How Invisibility Cloaks Work

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Introduction to How Invisibility Cloaks Work

Photo courtesy ©Tachi Laboratory, the University of Tokyo
What if you could simply throw on a cloak and disappear from sight?

Whether you're a Harry Potter fan or a mere fourth-level mage in a game of "Dungeons and Dragons," you're probably familiar with the concept of an invisibility cloak. Slip this magical garment over your head and, in the eyes of all onlookers, vanish.

This sort of thing tends to fly unquestioned on the grounds of Hogwarts or in the umber-hulk-infested depths of the Underdark. But what about in the real world, where even the "Predator" aliens' clunky sci-fi camouflage looks far-fetched? Well Muggles, science has some good news for you: Invisibility cloaks are a reality. The technology is far from perfect, but we've already reached the point where you even get to choose from two different invisibility technologies.

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On one hand, there's the optical camouflage technology developed by scientists at the University of Tokyo. This approach works on the same principles of the blue screen used by TV weather forecasters and Hollywood filmmakers. If you want people to see through you, then why not just film what's behind you and project it onto your body? Granted, it hardly matches up to the personal cloaking devices found in the likes of "Metal Gear Solid" video game series, but it produces similar (though more limited) results.

For a far more advanced approach, consider investing in a cloak made from metamaterials, tiny structures smaller than the wavelength of light. If properly constructed, such a cloak would actually guide rays of visible light around an object -- much like a rock diverting water in a stream. For now, however, the technology only works in two dimensions and only comes in the ultra-petite size of 10 micrometers across.

In this article, we'll explore both invisibility technologies, the science behind them and their possible applications outside of magical quests and sci-fi infiltration missions.

Optical Camouflage: Altered Reality

Photo courtesy ©Tachi Laboratory, the University of Tokyo
Optical camouflage technology won't make you invisible to multi-eyed Beholder monsters -- or even to stray cats and squirrels.

Optical camouflage delivers a similar experience to Harry Potter's invisibility cloak, but using it requires a slightly complicated arrangement. First, the person who wants to be invisible (let's call him Harry) dons a garment that resembles a hooded raincoat. The garment is made of a special material that we'll examine more closely in a moment.

Next, an observer (let's call him Professor Snape) stands before Harry at a specific location. At that location, instead of seeing Harry wearing a hooded raincoat, Snape sees right through the cloak, making Harry appear to be invisible. The above photograph shows you what Snape would see. And if Snape stepped to the side and viewed Harry from a slightly different location? Why, he'd simply see the boy wizard wearing a silver garment. Scowls and detentions would likely follow. Lucky for Harry, his fictional cloak affords 360-degree protection.

Optical camouflage doesn't work by way of magic. It works by taking advantage of something called augmented-reality technology -- a type of technology first pioneered in the 1960s by Ivan Sutherland and his students at Harvard University and the University of Utah. You can read more about augmented reality in How Augmented Reality Works, but a quick recap will be helpful here.

Augmented-reality systems add computer-generated information to a user's sensory perceptions. Imagine, for example, that you're walking down a city street. As you gaze at sites along the way, additional information appears to enhance and enrich your normal view. Perhaps it's the day's specials at a restaurant or the showtimes at a theater or the bus schedule at the station. What's critical to understand is that augmented reality is not the same as virtual reality. While virtual reality aims to replace the world, augmented reality merely tries to supplement it with additional, helpful content. Think of it as a heads-up display (HUD) for everyday life.

Most augmented-reality systems require a user to look through a special viewing apparatus to see a real-world scene enhanced with synthesized graphics. They also call for a powerful computer. Optical camouflage requires these things as well, but it also necessitates several other components. Here's everything needed to make a person appear invisible:

a garment made from highly reflective material
a digital video camera
a computer
a projector
a special, half-silvered mirror called a combiner

On the next page, we'll look at each of these components in greater detail.

Optical Camouflage: Invisibility Cloak Components

Light Surface Reflectivity

All right, so you have your video camera, computer, projector, combiner and wondrous reflective raincoat. Just how does augmented reality technology turn this odd shopping list into a recipe for invisibility?

First, let's take a closer look at the raincoat: It's made from retro-reflective material. This high-tech fabric is covered with thousands and thousands of small beads. When light strikes one of these beads, the light rays bounce back exactly in the same direction from which they came.

