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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.
“The Times”8 pages
UNCLASSIFIED/;'f81il 8ffllil.l1k WE&i Si'llk>C This idea comes from Fermat's principle of the shortest optical path. 7 Suppose that light travels from A to B. According to Fermat's principle, light follows the path that takes the shortest time. In refractive materials, the speed of light is modified-in most cases reduced-by the refractive index. The time it takes for light to pass through an infinitesimal element of space is proportional to the refractive index. So, if the index varies in an optical material or in the boundaries between materials, the optical measure of path length varies, which is the defining feature of a curved geometry. To provide a simple example, a lens focuses parallel light rays into a point; the parallels meet there, which is the hallmark of a non-Euclidian geometry. So something as familiar as a lens creates something as fantastic as a curved geometry. 6 Figure 7. The Invisible Woman. Invisibility by bending light, an inspiration for cloaking. UNCLASSIFIED ff f81il 8ffll1Als Wli&i SUlklf
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Report, from the dia collection. The PDF is mirrored here; the original link is under it. 29 pages are in the text index: search them above, or from the library's search.