Plasma lamp

Plasma lamps are a type of gas discharge lamp energized by radio frequency (RF) power. They are distinct from the novelty plasma lamps that were popular in the 1980s.

The internal-electrodeless lamp was invented by Tesla after his experimentation with high-frequency currents in evacuated glass tubes for the purposes of lighting and the study of high voltage phenomena. The first practical plasma lamps were 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 high-efficiency plasma (HEP) lamps have been introduced to the general lighting market. Plasma lamps with an internal phosphor coating are called external electrode fluorescent lamps (EEFL); these external electrodes or terminal conductors provide the radio frequency electric field.

Description

Modern plasma lamps are a family of light sources that generate light by exciting 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 atomic 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 that 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. High power demand – 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 of Sweden to be possible to extend the life of magnetrons to over 40,000 hours,^[1] making low-cost plasma lamps possible.

Size

In the year 2000, a system was developed that concentrated radio frequency waves into a dielectric waveguide made of ceramic, which energized light-emitting plasma in a bulb positioned inside. This system, for the first time, permitted an extremely compact yet bright electrode-less lamp. The invention has been a matter of dispute. Claimed by Frederick Espiau, Chandrashekhar Joshi and Yian Chang, these claims were disputed by Ceravision Limited.^[2] Recently, a number of the core patents have been assigned to Ceravision.^[3][4]

Heat and power

The use of a high-dielectric waveguide allowed the sustaining of plasmas at much lower powers—down to 100 W 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 that appears to resolve this issue.

High-efficiency plasma (HEP)

High-efficiency plasma lighting is the class of plasma lamps that have system efficiencies of 90 lumens per watt or more. Lamps in this class are potentially the most energy-efficient light source for outdoor, commercial and industrial lighting. This is due not only to their high system efficiency but also to the small light source they present enabling very high luminaire efficiency.

Luminaire Efficacy Rating (LER) is the single figure of merit the National Electrical Manufacturers Association has defined to help address problems with lighting manufacturers' efficiency claims ^[5] and is designed to allow robust comparison between lighting types. It is given by the product of luminaire efficiency (EFF) times total rated lamp output in lumens (TLL) times ballast factor (BF), divided by the input power in watts (IP):

LER = EFF × TLL × BF / IP

The "system efficiency" for a High Efficiency Plasma lamp is given by the last three variables, that is, it excludes the luminaire efficiency. Though plasma lamps do not have a ballast, they have an RF power supply that fulfills the equivalent function. In electrodeless lamps, the inclusion of the electrical losses, or "ballast factor", in lumens per watt claimed can be particularly significant as conversion of electrical power to radio frequency (RF) power can be a highly inefficient process.

Many modern plasma lamps, such as those manufactured by Ceravision and Luxim, have very small light sources—far smaller than HID bulbs or fluorescent tubes—leading to much higher luminaire efficiencies also. High intensity discharge lamps have typical luminaire efficiencies of 55%, and fluorescent lamps of 70%. Plasma lamps typically have luminaire efficiencies exceeding 90%.

Producers

Companies producing or developing plasma lamps include Hive Lighting, Ceravision, Luxim, Plasma International, i-Giant, Topanga USA, Solaronix and Lumartix.

Luxim's Li-Fi lamp, claims 120 lumens per RF watt (i.e., before taking into account electrical losses).^[6] The lamp is used by Stray Light Optical Technologies in their commercial lighting fixtures. The lamp has been used in Robe lighting's ROBIN 300 Plasma Spot moving head light.^[7] It was also used in a line of, now discontinued, Panasonic rear projection TV.^[8]

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 HEP lamp. Ceravision uses a magnetron to generate the required RF power and claims a life of 20,000 hours.

See also

• List of plasma (physics) articles

References