To understand why this is unique, look at how light reflects off other types of surfaces. A rough surface creates a diffused reflection because the incident (incoming) light rays scatters in many different directions. A perfectly smooth surface, like that of a mirror, creates what is known as a specular reflection -- a reflection in which incident light rays and reflected light rays form the exact same angle with the mirror surface.

In retro-reflection, the glass beads act like prisms, bending the light rays by a process known as refraction. This causes the reflected light rays to travel back along the same path as the incident light rays. The result: An observer situated at the light source receives more of the reflected light and therefore sees a brighter reflection.

Retro-reflective materials are actually quite common. Traffic signs, road markers and bicycle reflectors all take advantage of retro-reflection to be more visible to people driving at night. The movie screens found in most modern commercial theaters also take advantage of this material because it allows for high brilliance under dark conditions. In optical camouflage, the use of retro-reflective material is critical because it can be seen from far away and outside in bright sunlight -- two requirements for the illusion of invisibility.

Optical Camouflage: More Invisibility Cloak Components

Do not attempt: professional invisible scientist standing in the road.

AP Photo/Shizuo Kambayashi
As you can see in this image, the experience closely resembles walking directly in front of a movie projection screen, only with a real background.

For the rest of the setup, the video camera needs to be positioned behind the subject to capture the background. The computer takes the captured image from the video camera, calculates the appropriate perspective and transforms the captured image into the picture that will be projected onto the retro-reflective material.

The projector then shines the modified image on the garment, by shining a light beam through an opening controlled by a device called an iris diaphragm. This diaphragm is made of thin, opaque plates, and turning a ring changes the diameter of the central opening. For optical camouflage to work properly, this opening must be the size of a pinhole. Why? This ensures a larger depth of field so that the screen (in this case the cloak) can be located any distance from the projector.

Finally, the overall system requires a special mirror to both reflect the projected image toward the cloak and to let light rays bouncing off the cloak return to the user's eye. This special mirror is called a beam splitter, or a combiner -- a half-silvered mirror that both reflects light (the silvered half) and transmits light (the transparent half).

If properly positioned in front of the user's eye, the combiner allows the user to perceive both the image enhanced by the computer and light from the surrounding world. This is critical because the computer-generated image and the real-world scene must be integrated fully for the illusion of invisibility to seem realistic. The user has to look through a peephole in this mirror to see the augmented reality.

On the next page, we'll look at how this whole system comes together.

Optical Camouflage: The Complete Invisibility System

Invisibility Cloak System

Now let's put all of these components together to see how the invisibility cloak appears to make a person transparent. The diagram below shows the typical arrangement of all the various devices and pieces of equipment.

Once a person puts on the cloak made with the retro-reflective material, here's the sequence of events:

A digital video camera captures the scene behind the person wearing the cloak.
The computer processes the captured image and makes the calculations necessary to adjust the still image or video so it will look realistic when it is projected.
The projector receives the enhanced image from the computer and shines the image through a pinhole-sized opening onto the combiner.
The silvered half of the mirror, which is completely reflective, bounces the projected image toward the person wearing the cloak.
The cloak acts like a movie screen, reflecting light directly back to the source, which in this case is the mirror.
Light rays bouncing off the cloak pass through the transparent part of the mirror and fall on the user's eyes. Remember that the light rays bouncing off the cloak contain the image of the scene that exists behind the person wearing the cloak.

The person wearing the cloak appears invisible because the background scene is being displayed onto the retro-reflective material. At the same time, light rays from the rest of the world are allowed to reach the user's eye, making it seem as if an invisible person exists in an otherwise normal-looking world.

Optical Camouflage: Real-world Invisibility Applications

Mutual Telexistence

The words "invisibility cloak" tends to summon images of fantastic adventure, magical espionage and otherworldly deception. The actual applications for optical camouflage, however, are far less out there. You can forget hiding your Romulan starship or hanging out in the lady wizards' dormitory, but that doesn't mean there aren't a number of viable uses for the technology.

For instance, pilots landing a plane could use this technology to make cockpit floors transparent. This would enable them to see the runway and the landing gear simply by glancing down at the floor (which would display the view from the outside of the fuselage) Similarly, drivers wouldn't have to deal with mirrors and blind spots. Instead, they could just "look through" the entire rear of the vehicle. The technology even boasts potential applications in the medical field, as surgeons could use optical camouflage to see through their hands and instruments for an unobstructed view of the underlying tissue.

