Oak Ridge National LaboratoryOak Ridge, Tenn.

Dec. 21, 2004
Hybrid lighting
Doug Bartholomew, Samuel Greengard, Glenn Hasek, John Jesitus, Scott Leibs, Kristin Ohlson, Robert Patton, Barb Schmitz, Tim Stevens, and John Teresko contributed to this article. With artificial lighting consuming about 25% of the U.S. electric bill each year, researchers at the U.S. Dept. of Energy's Oak Ridge National Laboratory (ORNL) are developing hybrid lighting solutions -- technologies that incorporate both natural and artificial lighting -- to reduce those costs. In partnership with six companies -- Fiber Optic Technologies Inc., Pomphret, Conn.; Daylite Co., Ventura, Calif.; Remote Source Lighting International, San Juan Capistrano, Calif.; Shakespeare Monofilament Div., Columbia, S.C.; and Plylite Inc., Minneapolis -- ORNL is researching how best to use optical fibers to transport light. While skylights currently are used in some buildings to supplement artificial light sources, the light from them cannot be transported. Using technology developed by ORNL and its partners, however, the transportation of natural light is becoming feasible. Jeff Muhs, a research scientist who leads the photonics and fiber-optics group at ORNL, says that his group of researchers is studying how best to transport light from a collector positioned at the top of a building. Muhs says that in an optimal hybrid lighting arrangement, a collector, using some form of concentrator such as lenses or mirrors, would direct sunlight through optical fibers. Once the light is directed into a building through the fibers, it could be dispersed into each room, or areas of a room, just as cables or wires carry electricity into a room. Fluorescent lighting would supplement the natural light. Light sensors would constantly regulate the light levels in each room. The key to the success of hybrid lighting will be the ability of the optical fibers, often called optical light guides, to transport large amounts of light. In optical light guides today, 1% of the light is lost per foot of distance traveled. ORNL's goal is to stretch that 1% loss to every 10 feet. Optical fibers are more efficient when they are fabricated with a highly transparent core surrounded by a transparent cladding of material with a lower index of refraction than the core. At the interface of core and cladding, light impinging at less than a certain angle (called the critical angle), is perfectly reflected off the optical interface with the cladding and continues down the core to its ultimate destination. Muhs says hybrid lighting makes a lot more sense than solar collection systems that use the sun to charge batteries that, in turn, power illumination systems. "We believe it is much easier to redistribute light rather than convert it twice," he says. As the number of office buildings grows along with the cost of electricity, Muhs says it is imperative to figure out ways to better use sunlight in buildings. The benefits may be more than just lower costs. "There have been several studies that show that use of sunlight in buildings has a positive impact on morale and productivity," he says. Stores have even found that goods displayed in areas illuminated by skylights have sold better. Another benefit to reducing the amount of artificial lighting in a building is the savings in energy needed to cool the building. About 10% of cooling and ventilation costs result from clearing away the heat generated from lighting. While some hybrid lighting products currently are being developed by ORNL's partners, it may be another few years before such products are marketed fully and another eight to 10 years before the technology will have a major impact.

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