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Pt.16: Microwave Sulphur Lamps Electric Lighting Microwave sulphur lamps are a relatively new innovation in the lighting industry and the concept is quite simple: use a magnetron to excite sulphur to produce a high luminous output. The lamps are commercially available and are suitable for hollow light guide applications. By JULIAN EDGAR T HE MICROWAVE SULPHUR lamp was invented in 1990 by US scientist Michael Ury, at the end of a 4-year research period. Subsequently, in 1992, Fusion Lighting Inc, Maryland (USA) obtained all rights for the development of the light source, making it available commercially as the Fusion Light Drive 1000. 82  Silicon iliconCChip hip These sulphur lamps frequently use hollow light guides to distribute and transport the light, with 3M’s Light Pipe being a popular choice. We’ll talk more about light guides later on in this article. Lamp development The sulphur lamp bulb consists of a 35mm quartz sphere filled with a few milligrams of yellow sulphur powder and an inert gas (such as argon) which is weakly ionised using microwaves. When it is ionised, the argon heats the sulphur into a gaseous state, thereby forming diatomic sulphur molecules or “dimers”. These dimers emit a broad continuum of energy as they drop back to lower energy states. The light radiation that is produced is almost entirely within the visible spectrum, with very little undesirable ultraviolet or infrared radiation. The microwaves are generated by two magnetrons operating at a frequency of 2.45GHz. As it is irradiated, the lamp bulb is spun at 3400 rpm (apparently to stabilise the plasma’s position within the bulb), with two fans providing forced air FACING PAGE: microwave sulphur lamps are used in conjunction with long sections of 3M Light Pipe at the Volvo Bus Manufacturing Plant in Boras, Sweden. The system provides high output, significantly reduces energy costs and is said to be ideal for use where maintenance is difficult or hazardous. Fusion Lighting’s microwave sulphur lamp (below) has a tiny bulb compared with a 1000W metal halide lamp as seen at right. Not shown in this view is the extensive ancillary equipment needed to drive it! cooling. Fig.1 shows a schematic diagram of the lamp while Fig.2 is a more detailed view of the Fusion Light Drive 1000. Note that the sulphur bulb is installed in the optical centre of a reflector system, to direct the light output. Prior to the development of this lamp, sulphur had not been used because it quickly corrodes the electrodes used in conventional bulbs. However, while experimenting with a variety of substances, Ury and colleague James Dolan decided to replace the mercury used in UV lamps with sulphur and then subject the bulb to microwaves. They picked the correct magnetron and rotational speed for the bulb on the first try and Ury is quoted as saying “if we [hadn’t got] everything together like that, we might have missed it.” The first lamps were rated at 3.5kW and had a light output of 450,000 lumens. By contrast, the Light Drive 1000 sulphur lamp which was subsequently made commercially available is rated at 1kW and has a light output of 135,000 lumens. Together with the auxiliary system, this lamp requires a power input of 1.425kW which means that it has an efficacy of about 95 lumens/watt of the total lamp power. Lower power lamps that use radio frequencies (RF) instead of microwaves are also currently under development. An RF-driven sulphur lamp that produces up to 15,000 lumens with an RF input of only 100 watts has been demonstrated – a luminous efficacy of approximately 140 lumens per RF watt. As with the microwave-excited versions, these lamps still need to be rotated but their lower power operation allows the forced-air cooling to be eliminated. Lamp operation One of the most interesting characteristics of the microwave sulphur lamp is that its spectral output is remarkably similar to sunlight. This Fig.1: this diagram shows how the microwave sulphur lamp works. Microwaves generated by a magnetron are used to irradiate a quartz sphere containing argon and a small quantity of sulphur. The ionised argon heats the sulphur into a gaseous state, forming diatomic sulphur molecules which emit light. DECEMBER 1999  83 can under conventional high intensity gas discharge illumination. Warm-up time With the exception of fluorescent lamps, the warm-up time of the sulphur lamp is notably shorter than for other gas discharge lamps. A sulphur lamp reaches 80% of its final luminous flux within 20 seconds and the lamp can be restarted approximately 5 minutes after a power cut. By using a computer-controlled electronic power supply, the sulphur lamp can be dimmed to 20% of its maximum luminous flux, at which point its power consumption will have dropped by 60%. And unlike high intensity discharge lamps such as the metal halide design, the colour temperature of the sulphur lamp varies relatively little during the dimming process, dropping by only approximately 500K. The life of the lamp itself is expected to be at least 60,000 hours – in fact, Fusion Lighting suggest that the bulb itself may never wear out. However, the life of the magnetrons is much shorter, being about 15-20,000 hours. To overcome this problem, Fusion Lighting recently released a new Light­Drive 1000 with an electronic mag­ netron power supply and two spare magnetron kits. The new