Interestingly enough, one possible application of this technology actually revolves around making objects more visible. The concept is called mutual telexistence and essentially involves projecting a remote user's appearance onto a robot coated in retro-reflective material. Say a surgeon were operating on a patient via remote control robotic surgery. Mutual telexistence would provide the human doctors assisting the procedure with the perception that they're working with another human instead of a machine.

Right now, mutual telexistence is science fiction, but scientists continue to push the boundaries of the technology. For example, pervasive gaming is already becoming a reality. Pervasive gaming extends gaming experiences out into the real world, whether on city streets or in remote wilderness. Players with mobile displays move through the world while sensors capture information about their environment, including their location. This information delivers a gaming experience that changes according to where users are and what they are doing.

Optical camouflage is just one method of creating the sensation of invisibility. On the next few pages, we'll explore the latest breakthroughs in the science of disappearing.

Metamaterials: Bending Light Waves

A meteorologist makes the mistake of wearing a blue outfit to work one day. When he walks in front of the blue screen to film the weather report, his arms and torso appear to have vanished, leaving only a floating pair of hands and a disembodied head. This scenario, which you might recognize from the 1993 comedy "Groundhog Day," is optical camouflage in a nutshell.

Metamaterials offer a more compelling vision of invisibility technology, without the need for multiple projectors and cameras. First conceptualized by Russian physicist Victor Veselago in 1967, these tiny, artificial structures are smaller than the wavelength of light (they have to be to divert them) and exhibit negative electromagnetic properties that affect how an object interacts with electromagnetic fields.

The Smallest Frontier

Metamaterials, a creation of science, don't occur naturally. In order to create the minute structures required to redirect electromagnetic waves, scientists employ nanotechnology. Read How Nanotechnology Works to learn all about the world's smallest machines.

Remember, images reach us via light waves. Sounds reaches us via sound waves. If you can channel these waves around an object, you can effectively hide it from view or sound. Imagine a small stream. If you stick a teabag full of red dye into the flowing water, its presence would be apparent downstream, thanks to the way it altered the water's hue, taste and smell. But what if you could divert the water around the teabag?

In 2006, Duke University's David Smith took an earlier theory posed by English theoretical physicist John Pendry and used it to create a metamaterial capable of distorting the flow of microwaves. Smith's metamaterial fabric consisted of concentric rings containing electronic microwave distorters. When activated, these steer frequency-specific microwaves around the central portion of the material.

Obviously humans don't see in the microwave spectrum, but the technology demonstrated that energy waves could be routed around an object. Imagine a cloak that can divert a third grader's straw-fired spitball, move it around the wearer and allow it to continue on the other side as if its trajectory had taken it, unopposed, straight through the person in the cloak. Now how much more of a stretch would it be to divert a rock? A bullet?

Smith's metamaterials proved the method. The recipe to invisibility lay in adapting it to different waves.

Metamaterials: Hiding Tanks, Not Boy Wizards

Image courtesy the University of Maryland Department of Electrical and Computer Engineering
This optical image shows the University of Maryland metamaterials in action, steering light waves away from the central circle. The arrows indicate the direction of the light waves.

Take a good look at an object on your desk -- say the unwashed coffee cup or that canister of Red Bull you're nursing. Seriously, why all the caffeine? As you stare at this object, ponder this: You've never actually seen this object before and you're not really seeing it now. You're actually seeing light -- that's all our eyes can sense.

Your eyes are processing the light leaving that shiny silver canister, light originating from a lamp or the world outside a nearby window. If we could somehow keep the light from intercepting the object in question, you wouldn't be able to see it.

In 2007, the University of Maryland's Igor Smolyaninov led his team even farther down the road to invisibility. Incorporating earlier theories proposed by Purdue University's Vladimir Shaleav, Smolyaninov constructed a metamaterial capable of bending visible light around an object.

A mere 10 micrometers wide, the Purdue cloak uses concentric gold rings injected with polarized cyan light. These rings steer incoming light waves away from the hidden object, effectively making it invisible. Chinese physicists at Wuhan University have taken this concept into the audible range, proposing the creation of an acoustic invisibility cloak capable of diverting sound waves around an object.

For the time being, metamaterial invisibility cloaks are somewhat limited. They're not only small; they're limited to two dimensions -- hardly what you'd need to vanish into the scenery of a 3-D war zone. Plus, the resulting cloak would weigh more than even a full-grown wizard could hope to lug around. As a result, the technology might be better suited to applications such as hiding stationary buildings or vehicles, such as a tank.

Explore the links on the next page to learn more about invisible tanks and other cutting-edge technologies.

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