Report on Invisibility

8/8/2019 Report on Invisibility

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10/17/2008

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Submitted by | Dheeraj Raisinghani & Supriya Jala


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Index

ABSTRACT 3

INVISIBILITY 4

ACTIVE CAMOUFLAGE 7

COMPUTER GENERATED HOLOGRAPHY 8

PHASED RAY OPTICS 11

META-MATERIAL 13

THE CLOAK 20

STEALTH TECHNOLOGY 26

LIMITATIONS OF INVISIBILITY 31

BIBLIOGRAPHY 32

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REPORTON

INVISIBILITY FROM FICTIONTO REALITY


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ABSTRACT

Invisibility is the state of an object which cannot be seen. Term is usually used as

a fantasy/science fiction term, where objects are literally made invisible by magical or

technological means; however, its effects can also be seen in the real world, particularly

in physics and perceptional psychology. Visibility also depends on the eyes of the observer

and/or the instruments used. Thus an object can be classified as "invisible to" a person, animal,

instrument, etc

An object can be considered invisible if its so massive that its escape velocity exceeds the speed

of light, Emitting or reflecting light outside the wavelength range of visible light. A recent

breakthrough (2006) has shown that invisibility is possible by using specifically patterned

crystals made up of nano-scale boxes that hold electrons. Theoretically, it is possible to make an

object invisible, if the object has the same refractive index as the surrounding medium.

Making use of real-time image displayed on a wearable display, scientists are able to create a

see-through effect, if not invisibility. This is known as active camouflage. Active camouflage is

a technology which allows an object to blend into its surroundings by use of panels capable of

altering their appearance, color, luminance and reflective properties. Active camouflage has the

capacity to provide perfect concealment from visual detection.

A meta-material (or meta-material) is a material which gains its properties from its structure

rather than directly from its composition. To distinguish meta-materials from

other composite materials, the meta-materiallabel is usually used for a material which has

unusual properties

Thus this paper will be discussing on how using Technologies like active camouflage, time

reversal, negative refractive index and computer holography and high level stealth

technologies, and object can be made invisible to the observer.

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Invisibility from fiction to

reality

Invisibility

What Is Invisibility?

Invisibility is the state of an object which cannot be seen. An object in this state is said to

be invisible (literally, "not visible"). The term is usually used as a fantasy/science fiction term,

where objects are literally made invisible by magical or technological means; however, its effects

can also be seen in the real world, particularly in physics and perceptional psychology.

Since objects can be seen by light in the visible spectrum from a source reflecting off their

surfaces and hitting the viewer's eye, the most natural form of invisibility (whether real or

fictional) is an object which neither reflects nor absorbs light (that is, it allows light to pass

through it). In nature, this is known as transparency, and is seen in many naturally occurring

materials (although no naturally occurring material is 100% transparent).

Visibility also depends on the eyes of the observer and/or the instruments used. Thus an object

can be classified as "invisible to" a person, animal, instrument, etc. In the research of

sensorial perception invisibility has been shown to happen in cycles.

Ways to Invisibility

By environment

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An object may be classified as "invisible" if it cannot be noticed by use of sight due

to environmental factors other than the fact that it doesn't reflect light. An object that might

normally be seeable may be classified as invisible if it is:

Behind an object.

The same color or pattern as the background (camouflage)

In an environment which is too dark or too bright.

In a particular observer's blind spot.

utilizing video/image capture (background capture), dynamic modification of background

image data transmitted to object attached display causes invisibility to human sight withinthe human sight light/photonic frequency range.

By physics

Theoretical and practical physics offer several causes of invisibility. An object may be invisible

if it is:

So massive that its escape velocity exceeds the speed of light (such objects are

called black holes)

Transparent (such as air and many other gases)

Emitting or reflecting light outside the wavelength range of visible light. (Radiation is

generally invisible by this means.) Unfortunately, this would result in any obscured

human being becoming not invisible and transparent, but completely opaque and

resembling a human-shaped black hole.

A recent breakthrough (2006) at Imperial College London has shown that invisibility is

possible by using specifically patterned crystals made up of nano-scale boxes that hold

electrons. When light hits these crystals, it becomes entangled within the boxes, causing

the object to become transparent.

