The figure above is a schematic of the system installed at the National Air and Space Museum and the DOE headquarters in Washington, D.C., Light from the sulfur lamp is focused by a parabolic reflector so that it enters the light pipe within a small angular cone. Light travels down the pipe, reflecting off the prismatic film (A) that lines the outer acrylic tube. The prismatic film reflects the light through total internal reflection (C), an intrinsically efficient process. Some of the light striking the film (at A) is not reflected and "leaks out" of the pipe walls (B), giving the pipe a glowing appearance. A light ray that travels all the way down the pipe will strike the mirror at the end (D) and return back up the pipe. A special light-extracting surface (another type of reflecting film) is used to draw the light out of the pipe in a controlled manner to where it is most needed (E).
In 1994, DOE announced that a new, highly efficient lighting system was illuminating the exterior of the Forrestal Building in Washington, D.C., and the Space Hall of the Smithsonian's National Air and Space Museum. The new system is a technological breakthrough that couples high-power sulfur lamps to a light pipe system that distributes the light. The lighting of the two buildings is the first working U.S. example of the high-power version of the sulfur lamp. In these installations, a hollow pipe distributes focused light from the sulfur lamp evenly over large areas.
The sulfur lamp bulb consists of a spherical quartz envelope filled with a few milligrams of sulfur and an inert noble gas, such as argon, which is weakly ionized using microwaves. The argon heats the sulfur into a gaseous state, forming diatomic sulfur molecules, or dimers. The dimers emit a broad continuum of energy as they drop back to lower energy states-a process called molecular emission. Molecular sulfur emits almost entirely over the visible portion of the electromagnetic spectrum, producing a uniform visible spectrum similar to sunlight but with very little undesirable infrared or ultraviolet radiation. Conventional mercury lamps and most other high-intensity discharge (HID) sources are built around atomic emission and produce an artificial-looking light with many missing colors.
Unlike conventional sources whose outputs typically diminish 75% over time, sulfur lamps will maintain their efficiency and light output over their entire lifetimes. By eliminating the need to compensate for lamp lumen depreciation, fewer sulfur lamps can provide a required light level, possibly for long lives of up to 50,000 hours. In addition, sulfur lamps contain no mercury, an environmentally toxic substance used in all other conventional efficient sources.
The sulfur lamp was developed originally by scientists (now at Fusion Lighting in Rockville, Maryland) who discovered that sulfur excited by microwave energy could be used in place of mercury in ultraviolet industrial lamps to produce a high-quality white light. These lamps operated at power and light output levels (3.5 KW input and 450,000 lumens) too high for most commercial applications. The high wattage required air-cooling and spinning the lamps to operate them. Applying their expertise in electrodeless discharge lamps, LBL researchers developed lower-power lamps using radio frequencies instead of microwaves. In 1993, they demonstrated an RF-driven sulfur lamp that produced up to 15,000 lumens with an RF input of only 100 watts-a luminous efficacy of approximately 150 lumens per RF watt. While the lamps still needed to be rotated, lower-power operation allowed the air cooling to be eliminated.
Although they are prototypes, the first-generation lamps at the Forrestal Building and the National Air and Space Museum are nonetheless energy- efficient. The Forrestal Building's 280-foot light pipe and two sulfur lamps replaced about 280 mercury HID fixtures, resulting in a measured energy savings of more than 65% and saving DOE approximately $8000 annually in energy costs. Because the sulfur lamp system replaced an old mercury system at the end of its maintenance cycle, the new light levels were roughly four times those of the old system. Maintenance costs are also lower, saving an additional $1500 per year.
DOE is funding Fusion Lighting through LBL to develop a microwave- operated, high-power sulfur lamp of 1000 watts, producing 125,000 lumens. It is best suited for applications like sports stadiums, convention centers, aircraft hangars, large maintenance facilities, highway and street lighting, and shopping mall and industrial lighting. Another DOE-funded project at Fusion Lighting is aimed at developing a commercial RF-driven sulfur lamp at lower power (50-100 watts)-small enough for use in homes and commercial buildings.
Building Technologies Program
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