Plasma globe

Plasma lamps refer to a type of highly efficient electrodeless lamp energized by radio frequency (RF) power now being introduced to the general lighting market. In the past it has also referred to the novelty plasma lamps which were popular in the 1980s.

The plasma lamp was invented by Nikola Tesla after his experimentation with high frequency currents in an evacuated glass tube for the purpose of studying high voltage phenomena. Modern versions were first designed by Bill Parker. but had no practical use for lighting. The first practical lamp was the sulfur lamps manufactured by Fusion Lighting. This lamp suffered a number of practical problems and did not prosper commercially. These problems have gradually been overcome by manufacturers such as Ceravision and Luxim and recently High Efficiency Plasma (HEP) lamps have been introduced to the general lighting market.

Description

Modern 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. In modern plasma lamps 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 commercial plasma lamp was an ultraviolet curing lamp with a bulb filled with argon and mercury vapor developed by Fusion UV. That lamp led Fusion Lighting to the development of the sulfur lamp, a bulb filled with argon and sulfur which is bombarded with microwaves through a hollow waveguide. The bulb had to be spun rapidly to prevent it burning through. Fusion Lighting did not prosper commercially, but other manufacturers such as LG Group continue to pursue sulfur lamps. Sulfur lamps, though relatively efficient have had a number of problems, chiefly:

1. Limited life - magnetrons had limited lives
2. Large size
3. Heat - the sulfur burnt through the bulb wall unless they were rotated rapidly
4. Low power - they could not sustain a plasma in powers under 1000W

Limited Life

In the past, the life of the plasma lamps was limited by the magnetron used to generate the microwaves. Solid state RF chips can be used and give long lives. However, using solid state chips to generate RF is currently an order of magnitude more expensive than using a magnetron and so only appropriate for high value lighting niches. It has recently been shown by Dipolar [1] of Sweden to be possible to greatly extend the life of magnetrons to over 40,000 hours making low cost plasma lamps possible.

Size

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 compact yet bright electrodeless lamp. 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 have been assigned to Ceravision .

Heat and Power

The use of a high dielectric waveguide allowed the sustaining of plasmas at much lower powers - down to 100W in some instances. It also allowed the use of conventional gas-discharge lamp fill materials which removed the need to spin the bulb. The only issue with the ceramic waveguide was that much of the light generated by the plasma was trapped inside the opaque ceramic waveguide. In 2009 Ceravision introduced an optically clear quartz waveguide which appears to resolve this issue.

Producers

Companies producing or developing plasma lamps include Ceravision, Luxim and Topanga Technologies.

Luxim's LIFI, or light fidelity lamp, claims 120 lumens per RF watt (ie 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 TV..

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 High-intensity discharge fittings. Ceravision claims the highest luminaire efficacy rating 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.