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Theoretically, it is possible to make an object invisible, if the object has the

same refractive index as the surrounding medium (e.g. air.) (This is the mechanic used in

HG Well's The Invisible Man.)

By technology

Technology can be used theoretically or practically to render real-world objects invisible:

Making use of real-time image displayed on a wearable display, scientists are able to

create a see-through effect, if not invisibility. This is known as active camouflage.

Though stealth technology is cited as invisibility to radar, all officially disclosed

applications of the technology can only reduce the size and/or clarity of the signature

detected by radar.

In some science fiction stories, a hypothetical "cloaking device" is used to make objects

invisible. On Thursday October 19, 2006 a team effort of researchers from Britain and the

U.S announced the development of a real cloak of invisibility, though it is only in its first

stages.

In filmmaking, people, objects, or backgrounds can be made to look invisible on camera

through a process known as Chroma keying.

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Active camouflage

Active camouflage oradaptive camouflage is a group of camouflage technologies which allow

an object to blend into its surroundings by use of panels or coatings capable of altering their

appearance, color, luminance and reflective properties. Active camouflage has the capacity to

provide perfect concealment from visual detection.

Active camouflage differs from conventional means of concealment in two important ways:

firstly, it makes the camouflaged object appear not merely similar to its surroundings, but

effectively invisible through the use of mimicry; secondly, active camouflage changes the

appearance of the object as changes occur in the background. Ideally, active camouflage mimics

nearby objects as well as objects as distant as the horizon.

Active camouflage has its origins in the diffused lighting camouflage first tested on Canadian

Navy corvettes during World War II, and later in the armed forces of the United Kingdom and

the United States of America.

Active camouflage is poised to develop at a rapid pace with the development of organic light-

emitting diodes (OLEDs) and other technologies which allow for images to be projected onto

irregularly-shaped surfaces. With the addition of a camera, an object may not be made

completely invisible, but may in theory mimic enough of its surrounding background to avoid

detection by the human eye as well as optical sensors. As motion may still be noticeable, an

object might not be rendered undetectable under this circumstance but potentially more difficult

to hit. This has been demonstrated with videos of "wearable" displays where the camera could

see "through" the wearer.

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Optical Camouflage

In 2003, three professors at University of Tokyo Susumu Tachi, Masahiko Inami and Naoki

Kawakami created a prototypical camouflage system in which a video camera takes a shot of

the background and displays it on a cloth using an external projector.

Phased array optics (PAO) provides a more ideal implementation of optical camouflage. Instead

of producing a two dimensional image of background scenery on an object, PAO would

use computational holography to produce a three dimensional hologram of background scenery

on an object to be concealed. Unlike a two dimensional image, the holographic image would

appear to be the actual scenery behind the object independent of viewer distance or view angle.

Active camouflage is not a human invention. The most convincing example of active camouflage

in animals is the octopus, which can blend into its surroundings by changing skin color as well as

skin shape and texture. The cuttlefish, another cephalopod like the octopus, is also known for its

color changing capabilities. Cuttlefish can produce more colors than most octopi can.

The chameleon can also change its color to blend with its surroundings. However, a chameleon

more routinely changes color based on body temperature and how stressed it is. The ability is

also used to communicate with other chameleons. Color change is also communicative in

octopuses and cuttlefish.

Computer Generated Holography

Computer Generated Holography (CGH) is the method of digitally

generating holographic interference patterns. A holographic image can be generated e.g. by

digitally computing a holographic interference pattern and printing it onto a mask or film for

subsequent illumination by suitable coherent light source. Alternatively, the holographic image

can be brought to life by a holographic 3D display (a display which operates on the basis of

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interference of coherent light), bypassing the need of having to fabricate a "hardcopy" of the

holographic interference pattern each time.

Computer generated holograms have the advantage that the objects which one wants to show do

not have to possess any physical reality at all (completely synthetic hologram generation). On the

other hand, if holographic data of existing objects is generated optically, but digitally recorded

and processed, and brought to display subsequently, this is termed CGH as well. Ultimately,

computer generated holography might serve all the roles of current computer generated imagery:

holographic computer displays for a wide range of applications from CAD to gaming,

holographic video and TV programs, automotive and communication applications (cell phone

displays) and many more.

