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Thesulfur lamp (alsosulphur lamp) is a highlyefficientfull-spectrumelectrodeless lighting system whose light is generated bysulfurplasma that has beenexcited bymicrowave radiation. They are a particular type ofplasma 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.
The sulfur lamp consists of agolf ball-sized (30 mm)fused-quartz bulb containing severalmilligrams ofsulfur powder andargon gas at the end of a thin glass spindle. The bulb is enclosed in a microwave-resonant wire-mesh cage. Amagnetron, much like the ones in homemicrowave ovens, bombards the bulb, via awaveguide, with2.45 GHz microwaves. The microwave energy excites the gas to fiveatmospheres pressure, which in turn heats the sulfur to an extreme degree forming a brightly glowingplasma 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 thefocus of aparabolic reflector to direct all the light in one direction.
It would be impossible to excite the sulfur using traditionalelectrodes since the sulfur would quickly react with and destroy anymetallic electrode. A patent pending to employ coated electrodes is discussed inFuture 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 themagnetron 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 offluorescent lamps, even at low ambient temperatures. It reaches 80% of its finalluminous 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 asystem efficiency of 80lumens perwatt.[1] 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.[2]
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.
The sulfur plasma consists mainly ofdimer molecules (S2), which generate the light throughmolecular emission. Unlikeatomic emission, theemission spectrum is continuous throughout thevisible spectrum. As much as 73% of the emitted radiation is in the visible spectrum, with a small amount ininfrared energy and less than 1% inultraviolet light.
The spectral output peaks at 510 nanometres, giving the light a greenish hue. The correlatedcolor temperature is about 6,000kelvins with aCRI of 79. The lamp can be dimmed to 15% without affecting the light quality.
Amagenta filter can be used to give the light a warmer feel. Such a filter was used on the lamps at theNational Air and Space Museum inWashington, D.C.[3]
The addition of other chemicals in the bulb might improve color rendition. Sulfur lamp bulbs withcalcium bromide (CaBr2) added produce a similar spectrum plus a spike in red wavelengths at 625 nm.[4] Other additives such aslithium iodide (LiI) andsodium iodide (NaI) can be used to modify the output spectra.[5][6]
The technology was conceived byengineer Michael Ury,physicist Charles Wood and their colleagues in 1990. With support from theUnited States Department of Energy, it was further developed in 1994 by Fusion Lighting ofRockville, Maryland, a spinoff of the Fusion UV division of Fusion Systems Corporation. Its origins are in microwave discharge light sources used for ultravioletcuring in the semiconductor and printing industries. The Fusion UV division was later sold toSpectris plc, and the rest of Fusion Systems was later acquired by theEaton 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.[7] Fusion Lighting closed its Rockville, MD location in early 2002-February 2003, after consuming approximately $90 million inventure capital.[8] Their patents were licensed to theLG Group. Their lamps were installed in more than one hundred facilities worldwide, but many of them have since been removed.[citation needed]
In 2001, Ningbo Youhe New Lighting Source Co., Ltd, inNingbo,China, produced its own sulfur lamp version.[citation needed] 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.[citation needed]
The magnetrons in these lamps may causeelectromagnetic interference in the2.4 GHz wireless spectrum, which is used byWi-Fi, cordless phones andsatellite radio inNorth America. Fearing interference with their broadcasts,Sirius andXM satellite radio petitioned the United StatesFederal 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 metalshielding 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.[9] 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 the2.45 GHz band and cause harmful interference to satelliteradio receivers as a consequence, and our existing limits prove inadequate, we will at that time take appropriate action.
Unlike fluorescent andhigh-intensity discharge lamps, sulfur lamps contain nomercury. Therefore, sulfur lamps do not pose a threat to the environment nor require special disposal.[citation needed] In addition, use of sulfur lamps has the potential to reduce the total amount of energy required for lighting.
As sulfur lamps in current production have a high output, it is often necessary to distribute the light to areas removed from the lamp. This can be achieved by usinglight pipes as a conduit.
