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Electrodeless Induction Lamps

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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. There are two advantages of eliminating electrodes. The first is extended bulb life, because the electrodes are usually the limiting factor in bulb life. The second benefit is the ability to use light-generating substances that would react with metal electrodes in normal lamps.

Two systems are described below—one based on conventional fluorescent lamp phosphors, and a second based on the use of direct-radiating sulfur vapor.

History

Nikola Tesla demonstrated wired and wireless transfer of power to electrodeless fluorescent and incandescent lamps in his lectures and articles in the 1890’s, and subsequently patented a system of light and power distribution on those principles. In the lecture before the AIEE, May 20, 1891, titled Experiments with Alternating Currents of Very High Frequency and Their Application to Methods of Artificial Illumination and US patent 454622, among many other references in the technical and popular press are found countless records for Dr. Tesla’s priority in this field. A suit filed by Dr. Tesla against J.J. Thompson for priority on the patent was subsequently granted in Dr. Tesla’s favor. The transcripts of the case languish currently in archives, awaiting processing, and eventual publishing. Noting the diagrams in Tesla’s lectures and patents, a striking similarity of construction to electrodeless lamps that are available on the market currently is readily apparent. Further, a statement in 1929 by Dr. Tesla, published in The World :

“Surely, my system is more important than the incandescent lamp, which is but one of the known electric illuminating devices and admittedly not the best. Although greatly improved through chemical and metallurgical advances and skill of artisans it is still inefficient, and the glaring filament emits hurtful rays responsible for millions of bald heads and spoiled eyes. In my opinion, it will soon be superseded by the electrodeless vacuum tube which I brought out thirty-eight years ago, a lamp much more economical and yielding a light of indescribable beauty and softness.”

In 1967 and 1968, John Anderson General Electric 2, 3 applied for patents for electrodeless lamps. 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.

In 2006, AMKO SOLARA in Taiwan has introduced and started selling their induction lamp SOLARA with a dimmable ballast and up to 400 watts of output. Their products operate at both 2.65 MHz and 250 kHz.

In 1990, Michael Ury, Charles Wood and colleagues, formulated the concept of the sulphur lamp. 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.

Around 2001, Frederick Espiau, Chandrashekhar Joshi and Yian Chang invented a system that concentrates radio frequency in a solid dielectric waveguide thereby energizing a light emitting plasma (physics) in a bulb positioned inside. This invention was further developed by LUXIM and introduced in 2006 as a projector lamp product trade-named LIFI.

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 at a frequency of 50 or 60 Hz. Most ballasts can also be connected 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.

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