Volume 17, Number 3 .... September 1995

The Next Generation of Lights: Electrodeless

The electrodeless sulfur lamp produces 455,000 lumen and replaces nearly 100 conventional, high intensity lamps like the one on the right.

The next generation of light sources has started to take form in the lighting world. This generation promises better light and long life with less energy. The unique feature of these systems lies in how light is produced. The lamps that we are all familiar with, primarily incandescent, heat the electrode, usually a piece of metal until it incandesces, or "glows", producing light. Lamp burn-out is usually associated with the electrode failing, or breaking. The new generation replaces the weakest link in the chain--the electrode--and produces visible light with some innovative techniques. Some use radio frequencies to excite a coil while others use microwave energy directed at the element sulfur to produce the visible light. The new sources, spurred by demands for better quality electric lights using less energy, will slowly but surely penetrate our daily and professional lives.

Transferring this technology to the museum and art gallery lighting may take a little time, but eventually it will happen. The benefits are many and the applications at this point in the development of the new sources, need careful planning. The new light sources usually have high lumen output, producing lots of illumination or lux . The light emitted from one of these sulfur bulbs is equal to over 250 standard 100 watt incandescent lamps. This means that it would require only a few of these sources to provide quality ambient light in a gallery while contributing very little to the heating load. The quality could enhance the colors of the art work, balancing the red portions of the spectrum, the "incandescent look", with other portions of the spectrum, the "daylight" look. Finally, the maintenance would be reduced considerably, ensuring a safer environment for the artifacts.

A case study

One demonstration project combining an electrodeless light source with a light pipe has been operating at the Smithsonian Institution's National Air and Space Museum (NASM) in Washington, D.C., since August 1994. The new lighting system, called the Sulfur Lighting System (SLS), consists of an light injector, a specially designed reflector, and a light pipe.

It was invented four years ago by Fusion Lighting, a small high-technology firm in Rockville, Maryland. Fusion discovered that sulfur, stimulated by microwave energy, could be used in place of mercury in their ultraviolet industrial lamps to produce a very bright, near-sunlight quality light.

How it works

Light from the sulfur lamp is focused by a parabolic reflector so that most of the light enters the light pipe within a small angular cone. Light travels down the tube, reflecting off of the prismatic film (A) that lines the plastic light pipe. The prismatic film reflects the light via total internal reflection (C), an intrinsically efficient process. Some of the light striking the film (at A) is not reflected and "leaks out" of the pipe walls (B), giving the pipe a glowing appearance. A light ray that travels all the way down the pipe will strike the mirror at the end (D) and return up the pipe. A special light-extracting surface (another type of prismatic film) is used to direct most of the light flux downwards in a controllable manner. (E)

The components

Producing visible light: Sulfur and a noble gas are sealed in a glass bulb. The bulb is installed in the microwave cavity and rotated. Microwave energy is directed at the spinning bulb, producing visible light. A polished parabolic reflector directs the light into the light pipe. A constant flow of air is directed across the bulb wall to cool it. Future models will eliminate the need for forced cooling.

The pipe: The inside of a 266 mm diameter acrylic tube is lined with a prism light guide. This film uses total internal reflection to efficiently "carry" the light from the light injector through the length of the pipe. The NASM light pipe is over 27 meters. The efficient light transmission is made possible by a new technology known as micro-replication. Micro-replication enables thousands of precise prisms to be incorporated into the optical wall of the guide. The prism light guide was invented by Lorne Whitehead of the University of British Columbia, and the micro-replicated prismatic material was developed by Roger Appeldorn and Sanford Cobb of the 3M Company, St. Paul, Minnesota. The light pipes are of all acrylic construction. Each 3 meter section weighs approximately 23 kilograms.

Extracting light from the light pipe to the desired area requires another technique. Light is extracted by a fine and unique pattern of holes in the prismatic material, with the density of the holes varying over the length creating the desired output distribution pattern. After passing through the holes, the light is efficiently redirected downward by a secondary micro-replicated prismatic film located on the underside of the light pipe. This recently patented technique results in uniform, efficient illumination far below the light pipe.

System Specifications

The SLS has many advantages over high intensity discharge systems (HID). SLS is more energy efficient, environmentally benign (no mercury means no disposal problems), has excellent color rendition, unequaled color stability, superior lumen maintenance, short turn-on time, low infrared radiation, very low ultraviolet radiation, and long system life. The bulb has no filament, and therefore has a potentially infinite life expectancy. The magnetron has an expected life of over 10,000 hours. Future technology could probably double this life expectancy.

At the Air and Space Museum the Sulfur Lamp System installation in the Space Hall consists of three 27 meter pipes, located 3 meters from the ceiling and 18 meters from the gallery floor, each illuminated at one end by a single sulfur lamp. The system is used to light 1,150 square meters and replaced 94 HID lamps. Light levels at viewer level have been measured at 350 lux. Installation and retrofit costs of the sulfur lamp and light pipe combination were less than half those of a conventional lighting upgrade project.

Spectral irradiance comparison

In use, the system, which has effectively extended the "daylight" hours in the Space Hall, has resulted in the increase of light levels by a factor of three, energy usage has been cut by a fourth, and UV has been reduced. Furthermore, the light pipe significantly reduces maintenance costs. With the new system, the lamps are positioned for easy maintenance while the light pipe distributes the light throughout the exhibition area.

The future

Energy concerns as well as quality lighting demands will drive the research to have these new light sources miniaturized and enclosed in more familiar looking packages. Obvious avenues to explore are applications in smaller exhibition and display spaces, perhaps even in cases. Preparing for this future requires innovative designers, conservators, and curators willing to experiment to improve the museum experience. Their results will improve what we see, how we see, and how we appreciate artifacts.