Invisibility
splitter (part of the light is reflected, the other part is transmitted through the splitter), to enhance the computer-generated image to create a more realistic image. 11 Figure 5: Iris diaphragm. 12 Consisting of thin, opaque plates and a ring that, when turned, could change the diameter of the central opening. For optical camouflage to function correctly, the size of the opening must be that of a pinhole to ensure a larger depth of field (due to the small aperture), which ensures that the projected image on the cloak remains clear when the projector is at different distances to the cloak. 13
This results in a bulky setup (Fig.6). In 2003, researchers at the Tachi Laboratory at the University of Tokyo created a prototype ‘invisibility cloak’ u sing these principles. While this technique seems promising, there are some drawbacks. The bulky setup makes it difficult to implement into mobile systems, although advances in computer technology could reduce the bulkiness by implementing computer chips within the cloak itself. Another problem is that, currently, one can easily detect the optical illusion by simply viewing from another angle; to correct this deficiency one would have to consider creating a 3D image rather 2D image using holograms. 14 Despite these technical challenges, optical camouflage could be used in the future to hide stationary objects only. However, the biggest challenge faced by this technique is the quality of invisibility – it is easy to tell that there is an invisibility cloak present. While promising, it is unlikely that optical camouflage will be the overarching solution to invisibility.
Figure 6: System. 15 The entire setup of the invisibility cloak, based on optical camouflage, is bulky. However, in the future, with further development, the system can be condensed into a more mobile form.
Arguably the most promising technique for creating invisibility is via metamaterials, which have unique properties that cannot be found in nature. 16 In 2006, researchers at Duke University created a metamaterial which defied contemporary physics – this metamaterial
had a negative refractive index! 17 The principle is best explained with the analogy of a rock in a river. Water in a river when it encounters the rock, flows around the rock, and re-joins. Thus, further downstream, the presence of the rock disappears (Fig.7). 18 Researchers at Duke University implanted tiny electrical circuits within copper bands, arranged in concentric circles, to create magnetic fields to manipulate microwaves around an object, thus rendering it invisible. However, this technology also faces some technical issues. First, the tiny implants must be smaller than the wavelength of the 11 Bhardawaj, A. et al, p1059. 12 Definition: Diaphragm; Iris Diaphragm; Iris (n.d.), available at: https://www.photokonnexion.com/definition- iris-diaphragm/ (Accessed: 13 July 2020). 13 Bhardawaj, A. et al, p1059.
14 Kaku, M. (2009) Physics of the Impossible, England: Penguin Books Ltd, p32. 15 Harris, L., and Lamb, R., (2005), How invisibility cloaks work, available at: https://science.howstuffworks.com/invisibility-cloak.htm#pt5 (Accessed 14 July 2020). 16 Kaku, M. , p21.
17 Ibid., p23. 18 Ibid., p21.
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