Tuesday, 6 September 2011

Infrared invisibility cloak

RV Jones, in his excellent book on British scientific intelligence during the Second World War, called Most Secret War, tells the story of a German attempt to camouflage submarines so that they blended into the sea at both visible and far-infrared/thermal wavelengths. They developed a painting scheme which combined a black undercoat with a top layer of varnish that included powdered glass in suspension. To the naked eye the boat would look grey, presumably like the grey north Atlantic, but to an infrared viewer it would look dark, again like the sea. The properties of the undercoat, glass and varnish were such that infrared passed straight through and was absorbed by the undercoat while visible light was refracted by the glass and gave a grey appearance.

This was a neat trick, of which Jones was most complimentary in his book. Unfortunately the British were actually detecting submarines by using radar and not infrared.

This story comes to mind when seeing the news stories circulating about a thermal invisibility cloak called ADAPTIV produced by BAE systems (their press release). The plan is to enable a military object, such as a tank, to either blend into the background or to appear to be something innocuous. This is done by covering an appropriate surface or two with ingenious hexagonal panels that can be electronically set to give off suitable far-infrared radiation on demand. The trick being the ability to change temperature very rapidly and to adopt a temperature that can be lower as well as higher than ambient.

A thermal imaging camera can be used to pickup what is behind the object and use the ADAPTIV panels to show this on the opposite surface, thus rendering the object invisible. By replacing some of the output pixels with a different thermal pattern the object can be made to appear smaller and different. It's unclear whether such as system could yet be used by an individual soldier but objects larger than vehicles ... even buildings ... can be cloaked using this technology.

The promotional video [now no longer online] is rather fun, not only because of the 'so natural that you want to go to war' music but also for the shots of ADAPTIV in action. I particularly like how the panels can be used to display advertisements!

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