Holography is a technique originally invented by Hungarian physicist Dennis Gabor (1900-1979)

to improve the resolving power on electron microscopes. An object is illuminated with a

coherent (usually monochromatic) light beam; the scattered light is brought to interference with a

reference beam of the same source, recording the interference pattern. CGH as defined in the

introduction has broadly three tasks:

1. Computation of the virtual scattered wave-front

2. Encoding the wave-front data, preparing it for display

3. Reconstruction: Modulating the interference pattern onto a coherent light beam by

technological means, to transport it to the user observing the hologram.

Wave-front computation

Wave-front calculations are computationally very intensive; even with modern mathematical

techniques and high-end computing equipment, real-time computation is tricky. There are many

different methods for calculating the interference pattern for a CGH.

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Ray tracing method

Ray tracing is perhaps the simplest method of computer generated holography to visualize.

Essentially, the path length difference between the distance a virtual "reference beam" and a

virtual "object beam" have to travel is calculated; this will give the relative phase of the scattered

object beam

Fourier transforms method

In a Fourier Transform hologram the reconstruction of the image occurs in the far field. This isusually achieved by using the Fourier transforming properties of a positive lens for

reconstruction. So there are two steps in this process: computing the light field in the far observer

plane, and then Fourier transforming this field back to the lens plane. Instead of the Fourier

transform, one might also utilize the Fresnel transform to obtain near field holograms.

Interference pattern encoding

Once it is known how the scattered wave front of the object looks like or how it may be

computed, it must be fixed on a spatial light modulator (SLM), abusing this term to include not

only LCD displays or similar devices, but also films and masks. Basically, there are different

types of SLMs available: Pure phase modulators (retarding the illuminating wave), pure

amplitude modulators (blocking the illumination light), and SLMs which have the capability of

combined phase/amplitude modulation.

In the case of pure phase or amplitude modulation, clearly quality losses are unavoidable. Early

forms of pure amplitude holograms were simply printed in black and white, meaning that the

amplitude had to be encoded with one bit of depth only. Similarly, the kinoform is a pure-phase

encoding invented at IBM in the early days of CGH. Even if a fully complex phase/amplitude

modulation would be ideal, a pure phase or pure amplitude solution is normally preferred

because it is much easier to implement technologically.

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Reconstruction

The third (technical) issue is beam modulation and actual wave front reconstruction. Masks may

be printed, resulting often in a grained pattern structure since most printers can make only dots

(although very small ones). Films may be developed by laser exposure. Holographic displays are

currently yet a challenge (as of March 2008), although successful prototypes have been built. An

ideal display for computer generated holograms would consist of pixels smaller than a

wavelength of light with adjustable phase and brightness. Such displays have been called phased

array optics. Further progress in nanotechnology is required to build them.

Available CGH devices

Currently, several companies and university departments are researching on the field of CGH

devices:

MIT Media Lab has developed the "Holovideo" CGH display

SeeReal Technologies have prototyped a CGH display

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Phased array optics

Phased array optics (PAO) is the technology of controlling the phase of light waves

transmitting or reflecting from a two-dimensional surface by means of adjustable surface

elements. It is the optical analog of phased array radar. By dynamically controlling the

optical properties of a surface on a microscopic scale, it is possible to steer the direction

of light beams, or the view direction of sensors, without any moving parts. Hardware

associated with beam steering applications is commonly called an optical phased array

(OPA). Phased array beam steering is used for optical switching and multiplexing

in optoelectronic devices, and for aiming laser beams on a macroscopic scale.

Complicated patterns of phase variation can be used to produce diffractive optical

elements, such as dynamic virtual lenses, for beam focusing or splitting in addition to

aiming. Dynamic phase variation can also produce real-time holograms. Devices

permitting detailed addressable phase control over two dimensions are a type of spatial

light modulator (SLM).

