Electrodeless Lamps:
    Almost all of the light sources currently in use have one thing in common, metal electrodes sealed into the walls of the bulb to bring the electrical current inside the lamp chamber/envelope. Unsurprisingly, the main failure mechanisms in these typical lamps [other than breakage] is:

• Failure of the filament due to depletion of the filament material over time as atoms are stripped off by the electrical current;

• Vibration which breaks the filament, especially when it is hot;

• Failure of the seal integrity of the lamp; typically caused by thermal stresses in the area where the electrodes go through the glass walls.  The failure of the seal can either be sudden and complete, or a “slow leak” over time allowing the entry of atmospheric gasses which contaminates the interior.

    The dream of lighting inventors has been to produce a lamp with no internal electrodes so as to eliminate these common failure modes.  In an electrodeless lamp the envelope [bulb] is completely sealed and thus there is no chance of atmospheric contamination due to seal failure and no electrodes to wear out over time.  On 23 June 1891, Nicholas Tesla was granted US patent 454,622 to cover a very early form of Induction lamp.

    In an electrodeless lamp, the main failure mechanisms [other than breakage] are:

• Depletion of the mercury amalgam inside the envelope [bulb].  When the mercury ions are excited and bombard the phosphors [which then emit the light we see], a small percentage of them are absorbed by the phosphor coating over time.  Once the mercury ions inside the envelope are depleted, the lamp emits only a very dim light and has to be replaced.

• Failure of the electronics [ballast] used to drive the lamp.  This is not a catastrophic failure mode as typically the electronics [ballast] are external to the lamp and can easily be replaced.

 So how do you get an electrical current inside the bulb (glass envelope) of a lamp to excite the mercury ions within?

Magnetic Induction Lamps:

    Magnetic induction lamps are basically fluorescent lamps with electromagnets wrapped around a part of the tube (an External inductor lamp), or inserted inside the lamp (an internal inductor lamp).

n external inductor lamps, high frequency energy, from the electronic ballast, is sent through wires, which are wrapped in a coil around the ferrite inductor on the outside of the glass tube, creating a powerful magnet.
    The induction coil 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 up-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.
    The induction lamp system can be considered as a type of transformer where the inductor outside the glass envelope is the primary coil, while the mercury atoms in an inert gas-fill within the envelope/tube form a single-turn secondary coil.  

The advantages of Induction lamps are:

• Long lifespan due to the lack of electrodes - between 65,000 and 100,000 hours depending on the lamp type and model;

• Very high energy conversion efficiency of between 62 and 87 Lumens/watt [higher wattage lamps are more energy efficient];

• High power factor due to the high frequency electronic ballasts which are 98% efficient - less wasted energy in the ballast;

• Minimal Lumen depreciation (declining light output with age) compared to other lamp types;

• Instant-on and hot re-strike, unlike most conventional lamps used in commercial/industrial lighting;

• Environmentally friendly as the mercury is in a sold form and can be easily recovered if the lamp is broken, or for recycling at end-of-life;

• These benefits offer a considerable cost savings of between 30% and 70% in energy and maintenance costs compared to other types of HID lamps which they replace.

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