plastic having different index of refraction or thickness might be feasible. If one can accept for a given application power densities on the earth lower than those from the daytime sun, another approach to reducing spot size is available. Referring to Fig. 2 we may use light baffles with either a single lens (or reflector) or even a plane reflector. The light baffles must restrict the numerical aperture at the satellite location for light coming from the sun. Thus if a^ = /k a , the spot diameter on the earth will be reduced to = /k 331 km. Since the effective area of the sun is now reduced by k, the power density over the illuminated area on the earth will be reduced by k. The light baffle could consist of a thin (few cm) sheet of plastic made of optical fibers with very small numerical aperture. With this arrangement the single reflector would have to be curved only if its diameter D approaches D^; however one needs D to realize the maximum possible power density.
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