In nanotechnology, phased array optics refers to arrays of lasers or SLMs with

addressable phase and amplitude elements smaller than a wavelength of light. While still

theoretical, such high resolution arrays would permit extremely realistic three

dimensional image display by dynamic holography with no unwanted orders ofdiffraction. Applications for weapons, space communications, and invisibility by optical

camouflage have also been suggested.

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Meta-material

A meta-material (orMeta material) is a material which gains its properties from its structure

rather than directly from its composition. To distinguish meta-materials from

other composite materials, the meta-materiallabel is usually used for a material which has

unusual properties. The term was coined in 1999 by Rodger M. Walser of the University of

Texas at Austin. He defined meta-materials as:

Macroscopic composites having a manmade, three-dimensional, periodic cellular architecturedesigned to produce an optimized combination, not available in nature, of two or more

responses to specific excitation.

Electromagnetic researchers often use the term, quite narrowly, for materials which exhibit

negative refraction. W.E. Kock developed the first meta-materials in the late 1940s with metal-

lens antennas and metallic delay lenses.

Electromagnetic meta-materials

Meta-materials are of particular importance

in electromagnetism (especially optics and photonics). They show promise for a variety of

optical and microwave applications such as new types of beam steerers, modulators, band-pass

filters, lenses, microwave couplers, and antenna radomes.

In order for its structure to affect electromagnetic waves, a meta-material must have structural

features smaller than the wavelength of the electromagnetic radiation it interacts with. For

instance, if a meta-material is to behave as a homogeneous material accurately described by aneffective refractive index, the feature sizes must be much smaller than the wavelength.

For visible light, which has wavelengths of less than one micrometer typically

(560 nanometers for sunlight), the structures are generally half or less than half this size; i.e., less

than 280 nanometers. For microwave radiation, the structures need only be on the order of

one decimeter. Microwave frequency meta-materials are almost always artificial, constructed as

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arrays of current-conducting elements (such as loops of wire) which have suitable

inductive and capacitive characteristics.

Meta-materials usually consist of periodic structures, and thus have many similarities

with photonic crystals and frequency selective surfaces. However, these are usually considered to

be distinct from meta-materials, as their features are of similar size to the wavelength at which

they function, and thus cannot be approximated as a homogeneous material

Negative refractive index

(A comparison of refraction in a left-handed meta-material to that in a normal material)

The main reason researchers have investigated meta-materials is the possibility to create a

structure with a negative refractive index, since this property is not found in any naturally

occurring material. Almost all materials encountered in optics, such as glass or water, have positive values for both permittivity and permeability . However, many metals (such

as silver and gold) have negative at visible wavelengths. A material having either (but not

both) or negative is opaque to electromagnetic radiation.

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http://en.wikipedia.org/wiki/Image:Metarefraction.svghttp://en.wikipedia.org/wiki/Image:Metarefraction.svg


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Although the optical properties of a transparent material are fully specified by the

parameters and , in practice the refractive indexNis often used.Nmay be determined from

. All known transparent materials possess positive values for and . By

convention the positive square root is used forN.

However, some engineered meta-materials have < 0 and < 0; because the product () is

positive,Nis real. Under such circumstances, it is necessary to take the negative square root

forN. Physicist Victor Veselago proved that such substances can transmit light.

The foregoing considerations are simplistic for actual materials, which must have complex-

valued and . The real parts of both and do not have to be negative for a passive material to

display negative refraction.

Meta-materials with negativeNhave numerous startling properties:

Snell's law (N1sin1 =N2sin2) still applies, but asN2 is negative, the rays will be

refracted on thesame side of the normal on entering the material.

The Doppler shift is reversed: that is, a light source moving toward an observer appears

to reduce its frequency.

Cherenkov radiation points the other way.

The time-averaged Poynting vector is anti parallel to phase velocity. This means that

unlike a normal right-handed material, the wave fronts are moving in the opposite

direction to the flow of energy.

For plane waves propagating in such meta-materials, the electric field, magnetic field and wave

vector follow a left-hand rule, thus giving rise to the name left-handed (meta)materials. It should

be noted that the terms left-handed and right-handed can also arise in the study of chiral media,

but their use in that context is unrelated to this effect. Some researchers consider the qualifier

left-handed for achiral materials as particularly infelicitous.

