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 a collection of devices 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 via projector using a combiner, or a

half silvered mirror with 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, the observer 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 walking behind 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 as retro-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 a movie screen onto 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

an iris 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 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:

1. A digital video camera captures the scene behind the person wearing

the cloak.

2. 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.

3. The projector receives the enhanced image from the computer and shines

the image through a pinhole-sized opening onto the combiner.

4. The silvered half of the mirror, which is completely reflective, bounces the

projected image toward the person wearing the cloak.

5. 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, l, and the

diameter, d, of the observer’s aperture (A = l/d ).

• l = 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.

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.

Results:

Fig(a) shows the haptic display (real object) hiding the virtual object, but

Optical Camouflage techniques permit to make the haptic display to become

transparent.However, the operator’s hand is not made transparent,which

implies that it is possible to use this technique selectively.

Fig(b) shows a demonstration of ”Invisible Cloak”. It looks as if a red truck

can be seen through the body of a man who wear a retro-reflective coat.

Actually, he does not become transparent perfectly. The shape of the coat

isobserved clearly. Nevertheless, it looks like a very low refractive index

glasswork, which is enough to observe the background.

Fig(a).Optical camouflaged haptic display Fig (b). “Invisible cloak”

Head-mounted Displays:

Of course, making the observer stand behind a stationary combiner is not

very pragmatic -- no augmented-reality system would be of much practical

use if the user had to stand in a fixed location. That's why most systems

require that the user carry the computer on his or her person, either in a

backpack or clipped on the hip. It's also why most systems take advantage of

head-mounted displays, or HMDs, which assemble the combiner and optics in

a wearable device.

There are two types of HMDs: optical see-through displays and video see-

through displays. Optical see-through displays look like high-tech goggles,

sort of like the goggles Cyclops wears in the X-Men comic books and movies.

These goggles provide a display and optics for each eye, so the user sees the

augmented reality in stereo. Video see-through displays, on the other hand,

use video-mixing technology to combine the image from a head-worn camera

with computer-generated graphics.

In this arrangement, video of the real world is mixed with synthesized graphics

and then presented on a liquid-crystal display. The great advantage of video

see-through displays is that virtual objects can fully obscure real-world objects

and vice versa.

HEAD MOUNTED PROJECTOR

Two projectors -- one for each eye -- are required to produce a stereoscopic effect .

Conclusion

We have developed an Optical Camouflage system.Optical Camouflage

can be used on surgical globes or equipments so they don’t block surgeon’s

view during delicate operations. In aviation, cockpit floors couldbecome

'invisible' to assist pilots during landing. The weak point of this technique is

that the observer needs to look through a half-mirror. The current system

needs a half-mirror and projectors, which were fixed onthe ground. In the next

step of our research, an observer would be able to observe the background

image from various viewpoint with H.M.P. (Head-Mounted Projector).