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This Defense Intelligence Reference Document, dated 2 March 2010, was prepared by the Defense Intelligence Agency's Defense Warning Office under the Advanced Aerospace Weapon System Applications program. It reviews invisibility through camouflage, transparency, and cloaking, covering metamaterials, transformation optics, and non-Euclidean broadband cloaking designs. It concludes that perfect cloaking is impossible, but imperfect devices could be made. Microwave cloaking is within reach of present technology, while visible-light invisibility remains uncertain and depends mainly on new theoretical research.
From the source: Release of 2026-09-18 Incident: 3/2/10, Las Vegas, Nevada. Released with redactions. This document is a Defense Intelligence Reference Document (DIRD), a technical reference format used by the Defense Intelligence Agency (DIA) to capture baseline knowledge on a specific topic for later analytic use. DIRDs are best understood as reference and synthesis products rather than as original research. It is one of 38 DIRDs produced under the Advanced Aerospace Weapon System Applications Program (AAWSAP) between 2009 and 2011. Because AAWSAP’s scope permitted a broad range of supporting topics, not every DIRD in the series directly concerns aerospace systems or future threat assessment. The following summary reflects the DIRD’s scope and framing at the time of writing and should not be read as implying current validation of the concepts discussed. This DIRD surveys the theory and early experiments behind invisibility cloaking, describing several ways an object might be hidden from visual or sensor detection, including camouflage, transparency effects, and optical cloaking that bends light around an object. It focuses mainly on metamaterials, negative refraction, and transformation optics, and reviews experiments that had already demonstrated limited cloaking at microwave frequencies. The report argues that “imperfect” cloaking is physically achievable in some parts of the electromagnetic spectrum, especially for microwaves, but that “perfect” cloaking is not practical because it would require material properties that conflict with the underlying physics. Its overall conclusion is that cloaking is a scientific field with plausible narrow applications, but that useful visible-light cloaking depends more on future theoretical breakthroughs than on conventional advances in materials science.
UNCLASSIFIED/; POI\ OPPICIAL U.!I!! er•tY Transparency H. G. Wells's novel The Invisible Man represents another strategy for becoming invisible: transparency. In Wells's novel, the invisible man, a disgruntled college professor, invents a substance that somehow changes the refractive index 1 of his body. Most transparent substances, like glass, air, or water, modify the speed of light, because the atoms or molecules of these substances absorb and re-emit light, which takes time. The delay caused by the atoms and molecules results in a reduced speed of light and hence in a refractive index larger than 1. It is, however, also possible to achieve a refractive index smaller than 1, although only in narrow bands of the spectrum. In these cases, the atoms or molecules advance the wave fronts of light because they are excited such that their electron clouds oscillate ahead of the light. If the refractive index is uniform in a material, light is reflected and refracted at the boundary but otherwise is traveling straight through. On the other hand, if the refractive index varies, light is scattered at the index inhomogeneities and gets lost. Most white substances appear white because of such scattering . Milk, for example, consists of minuscule oily droplets-fat-in water. The refractive index of the droplets differs from water, and hence light is scattered at them; it does not penetrate the substance, and the diffused light appears as white. Now, human bodies are visible, because they absorb light. Most of the absorption is due to the scattering of light in biological tissue, in the cells of which the bodies are made. If the refractive indices of a person's cells could somehow be changed to the refractive index of air, the person would become transparent and disappear from view-like the Invisible Man. Some animals (for example, some jellyfish) are transparent, but the cells of higher order animals are usually much too complex and diverse for transparency to become a serious option for disguise. Exceptions are the transparent parts of the body, most notably eye lenses, which consist of uniform cells kept in a state between life and death. If this balance is upset, the lenses become opaque as a cataract develops. Transparency is the idea behind some proposed forms of invisibility by technology. For example, in plasmonic covering, 1 a particle should be surrounded by layers made of metals and transparent substances, such as glass. The layers are designed such that they cancel the scattering of light at the particle, hence making both the particle and the layers transparent-that is, invisible. Another proposal 2 exploits the resonance of the particle with a negative-refractive material that cancels out scattering. In a material with negative refraction, the wave fronts of light appear to move in the opposite 1 The refractive index is the ratio between the speed of light in vacuum and the speed of light in a material. UNCLASSIFIED//5O9 OEEICI0 .. Uili QPlkY Figure 4. H. G. Wells's The Invisible Man: Invisibility by Transparency 3
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Official release, from the pursue 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.