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pole would instead appear to jut out from the water's surface. Or, to give another example, a fish

swimming underwater would instead appear to be moving in the air above the water's surface.

The meta-material described in the Science paper takes another approach to the goal of bending

light backwards. It is composed of silver nano-wires grown inside porous aluminum oxide.

Although the structure is about 10 times thinner than a piece of paper - a wayward sneeze could

blow it away - it is considered a bulk meta-material because it is more than 10 times the size of a

wavelength of light.

The authors of the Science paper observed negative

refraction from red light wavelengths as short as 660

nanometers. It is the first demonstration of bulk media

bending visible light backwards.

The innovation of these nano-wire material,

researchers said is that it finds a new way to bend

light backwards without technically achieving a

negative index of refraction. For there to be a negative

index of refraction in a meta-material, its values for

permittivity - the ability to transmit an electric field -

and permeability - how it responds to a magnetic field - must both be negative.

The benefits of having a true negative index of refraction, such as the one achieved by the fishnet

meta-material in the Nature paper, is that it can dramatically improve the performance of

antennas by reducing interference. Negative index materials are also able to reverse the Doppler

Effect - the phenomenon used in police radar guns to monitor the speed of passing vehicles - so

that the frequency of waves decreases instead of increases upon approach.

But for most of the applications touted for meta-materials, such as nano-scale optical imaging or

cloaking devices, both the nano-wire and fishnet meta-materials can potentially play a key role,

the researchers said.

Super lens

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The first super lens with a negative refractive index provided resolution three times better than

the diffraction limit and was demonstrated at microwave frequencies at the University of

Toronto by A. Grbic and G.V. Eleftheriades. Subsequently, the first optical super lens (an optical

lens which exceeds the diffraction limit) was created and demonstrated in 2005 by Xiang

Zhang et al. of UC Berkeley, as reported that year in the April 22 issue of the journal Science,

but their lens didn't rely on negative refraction. Instead, they used a thin silver film to enhance

the evanescent modes through surface Plasmon coupling. This idea was first suggested by John

Pendry inPhysical Review Letters.

Cloaking devices

Meta-materials have been proposed as a mechanism for building a cloaking device. These

mechanisms typically involve surrounding the object to be cloaked with a shell which affects the

passage of light near it. On February 14, 2005, Andrea Al and Nader Engheta at the University

of Pennsylvania announced in a research paper that Plasmon could be used to cancel out visible

light or radiation coming from an object. This 'plasmonic cover' would work by suppressing light

scattering by resonating with illuminated light, which could render objects "nearly invisible to an

observer." The plasmonic screen would have to be tuned to the object being hidden, and would

only suppress a specific wavelength: An object made invisible in red light would still be visible

in multi-wavelength daylight.

A concept for a cloaking device was put forward by two mathematicians in one of

the UKs Royal Society journals. Shortly afterwards, blueprints for building a cloaking device

were put forward in the journal Science by researchers in the US and UK. However, "Scientists

not involved in the work said the plans appear feasible but that they would require more-

advanced substances than currently exist".

In October 2006, a US-British team of scientists created a meta-material which made an object

invisible to microwave radiation. Since light is just another form of electromagnetic radiation,

this was considered the first step towards a cloaking device for visible light, though more

advanced nano-engineering techniques would be needed due to visible light's short wavelengths.

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On April 2, 2007, two Purdue University engineers announced a theoretical design for an optical

cloaking device based on the 2006 British concept. The design deploys an array of tiny needles

projecting from a central spoke that would render an object within the cloak invisible in a

wavelength of 632.8 nanometers.

Duke University and Imperial College London are currently researching this use of meta-

materials and have managed to cloak an object in the microwave spectrum using special

concentric rings; the microwaves were barely affected by the presence of the cloaked object.In

early 2007, a meta-material with a negative index of refraction for visible light wavelengths was

announced by a joint team of researchers at the Ames Laboratory of the United States

Department of Energy and at Karlsruhe University in Germany. The material had an index of

-0.6 at 780 nanometers.

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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.

