If you're a fan of Harry Potter, then you're quite familiar with the concept of an invisibility cloak. In his first year at Hogwarts Academy, Harry receives an invisibility cloak that used to belong to his father. As its name suggests, the invisibility cloak renders Harry invisible when he slips beneath the shining, silvery cloth.
 Photo courtesy ©Tachi Laboratory, the University of Tokyo Optical-camouflage technology developed at the University of Tokyo |
This seems perfectly believable when you're reading about a fictional world filled with witches, wizards and centuries-old magic; but in the real world, such a garment would be impossible, right? Not so fast. With optical-camouflage technology developed by scientists at the University of Tokyo, the invisibility cloak is already a reality.
Optical camouflage delivers a similar experience to Harry Potter's invisibility cloak, but using it requires a slightly more complicated arrangement. First, the person who wants to be invisible (let's call her Person A) 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 (Person B) stands before Person A at a specific location. At that location, instead of seeing Person A wearing a hooded raincoat, Person B sees right through the cloak, making Person A appear to be invisible. The photograph on the right below shows you what Person B would see. If Person B were viewing from a slightly different location, he would simply see Person A wearing a silver garment (left photograph below).
 Photo courtesy ©Tachi Laboratory, the University of Tokyo |
Still, despite its limitations, this is a cool piece of technology. Not only that, but it's also a technology that's been around for a while.
Altered Reality
Optical camouflage doesn't work by way of magic. It works by taking advantage of something called augmented-reality technology -- a type of technology that was 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 Will Work, 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 show times at a theater or the bus schedule at the station. What's critical to understand here 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.
 Augmented-reality displays overlay computer-generated graphics onto the real world. |
Most augmented-reality systems require that users look through a special viewing apparatus to see a real-world scene enhanced with synthesized graphics. They also require a powerful computer. Optical camouflage requires these things, as well, but it also requires several other components. Here's everything needed to make a person appear invisible:
- A garment made from highly reflective material
- A video camera
- A computer
- A projector
- A special, half-silvered mirror called a combiner
Let's look at each of these components in greater detail.
The Cloak
The cloak that enables optical camouflage to work is made from a special material known as retro-reflective material.
 Photo courtesy ©Tachi Laboratory, the University of Tokyo Invisibility cloak |
A retro-reflective material 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 of other types of surfaces. A rough surface creates a diffused reflection because the incident (incoming) light rays get scattered 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. Movie screens used 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.
More Invisibility Cloak Components
Video Camera
 Photo courtesy ©Tachi Laboratory, the University of Tokyo |
Computer
All augmented-reality systems rely on powerful computers to synthesize graphics and then superimpose them on a real-world image. For optical camouflage to work, the hardware/software combo must take the captured image from the video camera, calculate the appropriate perspective to simulate reality and transform the captured image into the image that will be projected onto the retro-reflective material.
The Projector
 Photo courtesy ©Tachi Laboratory, the University of Tokyo |
The Combiner
The 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 fully integrated for the illusion of invisibility to seem realistic. The user has to look through a peephole in this mirror to see the augmented reality.
The Complete 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 of 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 of the cloak pass through the transparent part of the mirror and fall on the user's eyes. Remember that the light rays bouncing off of the cloak contain the image of the scene that exists behind the person wearing the cloak.
Nema komentara:
Objavi komentar