The internal electrodeless lamp or induction light is a gas discharge lamp in which the power required to generate light is transferred from outside the lamp envelope to the gas inside via an electric or magnetic field, in contrast with a typical gas discharge lamp that uses internal electrodes connected to the power supply by conductors that pass through the lamp envelope. There are three advantages to elimination of the internal electrodes:
- Extended lamp life, because the internal electrodes are usually the limiting factor in lamp life.
- The ability to use light-generating substances of higher efficiency that would react with internal metal electrodes in normal lamps.
- Improved collection efficiency because the source can be made very small without shortening life, a problem in internal electroded lamps.
Two systems are described below – plasma lamps, which use electrostatic induction to energize a bulb filled with sulfur vapor or metal halides, and fluorescent induction lamps, based upon a conventional fluorescent lamp bulb in which current is induced by an external coil of wire via electrodynamic induction.
Nikola Tesla demonstrated wired and wireless transfer of power to electrodeless fluorescent and incandescent lamps in his lectures and articles in the 1890s, 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 Tesla's priority in this field. A suit filed by Tesla against J. J. Thomson for priority on the patent was subsequently granted in Tesla's favor. As of 2011, the transcripts of the case languish 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 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 of General Electric   applied for patents for electrodeless lamps. In 1971 Fusion UV Systems installed a 300 Watt electrodeless microwave plasma UV lamp on a Coors can line. 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.
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.
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 the Sylvania Icetron. In 1997 PQL Lighting Introduced in the US the Superior Life Brand Induction Lighting Systems. Most induction lighting systems are rated for 100,000 hours of use before requiring absolute component replacements.
Since 2005, Amko Solara in Taiwan introduced induction lamps that can dim and use IP based controls. Their lamps have a range from 12 to 400 watts and operate at 250 kHz.
From 1995, the former distributors of Fusion, Jenton / Jenact, expanded on the fact that energised UV-emitting plasmas act as lossy conductors to create a number of patents regarding electrodeless UV lamps for sterilising and germicidal uses.
Around 2000 a system was developed that concentrated radio frequency waves into a solid dielectric waveguide made of ceramic which energized a light emitting plasma in a bulb positioned inside. This system, for the first time, permitted an extremely bright and compact electrodeless lamps. The invention has been a matter of dispute. Claimed by Frederick Espiau (then of Luxim now of Topanga Technologies), Chandrashekhar Joshi and Yian Chang, these claims were disputed by Ceravision Limited. Recently a number of the core patents were assigned to Ceravision.
In 2006 Luxim introduced a projector lamp product trade-named LIFI. The company further extended the technology with light source products in instrument, entertainment, street, area and architectural lighting applications among others throughout 2007 and 2008.
In 2009 Ceravision Limited introduced the first High Efficiency Plasma (HEP) lamp under the trade name Alvara. This lamp replaces the opaque ceramic waveguide used in earlier lamps with an optically clear quartz waveguide giving greatly increased efficiency. In previous lamps, though the burner, or bulb, was very efficient, the opaque ceramic waveguide severely obstructed the collection of light. A quartz waveguide allows all of the light from the plasma to be collected.
In 2012 Topanga Technologies introduced a line of advanced plasma lamps (APL), driven by a solid state RF driver, thereby circumventing the limited life of magnetron based drivers, with system power of 127 and 230 Watts and system efficacies of 96 and 87 lumen/Watt, with a CRI of about 70.
Plasma lamps are a family of light sources that generate light by exciting a plasma inside a closed transparent burner or bulb using radio frequency (RF) power. Typically, such lamps use a noble gas or a mixture of these gases and additional materials such as metal halides, sodium, mercury or sulfur. A waveguide is used to constrain and focus the electrical field into the plasma. In operation the gas is ionized and free electrons, accelerated by the electrical field collide with gas and metal atoms. Some electrons circling around the gas and metal atoms are excited by these collisions, bringing them to a higher energy state. When the electron falls back to its original state, it emits a photon, resulting in visible light or ultraviolet radiation depending on the fill materials.
The first plasma lamp was an ultraviolet curing lamp with a bulb filled with argon and mercury vapor developed by Fusion UV. That lamp led Fusion Systems to the development of the sulfur lamp, a bulb filled with argon and sulfur which is bombarded with microwaves through a hollow waveguide.
In the past, the reliability of the technology was limited by the magnetron used to generate the microwaves. Solid state RF generation can be used and gives long life. However, using solid state chips to generate RF is approximately fifty times more expensive currently than using a magnetron and so only appropriate for high value lighting niches. It has recently been shown by Dipolar  of Sweden to be possible to greatly extend the life of magnetrons to over 40,000 hours  making low cost plasma lamps possible. Plasma lamps are currently produced by Ceravision and Luxim and in development by Topanga Technologies.
Ceravision has introduced a combined lamp and luminaire under the trade name Alvara for use in high bay and street lighting applications. It uses an optically clear quartz waveguide with an integral burner allowing all the light from the plasma to be collected. The small source also allows the luminaire to utilize more than 90% of the available light compared with 55% for typical HID fittings. Ceravision claims the highest Luminaire Efficacy Rating (LER)  of any light fitting on the market and to have created the first High Efficiency Plasma (HEP) lamp. Ceravision uses a magnetron to generate the required RF power and claim a life of 20,000 hours.
