Tang Yau Hoong

Lighting designers have long sought to conceal luminaires, illuminating spaces so that an inhabitant notices only the light, not its source. Today, the notion that a luminaire could nearly disappear is closer than ever, thanks to evolutions in material technologies. The trend in LED lighting is toward smaller, thinner, more integrated systems that provide optimized performance but are practically invisible. “Our vision is where the fixture is the LED, where you don’t have LED components but where the whole thing is made as one manufactured piece,” says Julian Carey, a senior director of phosphor marketing at Intematix, which makes phosphor materials. Jim Anderson, the director of strategic marketing at Philips Color Kinetics, concurs. “Nobody wants to see the luminaire,” he says. “It’s been that way forever.”

Innovation in the field of LED lighting is currently taking the form of a race to the finish without a clearly marked route. Companies are charting their own course, investing in some combination of new and legacy materials with an eye toward producing the most powerful, most efficient LEDs for the market.

One example of this experimentation is in the development and implementation of substrates, a crucial component of an LED’s makeup. Today, companies use substrates made from various compounds, such as silicon, sapphire, polycarbonate, and gallium nitride, but no one material has emerged as the leader. And the same can be said for many of the materials in an LED fixture, from the optics to the light-emitting diodes themselves. “You keep looking for the next generation, and you’re either going to add performance or take costs out,” says Kevin Dunay, a market segment leader with Bayer MaterialScience.

Increased standardization would allow lighting companies greater flexibility in combining systems. On the other hand, Philips Color Kinetics’ Anderson suggests, too much standardization could stall innovation. These conditions aside, research and development certainly isn’t flagging. “LEDs are changing every day,” says Hugo da Silva, the global industry director for LED lighting at Dow Corning, whose moldable silicone products have gained traction in the past year.

Polycarbonate sheet material from Bayer MaterialScience.
Courtesy Bayer MaterialScince Polycarbonate sheet material from Bayer MaterialScience.

Remote Phosphor
One of the biggest innovations in the past two years has been remote phosphor technology, which separates the phosphor from the diode. Phosphor is nothing new. The light-emitting substance is the basis of glow-in-the-dark toys, and it’s been integral to solid-state lighting, typically in the form of a coating that is applied to blue LEDs to turn their light white. The key distinction with remote phosphor technology is that the phosphor material is integrated with the diffuser optic, increasing design freedom while decreasing manufacturing costs.

One company working in this realm is Intematix, which markets remote phosphor under the name ChromaLit. According to the company, remote phosphor can boost efficacy by as much as 30 percent because it effectively captures an LED’s spill light, extracting more lumens per blue LED than traditional designs. “The key thing to remember about phosphor is that it produces light [in] 360 degrees when stimulated so you have to figure out a way to reuse that light,” Intematix’s Carey says. Particularly in linear lighting applications, “remote phosphor very efficiently recycles that light so you have maximum efficiency.”

A major challenge for Intematix as they have pursued remote phosphor technologies has been achieving a consistency in color temperature. During initial investigations, researchers experienced subtle variations when their remote phosphor optic, typically made from a thermoplastic like polycarbonate, was combined with a customer’s LED. “The human eye is very sensitive to color changes,” Carey says. But with further development, these issues were resolved, and now remote phosphor offers a new way to control color, since hue and temperature now can be manipulated without altering the LED architecture.

“At Lightfair last year, there was a lot of interest from designers for very warm color temperatures for use in architectural lighting,” Carey says. “For example, 2400K. That’s not something that is easy to accomplish with a white LED. But with remote phosphor, you can just make that color. We dial in the formulation and you can have it.”

Remote phosphor has been adopted for applications ranging from undercabinet to high-bay spaces, but cost continues to be a barrier for certain types of luminaires. And manufacturers like Intematix continue to search for even greater efficiencies. Currently, the holy grail in the search for new phosphor materials is narrow-band red phosphor, which would allow manufacturers better access to certain portions of the light spectrum. “This is subject to maintaining CRI, but you can get much more efficiency if you have narrow-band red phosphor, because you’re not wasting light in the long portion of the red spectrum,” Carey says. “That’s one of the industry’s priorities.”