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 spectacular 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

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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

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 amoviescreen 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.

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/softwarecombo 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

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Photo courtesy Tachi

Laboratory, the University of

Tokyo
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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 CombinerThe 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.

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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.

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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 eyes, making it seem as if an invisible person exists in an otherwise

normal-looking world.

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.

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.

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Mutual telexistence

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.

Right now, mutual telexistence is science fiction, but it won't be for long as scientists continue to

push the boundaries of the technology

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Stealth technology

Stealth technology is also known as LOT (Low Observability Technology). The concept of

stealth is not new: being able to operate without the knowledge of the enemy has always been a

goal of military technology and techniques. However, as the potency of detection and

interception technologies (radar, IRST, surface-to-air missiles etc.) has increased, so too has the

extent to which the design and operation of military vehicles have been affected in response.

Stealth principles

Stealth technology (often referred to as "LO", for "low observability") is not a single technology

but is a combination of technologies that attempt to greatly reduce the distances at which a

vehicle can be detected; in particular radar cross section reductions, but

also acoustic, thermal and other aspects specifically:

Radarcross-section (RCS) reductions

Almost since the invention of radar, various techniques have been tried to minimize detection.

The term 'Stealth' in reference to reduced radar signature aircraft became popular during the late

eighties when the F-117 stealth fighter became widely known. Many countries nevertheless

continue to develop low-RCS vehicles because low RCS still offers advantages in detection

range reduction as well as increasing the effectiveness of decoys against radar-seeking threats.

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Vehicleshape

(Certain shapes offer better stealth)

The possibility of designing aircraft in such a manner as to reduce their radar cross-section was

recognized in the late 1930s, when the first radar tracking systems were employed, and it has

been known since at least the 1960s that aircraft shape makes a very significant difference in

how well an aircraft can be detected by a radar. Another important factor is the internal

construction. Behind the skin of some aircraft are structures known as re-entrant triangles. Radar

waves penetrating the skin of the aircraft get trapped in these structures, bouncing off the internal

faces and losing energy. This approach was first used on SR-71.

The most efficient way to reflect radar waves back to the transmitting radar is with orthogonal

metal plates, forming a corner reflector consisting of either a dihedral (two plates) or a trihedral

(three orthogonal plates). This configuration occurs in the tail of a conventional aircraft, where

the vertical and horizontal components of the tail are set at right angles. Stealth aircraft such as

the F-117 use a different arrangement, tilting the tail surfaces to reduce corner reflections formed

between them. The most radical approach is to eliminate the tail completely, as in the B-2 Spirit.

In addition to altering the tail, stealth design must bury the engines within the wing or fuselage,

or in some cases where stealth is applied to an existing aircraft, install baffles in the air intakes,

so that the turbine blades are not visible to radar. A stealthy shape must be devoid of complex

bumps or protrusions of any kind; meaning that weapons, fuel tanks, and other stores must not be

carried externally. Any stealthy vehicle becomes un-stealthy when a door or hatch is opened.

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Non-metallicairframe

Dielectric composites are relatively transparent to radar, whereas electrically conductive

materials such as metals and carbon fibers reflect electromagnetic energy incident on the

material's surface. Composites used may contain ferrites to optimize the dielectric and magnetic

properties of the material for its application.

Radarabsorbingmaterial

Radar absorbent material (RAM), often as paints, are used especially on the edges of metal

surfaces. One such coating, also called iron ball paint, contains tiny spheres coated with

carbonyl iron ferrite. Radar waves induce alternating magnetic field in this material, which leads

to conversion of their energy into heat. Early versions of F-117A planes were covered

with neoprene-like tiles with ferrite grains embedded in the polymer matrix, current models have

RAM paint applied directly. The paint must be applied by robots because of problems of solvent

toxicity and tight tolerances on layer thickness.

Similarly, coating the cockpit canopy with a thin film transparent conductor (vapor-

deposited gold or indium tin oxide) helps to reduce the aircraft's radar profile because radar

waves would normally enter the cockpit, bounce off something random (the inside of the cockpit

has a complex shape), and possibly return to the radar, but the conductive coating creates a

controlled shape that deflects the incoming radar waves away from the radar. The coating is thin

enough that it has no adverse effect on the pilot's vision.

