Now you see me … now you don’t

Posted: March 28, 2013 in New materials
Tags: , , , , ,

Let’s acknowledge it: we’ve all wanted an invisibility cloak at some point in our lives. And, since we can’t steal it from Harry Potter, we have to get imaginative, right?

In fact, Harry’s cloak is actually easy to build and it is based on a long time working engineering trick called chroma, that everyone has seen not only in the movies, but also on weather news every day. This technique is also used to create fantasy and scifi backgrounds that was obviously never there, like basically everything in Once Upon a Time or the whole 300 movie. You just need to film everything with a green background and replace anything green with whatever you want.


But back to invisibility cloaks … the trick works like this: we get a cloth (or surface) colored in something ghastly enough that you won’t find in anything in nature, like the usual radioactive green. Now, you get yourself a video camera and get a shot of the background, minus people, so the camera actually knows what’s behind any person walking in the field of view. After that, if someone covers in the green cloth and walks in front of the camera, you can tell a computer to simply remove any green pixel in the image and replace it with whatever color the pixel in the same position was in our background image. Voila! anything under the cloth is removed and replaced with the background. Bad news, though, is we are only invisible through the camera: to anyone looking at us in plain view we would look like Green Riding Hood.

If what we would like to have, in fact, is for everyone to see our invisible garment, we need something like the University of Tokyo invisibility cloak (or, rather, raincoat).


In this case, the previous idea is combined with Augmented Reality techniques. What we do to actually achieve the desired effect is to project whatever is behind the “invisible” guy on top of the garment, which includes a camera on its back to capture what we are occluding in real time. The key to a good projection is to use a retro-reflective material in our cloak, capable of bouncing back the light rays exactly in the same direction from which they came so that it is bright enough to see outside in bright daylight (think about the difference between projecting something on your wall or on a cinema screen with the lights on). In the Tokyo prototype, this is achieved via a beaded surface.

Image taken from How Stuff Work

Needless to say, the whole setup for a camouflage effect that only works from a fixed direction is quite bulky. Nice enough for something like the Avengers helicarrier, though 🙂

avengers helicarrier

Actually, the closest we have to invisibility gadgets are metamaterials, because they can actually bend electromagnetic radiation. Assuming that we actually see an object because light reflected by its surface reaches our eyes, it’s immediate to notice that if we place a second object in the middle of the reflected light path, some beams are blocked and replaced with the second object reflected beams instead. Thus, the image we perceive on the other side is a composition of the original object and the second one. If the second object is reflective, light gets bounced up and we get nice effects like nifty rainbows on the wall and stuff, but the fact is that blocked light doesn’t get to us, so we can’t perceive the original image as it was.


Metamaterials can actually do the trick for us. We can imagine that they divert the light, like a mirror, but they also have the skill to bring it back to its original position after the second object is left behind. Thus, when it reaches our eye, the original picture looks like it should be: as if nothing had been on the way. Our metamaterial object has rendered invisible to all effects.

Unfortunately, metamaterials to make invisibility cloaks need to be made of a lattice with the spacing between elements less than the wavelength of the light we wish to bend. Since we can’t physically make the lattice as small as we would like, thus far this has only worked for long-wavelength radiation such as microwaves. With a cape like this, we would be invisible to some wave detectors (think of good old invisible planes) but not to visual inspection.

In 2007, the University of Maryland’s Igor Smolyaninov managed to produce a metamaterial capable of bending visible light around an object, but quite small and heavy and limited to 2 dimensions. Some universities have also reported new structures that could work in the visible light spectrum and even in 3D (i.e. the cloaked object would be invisible from any direction), but, given the current limitations in materials, it’s not likely we are buying any invisible coat in Zara anytime soon.


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