page_title: Electrodeless lamp
section_title: Magnetic induction lamps
hierarchical_section_title: Electrodeless lamp / Magnetic induction lamps
context_page_description: The internal electrodeless lamp or induction lamp is a gas discharge lamp in which an electric or magnetic field transfers the power required to generate light from outside the lamp envelope to the gas inside. This is in contrast to a typical gas discharge lamp that uses internal electrodes connected to the power supply by conductors that pass through the lamp envelope. Eliminating the internal electrodes provides two advantages:
  Extended lamp life
  Ability to use higher-efficiency light-generating substances that would react with internal metal electrodes in conventional fluorescent lamps
  Two systems are common: plasma lamps, in which electrostatic induction energizes a bulb filled with sulfur vapor or metal halides, and fluorescent induction lamps, which are like a conventional fluorescent lamp bulb that induces current with an external coil of wire via electrodynamic induction.
context_section_description: 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 internal phosphors to produce visible light. While still relatively unknown to the public, these lamps have been available since 1990. 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 lamps: external core lamps and internal core lamps.  The first commercially available and still widely used form of induction lamp is the internal core type. The external core type, which was commercialized later, has a wider range of applications and is available in round, rectangular and "olive" shaped form factors.
  External core lamps are basically fluorescent lamps with magnetic cores wrapped around a part of the discharge tube. The core is usually made of ferrite, a ceramic material containing iron oxide and other metals. In external core lamps, high frequency energy from a special power supply called an electronic ballast passes through wires that are wrapped in a coil around a toroidal ferrite core placed around the outside of a portion of the glass tube. This creates a high frequency magnetic field within the ferrite core. Since the magnetic permeability of the ferrite is hundreds or thousands of times higher than that of the surrounding air or glass, and the ferrite core provides a closed path for the magnetic field, the ferrite core contains virtually all of the magnetic field. 
  Following Faraday's law of induction, the time varying magnetic field in the core generates a time varying electric voltage in any closed path that encloses the time varying magnetic field. The discharge tube forms one such closed path around the ferrite core, and in that manner the time varying magnetic field in the core generates a time varying electric field in the discharge tube,  There is no need for the magnetic field to penetrate the discharge tube. The electric field generated by the time varying magnetic field drives the mercury-rare gas discharge in the same way the discharge is driven by the electric field in a conventional fluorescent lamp.  The primary winding on the ferrite core, the core, and the discharge form a transformer, with the discharge being a one-turn secondary on that transformer.   
  The discharge tube contains a low pressure of a rare gas such as argon and mercury vapor. The mercury atoms are provided by a drop of liquid mercury, or by a semi-solid amalgam of mercury and other metals such as bismuth, lead, or tin. Some of the liquid mercury or the mercury in the amalgam vaporizes to provide the mercury vapor. The electric field ionizes some of the mercury atoms to produce free electrons, and then accelerates those free electrons.  When the free electrons collide with mercury atoms, some of those atoms absorb energy from the electrons and are “excited” to higher energy levels.  After a short delay, the excited mercury atoms spontaneously relax to their original lower energy state and emit a UV photon with the excess energy. As in a conventional fluorescent tube, the UV photon diffuses through the gas to the inside of the outer bulb, and is absorbed by the phosphor coating that surface, transferring its energy to the phosphor. When the phosphor then relaxes to its original, lower energy state, it emits visible light.  In this way the UV photon 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 photons because ordinary glass blocks UV radiation at the 253.7 nm and shorter wavelengths.
  In the internal core 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 coup
caption: A Philips QL induction lighting system, where (A) Discharge vessel, (B) Tube with power coupler and (C) Electronic ballast.