The 3Mlight pipe is a long, transparent, hollowcylinder with aprismatic surface developed by3M that distributes the light uniformly over its length. Light pipes can be as long as 40 metres (130 ft) and are assembled on site from shorter, modular units. The light pipe is attached to the parabolic reflector of the sulfur lamp. For shorter pipes, there will be a mirror at the opposite end; for longer ones, there will be a lamp at each end. The overall appearance of a light pipe has been compared to that of a giant-sizedfluorescent tube. One sulfur lamp with a light pipe can replace dozens ofHID lamps. In theNational Air and Space Museum, three lamps, each with a 27-metre (89 ft) pipe, replaced 94 HID lamps while greatly increasing the amount of light delivered.[3]
A reduced number of individual lamps may simplify maintenance and reduce installation costs but may also require a backup system for areas where lighting is critical. The light pipes allow the lamp to be placed in an easily accessible area for maintenance and away from places where the heat of the lamp may be a problem.
A secondary reflector is a structure with a mirrored surface placed directly into the path of the beam of light as it exits the parabolic primary reflector of the lamp. A secondary reflector can have a complex geometry which allows it to break up the light and direct it to where it is desired. It can spotlight an object or spread out the light for general illumination.
AtSundsvall-Härnösand Airport nearSundsvall,Sweden, airfield lighting is provided by sulfur lamps mounted on towers 30 metres tall. The lamps are directed upward and shine their light onto wing-shaped secondary reflectors that spread the light out and direct it downward. In this way, one lamp can illuminate an area 30 by 80 metres (100 by 260 ft).
At the headquarters ofDONG Energy, an energy company in Denmark, a single sulfur lamp directs its light onto numerous specular reflectors and diffusers to illuminate the entrance hall as well as several sculptures outside of the building.
At the entrance to University Hospital inLund,Sweden, secondary reflectors on the ceiling are clad with highly reflective films, but shaped so as to avoid any glare. Moreover, since these films have a microprismatic surface structure that splits up the beams, the risk of glare problems is further reduced. The fact that the reflectors move the light source far away from the eye of anyone who would happen to look into them helps to further eliminate glare problems.[10]
Indirect fixtures direct most of their luminous flux upward toward a ceiling. A highly reflective ceiling can then serve as a secondary source of diffusive, low luminance, high visual quality lighting for interior spaces. The primary advantages of indirect lighting are the opportunity to significantly reduce indirect glare potential and to eliminate direct source viewing.[11]
At theSacramento Municipal Utility District (SMUD) headquarters building, two sulfur lamps were installed in the tops of free-standingkiosks. The 4.2-metre (13 ft 9 in) high ceiling was retrofit with high reflectance (90%), white acousticceiling tile. The lamps direct their light upward, and it is reflected off the ceiling providing indirect light. Narrow, medium, or wide beam patterns can be created by choosing various reflector elements.[12]
Light pipes would not be necessary in applications such asstadium lighting, where a plain fixture can be mounted high enough so that the light can spread over a large area. The installation atHill Air Force Base contains lamps with light pipes as well asdownlight fixtures mounted high in an aircrafthangar.
Optical fibers have been studied as a distribution system for sulfur lamps, but no practical system has ever been marketed.[13]
Sulfur lamps can be used as light sources in scientific instruments.
The development of an affordable, efficient, and long-lived microwave source is a technological hurdle to cost reduction and commercial success. The lamp prototypes were only available in high wattages (1000+ W), which impeded adoption in applications where light output demands were not great. The sulfur lamp has problems with the life of the magnetron and the motor that rotates the bulb and noise from the cooling fan. Because most sulfur lamps have moving parts, reliability remains a critical issue, and system maintenance may impede market adoption, however newer-design lamps which no longer require active cooling are commercially available.[2]Researchers have had some success at eliminating the need to rotate the bulb by usingcircularly polarized microwaves to spin the plasma discharge instead.[14][15] Other experiments have usedsodium iodide,scandiumiodide,indium monobromide (InBr),[16][17] ortellurium[18] as the light-generating medium.
Many of the installations of the lamps were for testing purposes only, but there remain a few sites where the lamps are in use as the primary lighting source. Perhaps the most visible of these would be the glassatria in theNational Air and Space Museum.
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