power supply is claimed to have increased each magnetron’s service life to at least 20,000 hours, meaning that with the spare magnetrons, a service life of at least 60,000 hours for the complete lamp is achievable. Lamp applications Fig.2: the Fusion LightDrive 1000 is a commercially available lamp using microwave sulphur technology. It uses two magnetrons to irradiate the sulphur bulb plus two fans to keep everything cool. The output from the lamp is coupled to a light pipe. means that objects viewed under the light of a microwave sulphur lamp have practically the same appearance as when viewed under sunlight. Fig.3 shows the spectrum of the sulphur lamp compared with the sensitivity of the eye and the spectral output of the Sun. The colour temperature of the LightDrive 1000 lamp is 5700K and the colour reproduction index is Ra 79. By the way, the amount of UV radiation emitted by the lamp is lower than for sunlight. In fact, for wavelengths less than 380nm, it comprises only 84  Silicon Chip 0.14% of the total light output. A similar situation occurs for infrared radiation, which makes up less than 8% of the total output for wavelengths higher than 780nm. The low UV radiation figure means that materials exposed to the light will age at a slower rate than if they were exposed to sunlight. What’s more, the heat load will be more favourable than for sunlight due to the reduced amount of infrared radiation. These two properties mean that plastics can be used more widely under sulphur lamp illumination than they The sulphur lamp’s very high luminous intensity and near point-source construction makes it very suitable for hollow light guide applications. One of the first uses of the sulphur lamp has been in a demonstration lighting system installed at the Smith­ sonian Institute’s National Air and Space Museum in Washington DC, USA. This system was installed in August 1994 and uses three 27-metre long (266mm diameter) hollow light guides, located three metres from the ceiling and 18 metres from the gallery floor. A single 455,000 lumen sulphur lamp illuminates each light guide, with about 222,000 lumens passed into the light guide itself. The hollow light guides have an efficiency of 55% and the light is Silicon Chip Binders REAL VALUE AT $12.95 PLUS P&P Fig.3: how 3M’s light-pipe system works. The light from the lamp is reflected by the prismatic film lining the inside walls as it travels down the tube, with some of the light escaping through a fine pattern of holes along the way. Fig.4: the spectral output of the sulphur lamp closely coincides with the spectral output of sunlight. This means that objects viewed under a sulphur lamp and under sunlight have practically the same appearance. These binders will protect your copies of SILICON CHIP. They feature heavy-board covers & are made from a dis­tinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf.  Hold up to 14 issues  80mm internal width  SILICON CHIP logo printed in gold-coloured lettering on spine & cover extracted from them through a fine pattern of holes in the “micro-replicated” prismatic film material lining the inside surfaces. The density of the holes varies over the length of the tubes to create the desired light distribution pattern. After passing through the holes, the light is efficiently reflected downwards by a secondary micro-replicated prismatic film located on the underside of the guide. This results in uniform, efficient illumination far beneath the guide itself. The Smithsonian system is used to light 1150 square metres and replaced 94 high-intensity discharge lamps. It delivers around 350 lux at viewer level, a figure that’s about four times higher than the original light level – all this for less than half the cost of a conventional lighting upgrade. The US Department of Energy is a financial backer of the development of the sulphur lamp and also has its own installation. In this case, a single 85-metre long hollow light guide equipped with two sulphur lamps has been used to illuminate a plaza. This installation replaced a system that used 280 high-intensity mercury discharge luminaires, resulting in a measured energy reduction of 65% and saving the DOE US$8000 annually in direct energy costs. Reduced maintenance costs are said to save an additional US$1500 per year. At the same time, the new system provides light levels that are approximately four times higher than from the old mercury discharge system. Other installations of sulphur lamp light guides have been made in European subway stations, the Hill Air Force Base in the US, at a Volvo bus and truck assembly plant in Sweden, and at an indoor karting track at Westerholt in Germany. Finally, sulphur lights are being investigated by the motion picture industry due to the light’s daylight colour and high SC intensity. Footnote: as this was written, it appears that Fusion Lighting has withdrawn its current sulphur lamp from sale, pending the introduction of a new design. Price: $A12.95 plus $A5 p&p. Available only in Australia. Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. Use this handy form Enclosed is my cheque/money order for $________ or please debit my ❏ Bankcard ❏ Visa   ❏ Mastercard Card No: _________________________________ Card Expiry Date ____/____ Signature ________________________ Name ___________________________ Address__________________________ __________________ P/code_______ DECEMBER 1999  85