI light extraction performs best and is compatible with many other methods and device structures.
Ruth Shinar of Iowa State University discussed enhanced light extraction from OLEDs fabricated on flexible patterned substrates. She described how a periodic nanopatterned flexible substrate was used successfully to extract light trapped in the anode and organic layers and reduce the loss of photons to surface plasmon excitation. Shinar noted that an EQE of ~50% was obtained for green PHOLEDs, and that the OLEDs were largely conformal with the corrugation, likely dependent on the pattern’s pitch-to-height ratio. She said that an integrated metal micromesh with a continuous field conductor improves the anode electrical conductivity, and that R2R coating will likely be beneficial for uniformity. Shinar added that the corrugation and OLED design were supported by modeling.
Juan Russo of Luminit focused on light-shaping diffusers for OLED light extraction. He noted that although OLED IQE can approach 100%, the EQE is limited to 20–40%, hampered by low extraction efficiencies due to waveguiding and trapping of the light that either gets reabsorbed or exits the device in directions other than the viewing direction. Russo described Luminit’s light-extraction system for OLEDs (LESO), which features an advanced surface structure that increases light extraction as well as surface patterning in the interface between high and low refractive index layers, and that uses index-specific resins to reduce waveguide losses. He talked about the advantages of LESO’s light-shaping diffusers (LSDs), which are surface relief patterns that mimic diffuser function and that are made by holographic recording. The advantages of using this approach over scatterers for light extraction are that there is no loss from scattering sites and no wavelength dependence. LSD can also help with controlling uniformity and light distribution. While the LESO system has not yet been integrated with OLED devices, preliminary experiments with QD photoluminescent layers to mimic an OLED show promise.
Yue Qu of the University of Michigan talked about extracting light from OLEDs. He noted that only 20% of an OLED’s light will be extracted from a conventional glass surface, with the remaining amount of light lost in substrate mode, waveguide mode, and surface-plasmon mode. Qu described a sub-anode grid structure he and his colleagues are investigating to extract light from the waveguide and surface-plasmon modes. He noted that their grid enhances light output by up to 50%. Since the grid period is considerably larger than the optical wavelength, the outcoupling is independent of wavelength; and as the grid structure is planar, it’s not intrusive to the active region. Another approach Qu and his colleagues are researching is the use of flexible corrugated substrates, similar to the approach discussed by Shinar. Light-extraction enhancement using this approach is promising, but Qu explained that nonintrusive structures are more suitable for OLED manufacture.
ENHANCED FUNCTIONALITY AND BUILDING INTEGRATION
Monica Hansen moderated a panel that explored the technology challenges associated with building integration of new SSL form factors, a connected-lighting building platform, and the use of these technologies for enhancing user preference. Dorene Maniccia focused on the Edge, Deloitte’s headquarters in Amsterdam, widely considered to be the most connected and sustainable office building in the world. She gave a summary snapshot of the building’s advanced connected lighting system, which her company provided and which includes 6,800 Power over Ethernet (PoE) fixtures and 750 PoE switches. Maniccia explained that integrated sensors capture and distribute occupancy and human movement data, while data analytics enable the precise delivery of lighting, heating, cooling, and cleaning resources to reduce operational costs, optimize space, and reduce energy use. Maniccia noted that employees use an app to personalize their workspace lighting and temperature, improving comfort and well-being, and that Deloitte has seen a fourfold increase in job applicants, which they attribute to the building’s prestige and sustainability.
Kelly Gordon of PNNL focused on new and innovative form factors enabled by SSL. She showed photographs of various innovative SSL luminaires, both LED and OLED, some of which were entered in past Next Generation Luminaires design competitions, and which invited innovative approaches and form factors. Some of the OLED luminaires were integrated into the architecture or furniture, although Gordon observed that “the drivers have to go somewhere.” Among the ways to get a smaller lighting footprint, she noted, are reducing AC to DC conversion losses; using less materials; collecting and exchanging data with other devices and systems; significantly increasing luminous efficacy; leveraging renewable-energy generation; and designing for disassembly, remanufacturing, and recycling. Gordon stated that with SSL, “everything is possible,” but that anything nonstandard is more complicated and costly. She said that higher-efficacy LEDs will help because they make smaller heat sinks and less mass possible, and offer more freedom in design.
Richard Radke of Rensselaer Polytechnic Institute (RPI) spoke about occupant-aware, cognitive illuminated environments. He explained that RPI’s Center for Lighting Enabled Systems Applications (LESA) has set up a number of test beds, but the one he focused on was the one set up as a conference room. Radke said the conference room is set up with time-of-flight sensors that have been redesigned to track occupancy and adjust the lighting accordingly. He talked about the idea of using the data to enhance the comfort and productivity of the occupants, and how productivity might be measured. Radke also talked about how LESA is working on enhancing the room’s sensing capability and intelligence with acoustic sensors that could be programmed to distinguish tone of voice and even the content of what was being discussed, and that could provide helpful feedback to the occupants, based on that data.
Brodrick concluded the three-day workshop by thanking the attendees and speakers for their input and participation. He encouraged attendees to stay apprised of DOE SSL program activities by visiting www.ssl.energy.gov.