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Space-habitat illuminators with nonimaging optics

The strongest known material, carbon nanotube, has inspired designs of spinning space habitats reaching radii of 1000 km. Known as the Space Ring, a cylinder of such radius, with a 'short' 500 km length, has an 'open sky' geometry that only self-occludes a third of the view of the stars, as they spin around twice per hour. Sunlight is unsuitable for illuminating the Ring interior because it cannot be turned off. Instead, the Ring axis is sideways to the sun, and solar-cell concentrator troughs cover its exterior to provide power for a central illuminator. Because the inside living space is p times the projected cell area, illumination effectiveness is at a premium, even if the electrical efficiency of the cells is 50 percent, and enormous storage capacity accounts for 'night-time' sunlight. Only a quarter of the light output of a central isotropic emitter would fall on the Ring, so that nonimaging optics is called for. Overall luminaire size (and cost) is reduced by increased emitter luminance, but considerations of eye safety oppose this. An ideal CPC of revolution would produce a rectangular 'sun' with 4:1 aspect ratio, fixed overhead in a blue sky, unable to generate rainbows, sunrises, or sunsets.