Dow Corning's moldable silicone offers a new option for secondary lamp optics.
Courtesy Dow Corning Dow Corning's moldable silicone offers a new option for secondary lamp optics.

Moldable Silicone
While remote phosphor integrates the light source with the secondary optic, optics themselves are undergoing a material revolution. New LED configurations such as chip-on-board designs have resulted in achieving higher, more consistent temperatures than ever before, while the push to reduce costs has manufacturers on the hunt for materials that can be inexpensively produced.

One of the things that LED manufacturers do agree on is the superior thermal and optical performance of glass. At the same time, the demand for ever-thinner materials and for ever-smaller sources has made glass less desirable. As a material, glass is highly inflexible. (We won’t even bring up how expensive it can be.) The alternatives, thus far, have been polycarbonate and PMMA (polymethyl methacrylate).

Polycarbonate is a type of thermoplastic polymer that enjoys a certain ubiquity in the modern world (the plastic case for Apple’s iPhone 5c, for instance) and is equally versatile in the solid-state lighting industry. Makrolon, a polycarbonate manufactured by Bayer MaterialScience, can be injection molded into reflectors or secondary optics for LED lighting. Bayer also makes Makrolon diffuser sheet products that have become popular in troffers. But polycarbonate does have its limitations, especially as LEDs get smaller. “You can only thin walls so much,” says Bayer’s Dunay.

Enter moldable silicone. In the hierarchy of materials, silicone is second only to glass, says Dow Corning’s da Silva, who is an expert in lighting and electronics and an evangelist for his company’s line of moldable silicone products. “Silicone combines the proven properties of plastics and the good properties of glass,” he says.

From a performance perspective, moldable silicone withstands high temperatures, high levels of ultraviolet light exposure, and high lumen densities. But its stability with different wall thicknesses is perhaps its greatest advantage over polycarbonate, and this advantage allows for greater design flexibility, especially when it comes to details that require undercuts, micro optical features, and negative draft angles. “At the end of the day, designers [have more freedom] to design with silicone than they [do] with any other material,” da Silva says.

Moldable silicone has gained a tremendous amount of momentum in the past year-and-a-half as manufacturers grasp the material’s potential. “A lot of people believe that moldable silicone can completely take over the optic materials [industry],” da Silva says. “Being more realistic, I believe silicones can have a fair share of the market, maybe 20 to 30 percent. Today, it’s not even close to that.”

Gallium Nitride Substrates
At the micro level, the diode itself is evolving. Innovation at this scale has the farthest-reaching implications, since luminaires often contain dozens of LEDs. So if each diode produces more light at a lower cost, lighting companies see substantial benefits.

First-generation LEDs all used some combination of gallium nitride (GaN) and a substrate made of a different material. Those substrates were made from glass, silicon, silicon carbide, and sapphire. In recent years, however, several companies, including Soraa and Panasonic, have experimented with making the substrate out of GaN as well. Using the same material for both layers creates a much more reliable diode, even when running at incredibly high power densities.

This advantage was not unknown to most LED manufacturers, but the cost of GaN has been prohibitive. “The notion of using a GaN substrate like we do was considered crazy by most people because the substrates were on the order of 100 times more expensive than sapphire substrates,” says Mike Krames, chief technology officer for Soraa. “Even when I was at Philips 10 years ago, we all knew that GaN substrates would be developed for lasers, but we couldn’t convince ourselves you could ever develop an LED system on it.”

