The sulfur lamp (also sulphur lamp) is a highly efficient full-spectrum electrodeless lighting system whose light is generated by sulfur plasma that has been excited by microwave radiation. They are a particular type of plasma lamp, and one of the most modern. The technology was developed in the early 1990s and appeared promising, but was not a commercial success by the late 1990s. Since 2005, lamps are again being manufactured for commercial use.
Mechanism The sulfur lamp consists of a golf ball-sized (30 mm) fused-quartz bulb containing several milligrams of sulfur powder and argon gas at the end of a thin glass spindle. The bulb is enclosed in a microwave-resonant wire-mesh cage. A magnetron, much like the ones in home microwave ovens, bombards the bulb, via a waveguide, with 2.45 GHz microwaves. The microwave energy excites the gas to five atmospheres pressure, which in turn heats the sulfur to an extreme degree forming a brightly glowing plasma capable of illuminating a large area. Because the bulb heats considerably, it may be necessary to provide forced air cooling to prevent it from melting. The bulb is usually placed at the focus of a parabolic reflector to direct all the light in one direction. It would be impossible to excite the sulfur using traditional electrodes since the sulfur would quickly react with and destroy any metallic electrode. A patent pending to employ coated electrodes is discussed in Future prospects below. The absence of electrodes allows for a much greater variety of light-generating substances to be used than those used in traditional lamps. The design life of the bulb is approximately 60,000 hours. The design life of the magnetron has been improved by the Germany/England based Plasma International so it can also last for that same period. The warm-up time of the sulfur lamp is notably shorter than for other gas discharge lamps, with the exception of fluorescent lamps, even at low ambient temperatures. It reaches 80% of its final luminous flux within 20 seconds, and the lamp can be restarted approximately five minutes after a power cut. The first prototype lamps were 5.9 kW units, with a system efficiency of 80 lumens per watt. The first production models were 96.4 lumens per watt. Later models were able to eliminate the cooling fan and improve luminous efficacy to 100 lumens per watt. By comparison, cost-effective commercially available LED chips are available with around 160 lumens per Watt (2023) efficacy, with a typical light output depreciation of 10% after 50,000 hours, dependent on operating environment.
Quality of emitted light The sulfur plasma consists mainly of dimer molecules (S2), which generate the light through molecular emission. Unlike atomic emission, the emission spectrum is continuous throughout the visible spectrum. As much as 73% of the emitted radiation is in the visible spectrum, with a small amount in infrared energy and less than 1% in ultraviolet light. The spectral output peaks at 510 nanometres, giving the light a greenish hue. The correlated color temperature is about 6,000 kelvins with a CRI of 79. The lamp can be dimmed to 15% without affecting the light quality. A magenta filter can be used to give the light a warmer feel. Such a filter was used on the lamps at the National Air and Space Museum in Washington, D.C. The addition of other chemicals in the bulb might improve color rendition. Sulfur lamp bulbs with calcium bromide (CaBr2) added produce a similar spectrum plus a spike in red wavelengths at 625 nm. Other additives such as lithium iodide (LiI) and sodium iodide (NaI) can be used to modify the output spectra.
History The technology was conceived by engineer Michael Ury, physicist Charles Wood and their colleagues in 1990. With support from the United States Department of Energy, it was further developed in 1994 by Fusion Lighting of Rockville, Maryland, a spinoff of the Fusion UV division of Fusion Systems Corporation. Its origins are in microwave discharge light sources used for ultraviolet curing in the semiconductor and printing industries. The Fusion UV division was later sold to Spectris plc, and the rest of Fusion Systems was later acquired by the Eaton Corporation. Only two production models were developed, both with similar specifications: the Solar 1000 in 1994 and the Light Drive 1000 in 1997, which was a refinement of the previous model. Production of these lamps ended in 1998. Fusion Lighting closed its Rockville, MD location in early 2002-February 2003, after consuming approximately $90 million in venture capital. Their patents were licensed to the LG Group. Their lamps were installed in more than one hundred facilities worldwide, but many of them have since been removed. In 2001, Ningbo Youhe New Lighting Source Co., Ltd, in Ningbo, China, produced its own sulfur lamp version. In 2006, LG Electronics began production of its sulfur lamps, called Plasma Lighting System]. Sulfur lamps were produced in the 2010s by Hive Lighting as the Wasp 1000. It can be identified by the mesh that surrounds the glass bulb. It was later discontinued.
Electromagnetic interference The magnetrons in these lamps may cause electromagnetic interference in the 2.4 GHz wireless spectrum, which is used by Wi-Fi, cordless phones and satellite radio in North America. Fearing interference with their broadcasts, Sirius and XM satellite radio petitioned the United States Federal Communications Commission (FCC) to force Fusion Lighting to reduce the electromagnetic emissions of their lamps by 99.9%. In 2001, Fusion Lighting agreed to install metal shielding around their lamps to reduce electromagnetic emissions by 95%. In May 2003, the FCC terminated the proceeding that would have defined out-of-band emission limits for radio-frequency lights operating at 2.45 GHz, saying the record of the proceeding had become outdated and Fusion Lighting had stopped working on such lamps. The order concluded:
We therefore decline to provide the requested relief from the Satellite Radio Licensees to prohibit operation of all RF lights in the 2.45 GHz band, as we find that the requested prohibition is overarching and is not warranted based on the circumstances. If there is evidence that any entity will seek to operate RF lights in the 2.45 GHz band and cause harmful interference to satellite radio receivers as a consequence, and our existing limits prove inadequate, we will at that time take appropriate action.
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