Lighting is the single largest energy user in commercial buildings, consuming about one-third of the total electricity. The Hybrid Lighting Partnership -- at this point still an informal group of 10 companies and the US Department of Energy's (DoE) Oak Ridge National Laboratory -- plans to combine natural and artificial lighting into a much more efficient lighting system.Hybrid lighting includes a sunlight collector on the roof of a building to feed the natural light into large-core optical fibers. The partnership hopes to have prototypes ready to be tested in 18 months, and commercially viable hybrid lighting systems available by 2003, according to Jeff Muhs, co-developer of the project at Oak Ridge. The group has developed a strategic business plan and was expecting to receive initial assessment funds from the DoE by the end of 1998, he said. "In addition, our partners have committed to provide in-kind contributions to the partnership." The DoE's Office of Energy Efficiency and Renewable Energy will assess the project's viability over the next several months while continuing to complete an industry-wide lighting technology road map, Muhs said. Hybrid lighting essentially will involve a sunlight collector on a rooftop that will feed the natural light into large-core optical fibers. The sunlight will emerge at a hybrid luminaire inside buildings. Electric lamps such as fluorescent lamps will be in the hybrid fixture along with the light emerging from the optical fibers. Hybrid lighting will provide spatial and temporal illumination control, eliminate glare and optimize the use of sunlight, according to Muhs.Maintaining quality of lightThere are, however, issues relating to light quality. "Whenever you collect sunlight, that sunlight has differing temperatures, so when the sun is low in the sky it has a reddish tint, and it gets white the higher it gets," Muhs said. "The optical fiber will have to attenuate the light and shift with the sun, so we are looking at developing affordable models that predict the effective quality of light emerging and make sure it matches the electric lamps in the fixtures. You wouldn't want the light to go away when the sun goes behind a cloud, for example."The eventual goal is to have products that will cost about $300 per square meter -- or systems that will have an add-on cost of about $1 per square foot of building floor space -- although that will be several years down the road, Muhs said. If the researchers can reduce the cost of existing sunlight collection systems used to generate electricity by replacing the photovoltaic components with a passive interface to optical fibers, companies should have a reasonable payback period, he said. The Hybrid Lighting Partnership recently completed an initial market analysis of what will be required to make hybrid lighting a reality. The target market will be commercial buildings that are one or two stories tall, Muhs said. "Light has a very large percentage of electrical use (about 30 percent) in these buildings, with large floor plates that are not close to windows."In the US there is 70 billion feet of commercial building space. For a conventional office building, the operational cost per square foot is $180, which includes employee salaries. Lighting is only about $1 of that $180, but studies have shown that people are more productive when they have natural light in their work space. So if you can improve productivity by just a percent or so, the overall operation cost savings will be significant."