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This Defense Intelligence Reference Document, dated 2 March 2010 and produced by the Defense Intelligence Agency under its Advanced Aerospace Weapon System Applications (AAWSA) Program, is a technical survey of invisibility. It covers camouflage, including stealth aircraft and optical camouflage, then transparency, cloaking by coordinate transformation, metamaterials and non-Euclidean broadband cloaking. It concludes that perfect cloaking is impossible but imperfect microwave cloaks are within reach of present technology. Whether cloaking will work at visible wavelengths remains unclear.
UNCLASSIFIED//F'iA. 'iFFllilCk Wlilii 'i•lk>/ Broadband Invisibility To understand why non-Euclidean geometry comes to the rescue of invisibility, consider the following two-dimensional example. 35 Imagine a virtual space made of a flat space, a sheet of paper, and a curved space, the surface of a sphere. The two spaces touch at one line. Consider the fate of light rays in this two-dimensional virtual world. Light rays would either pass the sphere or enter, through the connecting line, the surface of the sphere, whereupon, after one loop, they would continue in the same direction as they entered, as if the tour on the sphere had never happened. The sphere is invisible; it shows only as a time delay of the light ray. Although the sphere is invisible, it does not make something else invisible yet. However, this is easily arranged. Imagine a mirror around the equator of the sphere. The ray bounces off at the mirror, but, after another bounce, is back on track. A mirror in this curved space reflects light back to itself! The mirror creates the illusion that the light performs a full great circle, whereas in reality it stays on one hemisphere. The other hemisphere is hidden. Alternatively, some lines on the sphere are never crossed by light rays. Such lines can be opened like an eye; the space they enclose is hidden from sight. C Figure 20. Non-Euclidean Cloaking Device in Two Dimensions. The dev,ce creates the illusion shown in A: light propagates through a virtual space that consists of a plane and the surface of a sphere, a curved space, which touch along a line. Some incident light rays venture from the plane to the sphere; they return after one loop and continue in the same direction. Note that the rays never cross the red zigzag line on the sphere. Plane and sphere carry a coordinate grid that ,s mapped onto physical space B. The magenta circle defines the boundary of the device. Its interior has been expanded to make space for the grid of the sphere. In particular, the line where plane and sphere touch has been opened like an eye (thick black lines) to include the sphere. This is not a cloaking device yet, but one could place a mirror around the equator of the virtual sphere C, making the northern 19 UNCLASSIFIED/ ,<EiOA: OEiEil&l11J.k W&li SU.kif
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Report, from the dia collection. The PDF is mirrored here; the original link is above. 29 pages are in the text index: search them above, or from the library's search.