Sulphur Plasma Light
The sulphur lamp (also sulfur lamp) is a highly efficient full-spectrum electrodeless lighting system whose light is generated by sulphur plasma that has been excited by microwave radiation. The technology was developed in the early 1990s, funded by NASA and the US Department of Defence for anticipated extra terrestrial operations. Star wars program and funding for sulphur lighting was terminated by 1999. Since 2005, lamps were again being manufactured for both research and commercial use by Island Systems in Essex, UK.
The sulphur lamp consists of a golf ball-sized fused-quartz bulb (1) containing several milligrams of sulphur powder and argon gas at the end of a thin glass spindle. The bulb is enclosed in a microwave-resonant wire-mesh cage (7). A magnetron (4), variably powered by a digital electronic PSU (5), bombards the bulb, via a waveguide (3), with 2.45 GHz microwaves. The microwave energy excites the gas to several atmospheres pressure, which in turn heats the sulphur to an extreme degree forming a brightly glowing plasma capable of illuminating a large area. Because the bulb heats up considerably, it is necessary for an electric motor (2) to spin the bulb while a fan (6) cools it to prevent it from melting.
The bulb can be placed at the focus of a parabolic reflector to direct all the light in one direction, deeming it probably the only light source small enough and powerful enough for practical and commercially viable light pipe applications
It would be impossible to excite the sulphur using traditional electrodes. The sulphur would quickly react with and destroy all metallic electrodes rendering them useless. The absence of electrodes allows for a far greater variety of light-generating substances to be used than those used in traditional lamps.
The design life of the bulb is greater than 60,000 hours. The bulb emits no electric or magnetic fields. The quality of light reduces by up to 5% in the first 100 hours of operation and then remains constant for the next 5 years.
With the exception of fluorescent lamps, the warm-up time of the sulphur lamp is notably shorter than for all other gas discharge lamps, even at low ambient temperatures. It reaches 90% of its final luminous flux within twenty seconds (video), and the lamp can be restarted less than five minutes after a power cut.
Our first prototype lamps were 1.18 kW units, with a system efficiency of just over 100 lumens per watt. The first production models were 1.36 kW with an output of 191,000 lumens.
Quality of Light Emitted
The sulphur plasma consists mainly of dimer molecules (S2), which generate the light through molecular emission. Because this, instead of atomic emission, is the mechanism of light generation, the emission spectrum is continuous throughout the visible spectrum.
The lamp's output is low in infrared energy, and less than 1% is ultraviolet light. As much as 73% of the emitted radiation is in the visible spectrum, far more than other types of lamps. The visible light output mimics sunlight better than any other artificial light source, and the lack of harmful ultraviolet radiation can be especially beneficial to museums and displays of art.
The spectral output peaks at 510 nanometres and the correlated colour temperature is approximately 6000 kelvins with a CRI of 79. The lamp can be dimmed to 15% without affecting the light quality, and light output remains constant over the life of the bulb.
The use of a magenta filter can be used to give the light a warmer feel. Such a filter was used on the lamps at the National Air and Space Museum in Washington, D.C.
Because the lamp is electrodeless, the addition of other chemicals in the bulb can be used to change or improve colour rendition. Sulphur lamp bulbs with calcium bromide added produce a similar spectrum but with the addition of a spike in red wavelengths at 625nm. Other additives such as selenium indium halide, lithium iodide and sodium iodide can and have been been used to modify the output spectra. (see SPD chart from Jena Uni)