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LIGHT BY MICROWAVE COOKING
JUST ADD A PINCH OF SALT
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By Chris Williams, UKDL
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I must start this month's column by declaring an interest as well as being Director of UKDL, I am also a Director of Ceravision Ltd, a small UK company that has developed a disruptive lighting technology that may revolutionise many of the world's higher power lighting designs as we move through 2008 into the rest of this century.
The technology that Ceravision has created has the ability to convert electrical energy into light. Nothing original in this, you might think converting electrical power into light is what the humble incandescent light bulb does, albeit not very efficiently. The key to the Ceravision system is in the efficiency of the two-stage process used to create light.
Firstly, electrical energy at the available supply level (can be anything: 240VAC, 12VDC etc) is used to power a selfoscillating microwave power source. This power is generated at the nominal frequency of 2.4GHz and it is then transmitted via a suitable antenna structure into a metalised dielectric waveguide, usually square, rectangular or circular in shape. Being a dielectric structure, at an operating frequency of 2.4GHz, this allows compact dimensions to be used.
The waveguide contains a hole into which an electrodeless quartz bulb containing a noble gas and carefully selected metal halide salts are placed. The microwave energy transmitted into the waveguide induces a very high frequency, high density alternating electrical field into the quartz bulb, ionising the noble gas and forming a plasma channel. The heat from this plasma channel then causes the metal halide salts to vaporise and the light creation process commences in earnest.
The process of forming a plasma channel and vaporising metal halide salts is common to every type of Metal Halide High Intensity Discharge lamp. These are commonly used in electronic projectors, car headlamps, and many other commercial and professional lighting applications.
The idea of using microwave energy to
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Figure 1: Schematic of the Ceravision system
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Figure 2: The upper graph demonstrates line spectra emission from burner placed in air-filled waveguide cavity, and the lower graph demonstrates molecular excitation from burner placed in dielectric waveguide
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Figure 3: The benchtop demonstrator of the Ceravision Continuum 2.4 light source For more details see www.ceravision.com
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stimulate the creation of a plasma channel is not new many of the larger lighting companies have been attempting to do this for many years, with the first demonstration of microwave powered lighting having occurred more than 50 years ago.
Where Ceravision has succeeded and others have failed is to translate the principles of microwave excitation of a metal halide lamp into a commercially viable system.
The key to this achievement lay in the invention of what is casually referred to by the company as the "Microwave Interface Unit". The historic stumbling block for companies operating in this technology space has been the need to cope economically with the quirks of microwave engineering. The main problem is the bulb itself, called a "burner" in the industry, which has an effective electrical impedance approaching infinity in the "off" state, and an effective impedance approaching zero in the "on" state. The power source that drives this wide dynamic impedance range has to swing from an almost "open circuit" to a "short circuit" condition, and maintain efficient power transfer in the process. This is difficult to do at audio frequencies; look at the "protection circuitry" designs that are commonly used in high power audio amplifiers to protect the output stages if we accidentally unplug the speakers, or manage to short-circuit the terminal directly.
Historically, the commercial problem has been that this protection circuitry is expensive when implemented at microwave frequencies. Good though a new lighting system may be, it is difficult to sell if it costs three times the price of the technology it is trying to replace.
Enter Ceravision with their Continuum 2.4 range of products. This system uses a very simple, novel, low-cost component between the power source and the waveguide assembly that allows 100% of the energy from the power source to be transmitted into the dielectric waveguide, yet prevents
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less than 0.1% of the incident power from being reflected back from the waveguide towards the power source when the burner is "on". This patented component has eliminated the need for any feedback circuitry from the waveguide itself to "monitor" the operation of the burner, and has drastically simplified the circuitry within the power source itself.
The result is a commercially viable, simple, high-reliability light source that will be in mass production during 2008.
In addition to the simple benefit of lowcost manufacture, serendipity steps in to offer users a brand new set of advantages when looking to adopt this light source. Conventional metal halide lamps operate under "atomic excitation", whereby the heat generated in the plasma channel causes the
THE HISTORIC STUMBLING BLOCK FOR COMPANIES OPERATING IN THIS TECHNOLOGY SPACE HAS BEEN THE NEED TO COPE ECONOMICALLY WITH THE QUIRKS OF MICROWAVE ENGINEERING
molecules of the metal halide salts to break down into their constituent atoms when the salts are vaporised. This results in the light emitted comprising the atomic spectra of the individual component elements. Within Ceravision's Continuum 2.4 products, the field strength intensity and very high operating frequency present within the waveguide induces a "molecular excitation" of the metal halide salts, and not just atomic excitation.
In this process, the complete molecules are stimulated to emit light, resulting in many more frequencies of light being generated, and a broad spectral output being created. Figure 2 demonstrates this. In the first graph, a simple burner containing Indium Bromide as the metal halide salts is excited to emit light within a conventional air-filled cavity. This exhibits lower electrical field intensity than the Ceravision Continuum 2.4 system, and results in conventional atomic
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excitation as shown by the measured line spectra. The same bulb was then excited to emit light by being placed in the Ceravision Continuum 2.4 waveguide assembly. The resulting molecular excitation is shown in the second graph. This stunning improvement in the quality of light output means that using simple burner chemistry, it is possible to accurately simulate daylight.
The benefits continue in the areas of quality and reliability. By removing the need to have metal electrodes within the quartz envelope of the burner, the main cause of device contamination and reduction in quality of light is eliminated. The temperature of the plasma channel is of order several thousands of degrees Centigrade, and when operating with metal electrodes, the tips of the electrodes are subject to erosion.
This evaporated metal can either combine with the metal halide salts, modifying the chemistry and thereby changing subtly the colours of the light being emitted, or it can be deposited onto the walls of the burner itself. In conventional burners, both effects may happen, so the colour of a burner may start changing as soon as it is operating, and the brightness of the burner will slowly reduce as the glass envelopes is coated by the eroded metal. The need for glass to metal seals at the point where the electrodes enter the glass body is also a potential cause of failure due to mismatch of thermal coefficients of expansion.
Getting rid of the electrodes eliminates these problems and can hugely extend the operating life for the light system. Operating lifetimes in excess of 10-25,000 hours can be readily predicted with a microwavepowered system, compared with 1500 to 5000 hours from conventional lighting systems.
Perhaps the key conclusion to take away this month is that whilst the bulk of the technical press coverage on high-efficiency lamps is exclusively centred around LEDs and compact fluorescent lamps, there are alternative new technologies available, and these technologies will compete head-on with the old and new alike in their target application areas. I am very pleased to say that the world will not become solely dependent on LEDs and CFLi's. Wherever you need a lot of lumens from a very small bulb, then the Ceravision light source may be the perfect solution.Chris Williams is Network Director at the UK Display & Lighting Knowledge Transfer Network (UKDL KTN)
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Electronics World - April 08
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