Luxim's LIFI, or light fidelity lamp, claims 120 lumens per RF watt (i.e. before taking into account electrical losses). The lamp has been used in Robe lighting's ROBIN 300 Plasma Spot moving headlight. It was also used in a line of, now discontinued, Panasonic rear projection TVs.
Magnetic induction lamps
Aside from the method of coupling energy into the mercury vapour, these lamps are very similar to conventional fluorescent lamps. Mercury vapour in the discharge vessel is electrically excited to produce short-wave ultraviolet light, which then excites internal phosphors to produce visible light. While still relatively unknown to the public, these lamps have been available since 1990. The first type introduced had 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.
There are two main types of magnetic induction lamp: external inductor lamps and internal inductor lamps. The original, and still widely used form of induction lamps are the internal inductor types. A more recent development is the external inductor types which have a wider range of applications and which are available in round, rectangular and "olive" shaped form factors.
External inductor lamps are basically fluorescent lamps with electromagnets wrapped around a part of the tube. In the external inductor lamps, high frequency energy, from the electronic ballast, is sent through wires, which are wrapped in a coil around a ferrite inductor on the outside of the glass tube, creating a powerful electromagnet called an inductor. The induction coil (inductor) produces a very strong magnetic field which travels through the glass and excites the mercury atoms in the interior. The mercury atoms are provided by the amalgam (a solid form of mercury). The excited mercury atoms emit UV light and, just as in a fluorescent tube, the UV light is down-converted to visible light by the phosphor coating on the inside of the tube. The glass walls of the lamp prevent the emission of the UV light as ordinary glass blocks UV radiation at the 253.7 nm and 185 nm range.
In the internal inductor form (see diagram), a glass tube (B) protrudes bulb-wards from the bottom of the discharge vessel (A), forming a re-entrant cavity. This tube contains an antenna called a power coupler, which consists of a coil wound over a tubular ferrite core. The coil and ferrite forms the inductor which couples the energy into the lamp interior
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. 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 (inductor) 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. Many ballasts are available in low voltage models so can also be connected to DC voltage sources like batteries for emergency lighting purposes or for use with renewable energy (solar & wind) powered systems.
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. For induction lamps with integrated ballast, the lifespan is in the 15,000 to 50,000 hours range. Extremely high-quality electronic circuits are needed for the ballast to attain such a long service life. Such lamps are typically used in commercial or industrial applications. Typically operations and maintenance costs are significantly lower with induction lighting systems due to their industry average 100,000 hour life cycle and five to ten year warranty.
- Long lifespan due to the lack of electrodes – Strictly speaking almost indefinite on the lamp itself but between 25,000 and 100,000 hours depending on lamp model and quality of electronics used;
- Very high energy conversion efficiency of between 62 and 90 Lumens/Watt [higher power lamps are more energy efficient];
- High power factor due to the low loss of the high frequency electronic ballasts which are typically between 95% and 98% efficient;
- Minimal Lumen depreciation (declining light output with age) compared to other lamp types as filament evaporation and depletion is absent;
- "Instant-on" and hot re-strike, unlike most HID lamps used in commercial-industrial lighting applications (such as mercury-vapor lamp, sodium-vapor lamp and metal halide lamp);
- Environmentally friendly as induction lamps use less energy, and use less mercury per hour of operation than conventional lighting due to their long lifespan. The mercury is in a solid form and can be easily recovered if the lamp is broken, or for recycling at end-of-life.
These benefits offer considerable cost savings of between 35% and 55% in energy and maintenance costs for induction lamps compared to other types of commercial and industrial lamps which they replace.
- Some models of internal inductor lamps that use high frequency ballasts can produce radio frequency interference (RFI) which interferes with radio communications in the area. Newer, external inductor type lamps use low frequency ballasts that usually have FCC or other certification, thus complying with RFI regulations.
- External inductor lamps tend to be quite large, especially in higher wattage models, thus they are not always suitable for applications where a compact light source is required.
- Some types of inductor lamps contain mercury, which is highly toxic if released to the environment.
- "Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination," AIEE, Columbia College, N.Y., May 20, 1891
- Electrodeless gaseous electric discharge devices utilizing ferrite cores
- High frequency electrodeless fluorescent lamp assembly
- Ceravision Steps up Legal Action Against Luxim to Recover IP
- Microwave Energized Plasma Lamp with Solid Dielectric Waveguide
- Plasma Lamp with Dielectric Waveguide
- RF solid state light announcement Opanga
- Procedure for Determining Luminaire Efficacy Ratings for High-Intensity Discharge (HID) Industrial Luminaires
- "A lightbulb powered by radio waves". cnet. 2007-08-23.
- "Robe Launches ROBIN 300 Plasma Spot". Robe lighting. 2009-04-27.
- "The gift of LIFI: Panasonic projection TVs don't burn out". cnet. 2007-01-09.