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In contrast with all other electrical lamps that use electrical connections through the lamp envelope to transfer power to the lamp, in electrodeless lamps the power needed to generate light is transferred from the outside of the lamp envelope by means of (electro)magnetic fields. Two systems are described below - one based on the conventional fluorescent lamp phosphors, and a second based on the use of direct-radiating sulfur vapor.

Fluorescent induction lamps

Aside from the method of coupling energy into the mercury vapor, these lamps are very similar to conventional fluorescent lamps. Mercury vapor in the discharge vessel is electrically excited to produce short-wave ultraviolet light, which then excites the phosphors to produce visible light. While still relatively unknown to the public, these lamps have been available since 1990. The most common form has the shape of an incandescent light bulb. Unlike an incandescent lamp or conventional fluorescent lamps, there is no electrical connection going inside the glass bulb; the energy is transferred through the glass envelope solely by electromagnetic induction.

In the most common form, a glass tube (B) protrudes bulb-wards from the bottom of the discharge vessel (A). This tube contains an antenna called a power coupler, which consists of a coil wound over tubular ferrite core.

In lower-frequency versions of induction systems, the lamp consists of two long parallel glass tubes, connected by two short tubes that have coils mounted around them.

The antenna coils receive electric power from the electronic ballast (C) that generates a high frequency. The exact frequency varies with lamp design, but popular examples include 13.6 MHz, 2.65 MHz and 250 kHz (in physically large lamps). A special resonant circuit in the ballast produces an initial high voltage on the coil to start a gas discharge; thereafter the voltage is reduced to normal running level.

The system can be seen as a type of transformer, with the power coupler forming the primary coil and the gas discharge arc in the bulb forming the one-turn secondary coil and the load of the transformer. The ballast is connected to mains electricity, and is generally designed to operate on voltages between 100 and 277 VAC on the frequency of 50 or 60 Hz. Most ballasts can also be conneceted to DC voltage sources like batteries for emergency lighting purposes.

In other conventional gas discharge lamps, the electrodes are the part with the shortest life, limiting the lamp lifespan severely. Since an induction lamp has no electrodes, it can have a very long service life. For induction lamp systems with a separate ballast, the service life can be as long as 100,000 hours, which is 11.4 years continuous operation, or 22.8 years used at night or day only. For induction lamps with integrated ballast, the life is 15,000 to 30,000 hours. Extremely high-quality electronic circuits are needed for the ballast to attain such a long service life. Such expensive lamps have special application areas in situations where replacement costs are high.

Philips introduced their QL induction lighting systems, operating at 2.65 MHz, in 1990 in Europe and in 1992 in the US. Matsushita had induction light systems available in 1992. Intersource Technologies also announced one in 1992, called the E-lamp. Operating at 13.6 MHz, it was to be available on the US market in 1993 but as of July 2005 very few of these lamps have been manufactured.

Since 1994, General Electric has produced its induction lamp Genura with an integrated ballast, operating at 2.65 MHz. In 1996, Osram started selling their Endura induction light system, operating at 250 kHz. It is available in the US as Sylvania Icetron.

Research on electrodeless lamps continues, with variations in operating frequency, lamp shape, the induction coils and other design parameters, such as amalgam reservoirs for mercury absorption. Low public awareness and the relatively high prices have so far kept the use of such lamps highly specialized.

Direct-radiating sulfur lamps

The sulfur- or sulphur lamp is a microwave-powered electrodeless lighting system conceived by engineer Michael Ury, physicist Charles Wood and their colleagues in 1990. It was further developed in 1994 by Fusion Lighting (USA) with support from the U.S. Department of Energy.

Approximately the size of a golf ball, the sulfur lamp consists of a quartz bulb containing non-toxic sulfur and inert argon gas at the end of a thin glass spindle. A microwave magnetron energy source of 2.45 GHz bombards the lamp. The microwave energy excites the gas, which in turn heats the sulfur to an extreme degree, forming a brightly-glowing plasma capable of illuminating great areas. Because the bulb heats up considerably, it is necessary for an electric motor to spin the quartz bulb around while a fan cools it in order to keep it from melting.

The sulfur plasma consists mainly of dimer molecules (S₂), which generate the light through molecular emission. Because this (instead of atomic emission) is the mechanism of light generation, the emission spectrum is broad-based and continuous throughout the visible spectrum (much like sunlight), however the source is low in both the ultraviolet and infrared energy. The lack of harmful ultraviolet radiation can be especially beneficial in places like museums.

The design life of the lamp is currently approximately 60,000 hours. However, the design life of the magnetron is currently only about 15,000 to 20,000 hours. The sulfur lamp starts within seconds even at low ambient temperatures, and can be dimmed. The lamp emits no electric or magnetic fields, and the light output remains constant over its life.

The first prototype lamps were 5.9 kW units, with a system efficacy of 80 lumens per watt. Correlated color temperature was about 6000 Kelvins with a CRI of 79.

Fusion Lighting closed its doors in early 2002, after having used up about 90 million dollars in venture capital, but the patents have been licensed to the LG Group and Samsung.

These lamps may very well cause interruption in 2.4Ghz wireless spectrum. They have been lauded as an Extinction Level Event for the 802.11b forms of wireless networking because of their use of microwave generators over extended periods of time.

External links

• General overview Induction systems
• IAEEL publication Philips QL and Matshushita induction light
• IAEEL publication Intersource Tecnologies E-lamp
• IAEEL publication General Electric Genura lamp
• IAEEL publication Osram Endura
• IAEEL publication microwave excited lamps
• Radio frequency lamps overview
• Sulphur lamp information

Sources of light / lighting:
Natural/prehistoric light sources:	Bioluminescence | Celestial objects | Lightning
Combustion-based light sources:	Acetylene/Carbide lamps | Candles | Davy lamps | Fire | Gas lighting | Kerosene lamps | Lanterns | Limelights | Oil lamps | Rushlights
Direct chemical light sources:	Chemoluminescence (Lightsticks)
Nuclear light sources:	Betalights/Trasers | | Radium paint | Cherenkov radiation
Electric light sources:	Arc lamps | Incandescent light bulbs | Fluorescent lamps
High-intensity discharge light sources:	Ceramic Discharge Metal Halide lamps | HMI lamps | Mercury-vapor lamps | Metal halide lamps | Sodium vapor lamps | Xenon arc lamps
Other electric light sources:	Electroluminescent (EL) lamps | Globar | Inductive lighting | Discrete LEDs/Solid State Lighting (LEDs) | Neon and argon lamps | Nernst lamp | Sulfur lamp | Xenon flash lamps | Yablochkov candles

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