Lighting is poised to be the next great solid-state frontier. The technology is already making inroads in the markets for architectural lighting, signage, and specialty residential and retail applications.According to Robert Steele, director of optoelectronics programs at Strategies Unlimited (Mountain View, CA), the overall lighting market for LEDs in 2005 was an estimated $250 million. Hard numbers aren't yet available, but he projects the market to reach roughly $1 billion by 2010, with steady year-over-year growth rates of 30 to 40% annually. "It's still small but it's emerging," he says. "There's a huge amount of activity." One of the strengths of LED lighting is reliability, making the industrial market a natural target of opportunity. In an industrial setting, replacing burned out bulbs doesn't just incur parts costs but also production downtime costs. Given the industrial setting's voracious appetite for lumens, however, LED lighting is going to have to meet some pretty strict performance standards.

Industrial Strength
LEDs generate white light through a mix of red, green, and blue output or by downconversion via phosphors. In theory, the technology offers high efficiency; the reality is lagging a bit.

According to "Solid-State Lighting Research and Development Portfolio," a U.S. Department of Energy (DoE) roadmap compiled in conjunction with a panel of industry experts, current white-light LEDs can produce from 20 to 45 lm with a luminous efficiency of 20 lm/W or 45 lm/W, respectively. In contrast, a typical fluorescent bulb produces 5300 lm at a luminous efficiency of 83 lm/W, and a high-intensity discharge bulb produces 24 klm at a luminous efficiency of 80 lm/W. Clearly, LEDs are at an enormous disadvantage in terms of overall output, despite boasting lifetimes a factor of two or more higher than the other technologies.

Industry doesn't care as much about lifetime if it can't get the light it needs for an assembly line or a high bay, though. "For these particular applications, I think the requirement is still tens of thousands of lumens," says Srinath Aanegola, director for white LED technology at GELcore (Valley View, OH), and a member of the technical committee for the DoE report. "Whether solid-state lighting will be the right choice is still a question mark."

Power On
Of course, the aforementioned DoE roadmap carries two important milestones: reaching an efficiency of better than 100 lm/W by 2010 and reaching a value of $3/klm by 2015 (compared to $0.6/klm for fluorescents). Companies like Nichia (Tokyo, Japan) have reported reaching 100 lm/W for small white-light LEDs, but efficiencies typically drop by 50% when devices are scaled up to the large chips and color temperatures appropriate for industrial applications.

Past the magic 100 lm/W barrier, a whole host of interesting things happen. LEDs will provide better luminous efficiency and lifetime than any lighting alternative, on a competitive cost basis.

"In an industrial or a commercial setting, people pay attention to life cycle costs," says Steele. "If you're saving on labor and saving on electricity, you're willing to pay more up front."

With this efficiency comes the opportunity to achieve high-lumen output. Consider a white-light LED that produces 1000 lm at 100 lm/W. "Because I have more efficiency, can I drive it at 30 W and get three times the light output, make this a 3 klm source?" Aanegola asks. "I think that's what most people will try to do because what is going to drive penetration is how many lumens you're getting for every dollar you spend."

Of course, this approach raises the pesky issue of thermal management. Incandescent and fluorescent bulbs dissipate heat radiatively; LEDs are limited to conduction and convection. Moreover, incandescents generate and release most of their heat through the front of the bulb, which is available to the open air; LEDs generate most of their heat at the back, where they would presumably be plugged into a fixture. When you're talking about 4 or 5 W, it's not such a challenge. When you're talking about dissipating the tens or even hundreds of watts that could be generated by an industrial fixture, however, conductive and convective cooling are simply not feasible. Not only are there safety issues, there's the nasty little fact that elevated heat slashes device lifetime, which is a key part of the LED lighting value proposition.

"Then you've got to maybe change the shape, the fixture, and include active cooling techniques so you're not impacting lifetime, which is a key requirement," says Aanegola. But he's quick to point out that adding a thermoelectric cooler or a fan, say, in turn adds cost or failure points.

In the case of office lighting, for example, which would face far lower output requirements, cooling via heatsinks presents less of a challenge, says Kathryn Conway, LED Consulting (Nassau, NY). "Because they have to have a lot of surface area, they can be made in decorative shapes so it doesn't really detract. It can be made kind of aesthetically pleasing."

Fitting the Niche
If LED lighting for general industrial illumination has yet to take off, the technology is still moving along one niche at a time. "You have to find where LEDs will bring some value that you can't get with other light sources," Steele says. LED reliability, for example, has gained the technology a strong foothold in machine vision. "If the lamp on your machine vision system fails and you've got to shut the production line down for even 10 minutes to change it, that could be thousands of dollars of lost production time."

Conway recently scouted the Light+Building show (Frankfurt, Germany; 23-27 April) and discovered new trends in industrial lighting, including office-type fixtures that feature LEDs on the edges, mixing their output with that of other sources. "They're using the molded plastic that is there to reinforce the fixture for the industrial environment, but with the color LEDs, they're able to get kind of a cool decorative effect because the light travels on the edges of the reinforcing element. What they're doing with the color is adding some interest to it."

She also saw demonstrations of LED footpath systems, for example, to mark a pathway or illuminate a perforated steel element such as an electrical-cord cover. LEDs also showed up in handheld inspection devices.

As to applications beyond the niches, they still await the crossing of the 100 lm/W bar. "The earliest I could see LEDs enter the mainstream of manufacturing environment would be probably be around 2012, when we would have at least 100 lm/W and preferably maybe 150 lm/W. Then things change dramatically," says David Pelka, president of Tailored Optics Inc. (Los Angeles, CA).

For now, the industry just keeps moving. "There's been a lot of progress," says Steele. "The best white LEDs are now three times as efficient as incandescent and getting pretty close to compact fluorescents. After that, they'll be hitting the efficiency of linear fluorescents, and I think that's going to have an impact."

Learn more about the latest breakthroughs in LEDs at the Illumination Engineering conference at Optics & Photonics this August in San Diego, CA. Find out more at spie.org/events/op.

From SPIE Professional July 2006 - Kristin Lewotsky