While New High-Performance, Light-Transmitting Materials Such As Aerogel and Light-Transmitting

ABSTRACT

While new high-performance, light-transmitting materials such as aerogel and light-transmitting concrete compel us to question the nature of solidity, a new technology developed by University of Tokyo seeks to make matter disappear altogether. Scientists at Tachi Laboratory have developed Optical Camouflage, which utilizes acollection ofdevices working in concert to render a subject invisible. Although more encumbering and complicated than Harry Potter’s invisibility cloak, this system has essentially the same goal, rendering invisibility by slipping beneath the shining, silvery cloth. Optical Camouflage requires the use of clothing - in this case, a hooded jacket - made with a retro-reflective material, which is comprised by thousands of small beads that reflect light precisely according to the angle of incidence. A digital video camera placed behind the person wearing the cloak captures the scene that the individual would otherwise obstruct, and sends data to a computer for processing. A sophisticated program calculates the appropriate distance and viewing angle, and then transmits scene viaprojectorusing a combiner, or a half silveredmirrorwith an optical hole, which allows a witness to perceive a realistic merger of the projected scene with the background - thus rendering the cloak-wearer invisible..The concept of optical camouflage is straight forward to create the illusion of invisibility by covering an object with something that projects the scene directly behind that object. This system was conceived with the primary view in mind of concealing stationary or moving objects such as men, vehicles, or aircraft from view and has practical military, law enforcement, and security applications.

1.Introduction

Various methods have been proposed to integrate the visual space. In the field of Mixed Reality, one of the most popular topics is about displaying a virtual object into real world However making objects virtually transparent, like in H.G. Wells’ “Invisible Man” can also be Seen as dream of human being. In this paper, we describe what could be called a camouflage Technique named Optical Camouflage.

Camouflage :-

Camouflage is the method which allows an otherwise visible organism or object to remain indiscernible from the surrounding environment. Examples include a tiger's stripes and the battledress of a modern soldier. Camouflage is a form of deception. The word camouflage comes from the French word 'camoufler' meaning 'to disguise'.

Natural camouflage :-

In nature, there is a strong evolutionary pressure for animals to blend into their environment or conceal their shape; for prey animals to avoid predators and for predators to be able to sneak up on prey. Natural camouflage is one method that animals use to meet these aims.

(Anolis caroliensis showing blending camouflage and counter shading.)

Military camouflage :

These were intended to daunt the enemy, attract recruits, foster unit cohesion, allow easier identification of units in the fog of war.The British in India in 1857 were forced by casualties to dye their red tunics to neutral tones, initially a muddy tan called khaki.

The United States was quick to follow the British, going khaki in the same year.Later in 20thcentury, digital camouflage patterns have been exprerimented on helicopters, battledresses &other vehicles. It is termed "digital" because much of the design was done on a computer and unlike other camouflage patterns, it is blocky and appears almost pixelated.

Theory of Camouflage:

MacKay's statement above remains one of the most important elements in the theory of camouflage - an exact match with the environment's colours is less crucial than the patterning of the regions of colour themselves. Ideally, camouflage should be made to break up and thereby conceal the structural lines of the object which it hides. Thus, the patterns often seen on camouflage clothing, masking cloth and vehicle paints are carefully constructed to deceive the human eye by breaking up the boundaries that define sharp edges and human silhouettes. This is called high difference or disruptive camouflage. This mix of blending and disruptive patterns is called coincident disruption - the aim of modern military camouflage.

The opposite of camouflage is making a person or object more visible and easier to recognize,for example with retroreflectors and high-visibility clothing.

What is Optical Camouflage?

Optical camouflage is a kind of active camouflage. This idea is very simple. If you project background image onto the masked object, you can observe the masked object just as if it were virtually transparent. Although optical is a term that technically refers to all forms of light, most proposed forms of optical camouflage would only provide invisibility in the visible portion of the spectrum. The most intriguing prototype uses

an external camera placed behind the cloaked object to record a scene, which it then transmits to a computer for image processing. The computer feeds the image into an external projector which projects the image onto a person wearing a special retroreflective coat. This can lead to different results depending on the quality of the camera, the projector, and the coat, but by the late nineties, convincing illusions were created.

The downside is the large amount of external hardware required, along with the fact that the illusion is only convincing when viewed from a certain angle.

Creating complete optical camouflage across the visible light spectrum would require a coating or suit covered in tiny cameras and projectors, programmed to gather visual data from a multitude of different angles and project the gathered images outwards in an equally large number of different directions to give the illusion of invisibility from all angles. For a surface subject to bending like a flexible suit, a massive amount of computing power and embedded sensors would be necessary to continuously project the correct images in all directions. This would almost certainly require sophisticated nanotechnology, as our computers, projectors, and cameras are not yet miniaturized enough to meet these conditions.

Although the suit described above would provide a convincing illusion to the naked eye of a human observer, more sophisticated machinery would be

necessary to create perfect illusions in other electromagnetic bands, such as the infrared band. Sophisticated target-tracking software could ensure that the majority of computing power is focused on projecting false images in those directions where observers are most likely to be present, creating the most realistic illusion possible.

This figure shows the principle of the optical camouflage using X’tal vision. You can select camouflaged object to cover with retroreflector. Moreover, to project a stereoscopic image, theobserver looks at the masking object more transparent.

