Artemis Project: Fiber Optic Lighting Systems

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Fiber Optic Lighting Systems

Introduction

Recently, a new mode of lighting has been introduced for interior space lighting which replaces multiple electrically powered lights with a single, high efficiency, long-life light source and a collection of light guides which pipe the light to the desired area. The light source may be of various types; currently, metal-halide lamps and the newly developed RF-pumped sulfur lamps, are the most popular sources. It is also possible to use solar concentrators during daylight hours to pump the optical fibers directly from sunlight - saving electricity.

Piped Lighting Systems

The light may be carried to the destination in a number of ways. In the Air & Space Museum a 280-foot transparent light pipe covered with a special film is used. For the Artemis Project's purposes, a more suitable method would be to use low-cost, low-weight plastic fibers,

With such fibers, bundles totaling up to 60 fibers are end-illuminated with a high-intensity light source. The end of each fiber is then simply aimed towards the surface of the area to be illuminated, or perhaps towards the ceiling if a more diffuse light is desired.

There are several advantages to piped lighting over more conventional lighting schemes:

The smaller number of higher-reliability, albeit more expensive, light sources results in a much longer MTBF (Mean Time Between Failures).
The system is considerably more reliable and easier to maintain, since the distribution fibers themselves have indefinite lifetimes and all maintenance action is concentrated in one location.
For reliability and fail-safe purposes, a number of central light sources would be used and placed in several separated locations.
The illumination system should be less massive than a system using many separate bulbs and copper wiring.
Safety is enhanced, since the fibers carry no electrical current.
During the day, the fibers can be pumped directly from solar concentrators, using no electricity at maximum efficiency. Switching between light sources involves a simple mechanical fiber-optic switch, resembling a train track switch.

RF-Pumped Sulfur Bulbs

Microwave-pumped sulfur bulbs are a relatively new, although proven, technology that offers many advantages for illuminating lunar bases. These bulbs are already being used for applications such as the Smithsonian Air & Space Museum and the DOE headquarters in Washington DC. Sulfur bulbs are somewhat more complicated than incandecent bulbs, since they use a microwave RF source to drive the bulb and require physical rotation of the bulb, but their efficiency is very high, they do not dim with age, and their lifetimes are considerably longer - over 10,000 hours, primarily limited by the magnetron used to excite the bulb. The following table compares RF-sulfur bulbs with several other commercial lighting sources.

*Comparison of Bulb Characteristics. Courtesy of Fusion Lighting Inc.*
Bulb Type	MTBF, Hours	Output, Lumens/Watt	Comments
RF sulfur	60,000	80-100	Cooler than Metal Halide; Magnetron replacement at 15,000 hours; needs EMI shielding; very high output intensities available
Metal Halide	20,000	80-105	Very hot running; high output intensities available
Fluorescent	10,000-20,000	80-90	Very cool running; not available for high output intensities
Incandescent	1,000-5,000	17-18	Hot running, most output is in IR

Example Calculation for Greenhouse

One primary application for this lighting system will be to provide illumination for the agricultural greenhouse providing food for the settlement. See the essay on modular greenhouses in Section 6.7.2.1.4 as an example of an installation using this lighting system.

As an example calculation, let's suppose a 100-square-meter greenhouse tray. (This is for calculation purposes only, the system is easily scalable.) Let us then suppose that the average cable run for a light-pipe system in a dome, toroid, or cylinder is 25 meters.

	Plants require between 1,000 and 10,000 lux of light in order to grow, depending greatly upon the type of plant and the particular growth phase that it's in at the moment. See the Alberta Agriculture, Food and Rural Development Home Page's Artificial Lighting page for further information. This article will use a 5,000 lux requirement as an average value.
	(1 lux = 1 lumen/square meter)
	A good bundled-fiber light guide using high-quality plastic fiber, has a loss of approximately .5%/ft, or 1.63% per meter - equating to an efficiency of .984 per meter. An additional efficiency of .85, worst case, is introduced by the source-fiber coupling. For a 25-meter stretch this equates to 0.57% total transmission efficiency.
	A requirement for 5,000 lux over 100 square meters = a total requirement of 5x10⁵ lumens. Dividing by the transmission efficiency of .57 results in a source requirement for 8.8x10⁵ lumens.
	Remote Source Lighting International 1 of California is introducing an RF sulfur light source, specifically designed for piped lighting of this kind, which produces 120,000 lumens (1.2x10⁵) for a total power input of 1,500 Watts. The above source requirement translates into a requirement of 8 such sources, for a total power requirement of 12 kW.
	Assuming the retention of a terrestrial 24-hour diurnal cycle, this translates to a requirement for 1.8x10³ kW hours over the 14-day lunar night.
	During the lunar day, the light pipes may be switched over to solar concentrators. Using an approximation of 2x10⁵ lux of solar light available at the lunar surface, this translates to a 4.4 square meter concentrator, or a dish/lens approximately 1.25 meters in diameter.

Conclusions

The advantages to centrally-lit light-pipe illumination system are many. The fibers are light-weight and safe in any atmospheric or environmental condition. They are also highly reliable and difficult to damage. Fixturing is minimal at the illumination site and may be as simple as holding the fiber end in a clip and aiming it at the ceiling.

During the night, the light is obtained from a few, highly reliable, high-efficiency sources that are easily maintained. RF-pumped sulfur bulbs are recommended due to their high efficiency and reliability, as well as their low thermal output.

During the day, no electrical power is required to run the system, and all photovoltaic systems may be used for other purposes (e.g., charging storage systems for night use). The only electro-mechanical parts are the solar concentrators, which would use simple single-axis drive motors.

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