Sulphur lamp
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The sulfur lamp (also sulphur lamp) is a highly efficient
full-spectrum Full-spectrum light is light that covers the electromagnetic spectrum from infrared to near-ultraviolet, or all wavelengths that are useful to plant or animal life; in particular, sunlight is considered full spectrum, even though the solar spectral ...
electrodeless lighting system whose light is generated by
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral), a green translucent silica mineral * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood pla ...
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, but, although it appeared initially to be very promising, sulfur lighting was a commercial failure 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 Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
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 The standard atmosphere (symbol: atm) is a unit of pressure defined as Pa. It is sometimes used as a ''reference pressure'' or ''standard pressure''. It is approximately equal to Earth's average atmospheric pressure at sea level. History The s ...
pressure, which in turn heats the sulfur to an extreme degree forming a brightly glowing
plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral), a green translucent silica mineral * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood pla ...
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 In photometry, luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light), in th ...
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.


Quality of emitted light

The sulfur plasma consists mainly of
dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ( ...
molecules (S2), which generate the light through
molecular emission The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an electron making a transition from a high energy state to a lower energy state. The photon energy of t ...
. Unlike
atomic emission Atomic may refer to: * Of or relating to the atom, the smallest particle of a chemical element that retains its chemical properties * Atomic physics, the study of the atom * Atomic Age, also known as the "Atomic Era" * Atomic scale, distances comp ...
, the
emission spectrum The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an electron making a atomic electron transition, transition from a high energy state to a lower energy st ...
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
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in Washington, D.C. The addition of other chemicals in the bulb might improve color rendition. Sulfur lamp bulbs with
calcium bromide Calcium bromide is the name for compounds with the chemical formula Ca Br2(H2O)x. Individual compounds include the anhydrous material (x = 0), the hexahydrate (x = 6), and the rare dihydrate (x = 2). All are white powders that dissolve in water, ...
(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 Sodium iodide (chemical formula NaI) is an ionic compound formed from the chemical reaction of sodium metal and iodine. Under standard conditions, it is a white, water-soluble solid comprising a 1:1 mix of sodium cations (Na+) and iodide anions (I ...
(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
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, 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. Th
Fusion UV
division was later sold to
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, and the rest of Fusion Systems was later acquired by the
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. 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 February 2003, after consuming approximately $90 million in venture capital. Their patents were licensed to the
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. The Internet Archive has a copy o
Fusion Lighting's defunct website
Their lamps were installed in more than one hundred facilities worldwide, but many of them have already been removed. In 2001, Ningbo Youhe New Lighting Source Co., Ltd, in Ningbo,
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
, produced its own sulfur lamp version. The company's website is no longer online and may be out of business, but information on these lamps is available from it
archived copy
at the Internet Archive. In 2006, LG Electronics began production of its sulfur lamps, calle
Plasma Lighting System
(PLS). Sulfur lamps were produced in the 2010s by Hive Lighting as th
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 Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electros ...
in the 2.4 GHz wireless spectrum, which is used by Wi-Fi, cordless phones and
satellite radio Satellite radio is defined by the International Telecommunication Union (ITU)'s ITU Radio Regulations (RR) as a ''broadcasting-satellite service''. The satellite's signals are broadcast nationwide, across a much wider geographical area than ter ...
in
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. 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:


Environmental issues

Unlike fluorescent and high-intensity discharge lamps, sulfur lamps contain no
mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...
. Therefore, sulfur lamps do not pose a threat to the environment nor require special disposal. In addition, use of sulfur lamps has the potential to reduce the total amount of energy required for lighting.


Light distribution systems

Because the amount of light produced from one bulb is so great, it is usually necessary to distribute the light to areas far removed from the lamp. The most common method used is light pipes.


Light pipes

The 3M light pipe is a long, transparent, hollow cylinder with a
prismatic An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled — elements with two parallel surfaces are ''not'' prisms. The most familiar type of optical ...
surface developed by 3M that distributes the light uniformly over its length. Light pipes can be as long as 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-sized
fluorescent tube A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet ligh ...
. One sulfur lamp with a light pipe can replace dozens of
HID lamp High-intensity discharge lamps (HID lamps) are a type of electrical gas-discharge lamp which produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tub ...
s. In the
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, three lamps, each with a pipe, replaced 94 HID lamps while greatly increasing the amount of light delivered. The greatly reduced number of 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.


Secondary reflectors

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. At Sundsvall-Härnösand Airport near
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,
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, 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 . At the headquarters of
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, 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 in
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,
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, 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 micro prismatic 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.


Indirect lighting

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. At the Sacramento Municipal Utility District (SMUD) headquarters building, two sulfur lamps were installed in the tops of free-standing
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s. The high ceiling was retrofit with high reflectance (90%), white acoustic ceiling 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.


Direct lighting

Light pipes would not be necessary in applications such as
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lighting, where a plain fixture can be mounted high enough so that the light can spread over a large area. The installation at
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contains lamps with light pipes as well as downlight fixtures mounted high in an aircraft hangar.


Optical fibers

Optical fibers have been studied as a distribution system for sulfur lamps, but no practical system has ever been marketed.


Other uses

Sulfur lamps can be used as light sources in scientific instruments.


Future prospects

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. Researchers have had some success at eliminating the need to rotate the bulb by using circularly polarized microwaves to spin the plasma discharge instead. Other experiments have used
sodium iodide Sodium iodide (chemical formula NaI) is an ionic compound formed from the chemical reaction of sodium metal and iodine. Under standard conditions, it is a white, water-soluble solid comprising a 1:1 mix of sodium cations (Na+) and iodide anions (I ...
, scandium iodide,
indium monobromide Indium(I) bromide is a chemical compound of indium and bromine. It is a red crystalline compound that is isostructural with β-TlI and has a distorted rock salt structure. Indium(I) bromide is generally made from the elements, heating indium meta ...
(InBr), or tellurium as the light-generating medium.


Prominent installations

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 glass atria in the
National Air and Space Museum The National Air and Space Museum of the Smithsonian Institution, also called the Air and Space Museum, is a museum in Washington, D.C., in the United States. Established in 1946 as the National Air Museum, it opened its main building on the Nat ...
.


See also

* Electrodeless lamp * Plasma lamp * List of light sources *
Timeline of lighting technology Artificial lighting technology began to be developed tens of thousands of years ago and continues to be refined in the present day. Antiquity * 125,000 BC Widespread control of fire by early humans. * 17,500 BC oldest documented lamp, utilizi ...


Notes


Further reading

* Suplee, Curt, "Energy Dept. Brings Dazzling Bulb to Light", '' The Washington Post'', October 21, 1994 * Suplee, Curt, "A New Kind of Illumination That Burns Brightly, but Not Out", ''The Washington Post'', October 24, 1994 * Holusha, John, "Light Source To Replace Many Bulbs", '' The New York Times'', October 26, 1994 * "Sulfur Lighting on Track", ''Environmental Building News'', July 1995 * Schroeder, Michael, and Dreazen, Yochi, "Energy-Saving Light Bulbs Mar Satellite Radio", '' The Wall Street Journal'', August 6, 2001 * "Sulfur Lighting No Longer on Track", ''Environmental Building News'', August 2005 {{Artificial light sources American inventions Gas discharge lamps Sulfur