In the above shown figure,This transparent cloak makes you see as if the cloak is transparent by projecting the shooting image behind the person onto the cloak i.e. It looks like three men walkingbehind are seen through the body of the person. So, actually, the cloak is not really transparent.

How does it work?

First, putting the video camera behind the person in the cloak, and capturing his background. Then, projecting the captured image onto the cloak from the projector. So, if you see from the peephole, you will see as if the cloak is transparent. Because the image is projected by the technology called Retro-reflective Projection Technology (RPT), you can see the reflection only on the cloak and clearly even in brightness.

Retro-reflective Projection Technology(RPT):-

Now that we ‘ve seen how does optical camouflage works using RPT & X’stal vision let us illustrate RPT. When using a See-Through Head-mounted Display (STHMD) to merge virtual and real environments, the operator may see the image of a virtual object that is meant to be located behind a real object. This contradicts our intuition of depth, since the projected image of an object located behind another object in one's field of view will be obstructed at least partially. This depth cue is called occlusion, and is critical for the effectiveness of the presentation of virtual objects in three dimensions. To solve the occlusion contradiction problem, we developed RPT.

The three key techniques of RPT are the followings:

1-To use an object covered by retro-reflective material as a screen;

2-To place a projector into a position optically conjugated with the observer's eye by using

a half-mirror;

3-To make the projector's iris as small as possible (by using a pinhole).

Each of these points provides the following advantages, respectively:

Fig.5

Fig 6

Fig.5 and Fig.6 shows the principles of RPT. The image of a virtual object is projected through a pinhole. The projected image is reflected by the half-mirror on a right angle and then retro-reflected by the retro-reflective screen.

( no need)

Requirements of an optical camouflage system

The things needed to make a person appear invisible are:

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 asretro-reflective material.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.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.

The Video Camera:

The retro-reflective garment doesn't actually make a person invisible -- in fact, it's perfectly opaque. What the garment does is create an illusion of invisibility by acting like amovie screenonto which an image from the background is projected. Capturing the background image requires a video camera, which sits behind the person wearing the cloak. The video from the camera must be in a digital format so it can be sent to a computer for processing.

The 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 stimulate reality and transform the captured image into the image that will be projected onto the retro-reflective material. The projected image is composed by computer using animage-based rendering method.

The Projector:

The modified image produced by the computer must be shone onto the garment, which acts like a movie screen. A projector accomplishes this task by shining a light beam through an opening controlled by a device called aniris diaphragm. An iris 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.

The Combiner:

The system requires a special mirror to both reflect the projected image toward the cloak and to letlight raysbouncing off the cloak return to the user'seye. 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:

Adigital video cameracaptures 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.

6.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.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 reach the user's eye, making it seem as if an invisible person exists in an otherwise normal-looking world.

Inherent Physical Problems With Cloaking An Object :-

•Parametrical design considerations

•Resolution Factors

Parallax, View angle and range dependency, Tilt angle, and Perspective.

•Reflections and Glint

•Parameters were treated in depth by Schowengerdt and Schweizer in 1993 8 - Parallax is most critical and is summarized below and on next page .

•Angular resolution, A, is basically a function of the wavelength, , and the diameter, d, of the observer’s aperture (A = /d ).

•  = 500 nm for the effective central wavelength of visible light

• For human eye, A = 1 minute of arc = 0.0003 radian

• For 10 inch (25 cm) diameter telescope, A = 0.000002 radian

•Minimum range of an object necessary to escape detection is a function of the observer’s resolution, distance of the object from the observer, the

distance of the object from the background, and lateral motion of the observer necessary to detect the target.

Parallax problem:-

Using the trigonometric relationships below one may solve for various factors. For example, if an observer with naked eye moves his head laterally by only 1 foot, he will be unable to distinguish a camouflaged object from a background 10 feet behind it, provided that the concealed object is at a range of more than 180 feet from the observer. If the background object is 20 feet behind the object, then the range from the concealed object to the observer must be at least 250 feet. If the observer has a ten-inch telescope, then the minimum ranges become 2,200 and 3,200 feet respectively

β C

X = minimum lateral motion of observer necessary to detect target.

R = distance or range from observer to target.

D = distance from target to background plane.

T = position of target (concealed object).

B = location of object in background plane behind target

when observer is at Origin (O).

O = original position of observer before moving to X.

Real-World Applications:

While an invisibility cloak is an interesting application of optical camouflage, it's probably not the most useful one. Here are some practical ways the technology might be applied:

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.

Doctors performing surgery could use optical camouflage to see through their hands and instruments to the underlying tissue. See Tachi Lab: Optical Camouflage: oc-phantom.mpg to watch a video of how this might work.

Providing a view of the outside in windowless rooms is one of the more fanciful applications of the technology, but one that might improve the psychological well-being of people in such environments.
Drivers backing up cars could benefit one day from optical camouflage. A quick glance backward through a transparent rear hatch or tailgate would make it easy to know when to stop.

One of the most promising applications of this technology, however, has less to do with making objects invisible and more about making them visible. The concept is called mutual telexistence: working and perceiving with the feeling that you are in several places at once. Here's how it works:

Human user A is at one location while his telexistence robot A is at another location with human user B.
Human user B is at one location while his telexistence robot B is at another location with human user A.
Both telexistence robots are covered in retro-reflective material so that they act like screens.

With video cameras and projectors at each location, the images of the two human users are projected onto their respective robots in the remote locations.

This gives each human the perception that he is working with another human instead of a robot.