Krames, however, was soon persuaded. “When talking to the founders after first joining Soraa,” he says, “I realized that if we could develop an LED system that could operate at 10 times the power density, then you’d have a huge lever on cost because the substrate is only about one-tenth of the cost of the LED. The rest of it is all in the downstream fabrication. So if I can use 10 times less LED material, I can afford to pay about 10 times the price per square area of the substrate. And since I only need one-tenth of the area of the substrate, I can afford to pay about 100 times the price of sapphire.”

GaN-on-GaN has also made possible full-spectrum LEDs that include violet emission. Until now, that’s only been a pipe dream. “Violet-emitting LEDs do not tolerate the lower quality of GaN-on-foreign-substrate,” Krames says. Blue LEDs create light that appears white but, unlike [the] incandescence of sunlight, is not full-spectrum because it lacks violet emission. By adding violet and harnessing the full spectrum, LEDs will be one step closer to imitating natural light.

Like several other LED companies, Soraa is vertically integrated. It oversees the manufacturing process from the fabrication of the LED to the assembly of the lamp. This helps drive down costs and is, in part, what allows the company to leverage its GaN-on-GaN technology. But Soraa’s not alone. Other companies seem to be confident that the math for GaN-on-GaN pencils out. Panasonic has invested heavily in making LEDs on GaN substrates, and late last year the company unveiled an automotive headlight that uses the technology. Although automotive applications differ substantially from architectural lighting, the release of the GaN-on-GaN product could mean Panasonic is moving in that direction for all of its LED products.

The Rate of Adoption
LEDs have evolved so quickly that material innovation has struggled to keep up. But in many ways, today’s technologies already have the power to transform the lighting industry. And now it is the industry itself, Dow Corning’s da Silva says, that is lagging behind. LED lamps, for the most part, continue to be designed to mimic existing lamp form factors, and da Silva sees the romanticism of outdated technologies as a fatal flaw, noting that his company has run up against this misplaced nostalgia as it has marketed its moldable silicones. “It’s been a journey the last four years to [create] new technologies that bring a lot of flexibility under the same technology umbrella,” he says. “The challenge is how conservative the industry is.”

Philips Color Kinetics’ Anderson, however, argues that while the average consumer may be attached to the shape of the incandescent lamp, the architectural lighting community is much more willing to try new technologies in order to get the effects it wants.

Anderson is part of the LED Systems Reliability Consortium, a committee of the Next Generation Lighting Industry Alliance (NGLIA). A white paper from NGLIA, in conjunction with the U.S. Department of Energy, first released in 2011 and updated this past September, and titled “LED Luminaire Lifetime: Recommendations For Testing And Reporting,” proposed an accelerated verification process for new technologies in order to spur faster innovation. The proposal is modeled on what’s used in the electronics industry. LED lighting, Anderson says, could be treated the same way, removing the inertia that comes with a validation process that requires 3,000 to 6,000 hours of testing.

This shift would be momentous—and symbolic. Early on, LEDs overpromised and underdelivered when it came to lighting performance. Manufacturers entering the lighting industry from the electronics world had little understanding of the unique and nuanced requirements of architectural lighting. But that is changing. “What people claim compared to the actual performance has significantly improved,” Anderson says. “There’s a better match there. The industry is maturing.”

A list of reference sources that address LED materials.

Next Generation Lighting Industry Alliance, LED Systems Reliability Consortium, “LED Luminaire Lifetime: Recommendations For Testing And Reporting,” Updated in September 2014, 1.usa.gov/1Bp4QsV

Ludwig Maximilian University of Munich, “LED Phosphors: Better Red Makes Brighter White,” ScienceDaily, June 2014, bit.ly/195t6qY

Michael J. Cich, Rafael I. Aldaz, Arpan Chakraborty, Aurelien David, Michael J. Grundmann, Anurag Tyagi, Meng Zhang, Frank M. Steranka, and Michael R. Krames, “Bulk GaN Based Violet Light-Emitting Diodes With High Efficiency at Very High Current Density,” Applied Physics Letters 101, 223509, American Institute of Physics, November 2012, bit.ly/1CVGFpc