Radarstealthcountermeasuresandlimitations

Lowfrequencyradar

Shaping does not offer stealth advantages against low-frequency radar. If the radar wavelength is

roughly twice the size of the target, a half-wave resonance effect can still generate a significant

return. However, low-frequency radar is limited by lack of available frequencies which are

heavily used by other systems, lack of accuracy given the long wavelength, and by the radar's

size, making it difficult to transport. A long-wave radar may detect a target and roughly locate it,

but not identify it, and the location information lacks sufficient weapon targeting accuracy. Noise

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poses another problem, but that can be efficiently addressed using modern computer technology;

Chinese "Nantsin" radar and many older Soviet-made long-range types of radar were modified

this way. It has been said that "there's nothing invisible in the radar frequency range below 2

GHz".

Multipletransmitters

Much of the stealth comes from reflecting the transmissions in a different direction other than a

direct return. Therefore detection can be better achieved if the sources are spaced from the

receivers, known as bistatic radar , and proposals exist to use reflections from sources such as

civilian radio transmitters, including cellular telephone radio towers.

Acoustics

Acoustic stealth plays a primary role in submarine stealth as well as for ground vehicles.

Submarines have extensive usage of rubber mountings to isolate and avoid mechanical noises

that could reveal locations to underwater passive sonar arrays.

Early stealth observation aircraft used slow-turning propellers to avoid being heard by enemy

troops below. Stealth aircraft that stay subsonic can avoid being tracked by sonic boom. The

presence of supersonic and jet-powered stealth aircraft such as the SR-71 Blackbird indicates

that acoustic signature is not always a major driver in aircraft design, although the Blackbird

relied more on its extremely high speed and altitude.

Visibility

Most stealth aircraft use matte paint and dark colors, and operate only at night. Lately, interest on

daylight Stealth (especially by the USAF) has emphasized the use of gray paint in disruptive

schemes, and it is assumed that Yehudi lights could be used in the future to mask shadows in

the airframe (in daylight, against the clear background of the sky, dark tones are easier to detect

than light ones) or as a sort of active camouflage. The B-2 has wing tanks for a contrail-

inhibiting chemical, alleged by some to be chlorofluorosulphonic acid, and mission planning also

considers altitudes where the probability of their formation is minimized.

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Infrared

An exhaust plume contributes a significant infrared (IR) signature. One means of reducing the IR

signature is to have a non-circular tail pipe (a slit shape) in order to minimize the exhaust cross-

sectional volume and maximize the mixing of the hot exhaust with cool ambient air. Often, cool

air is deliberately injected into the exhaust flow to boost this process. Sometimes, the jet exhaust

is vented above the wing surface in order to shield it from observers below, as in the B-2 Spirit,

and the unstealthly A-10 Thunderbolt II. To achieve infrared stealth, the exhaust gas is cooled to

the temperatures where the brightest wavelengths it radiates on are absorbed by atmospheric

carbon dioxide and water vapor, dramatically reducing the infrared visibility of the exhaust

plume.

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Limitations of Invisibility

Invisiblecolorparadox

Since something that is invisible has no color associated with it, it is somewhat paradoxical to

imagine an object that is both invisible and colored. This idea is most famous in

the parody goddess, the Invisible Pink Unicorn.

Sightwhileinvisible

According to the laws of physics, a perfectly invisible person would necessarily be blind, no

matter how their invisibility was achieved. In order to see light, it must be absorbed by the retina,

but in order for a person to be invisible, the body must not absorb light. So to retain sight at least

pupil sized holes in the cloak would be necessary in front of the pupils and directly behind them

on the back of the person as light isn't being transmitted through. In fact, according to the no

cloning theorem of quantum mechanics, they could not even make a copy of the photons so they

could see one copy and allow the other copy to pass through or around them. This idea was first

discussed by Mat Ryer a computer software engineer based in London.

This physical barrier appears to offset the advantage of any perfect invisibility method, unless

one's intent was simply to hide and be still, letting the danger pass. On the other hand, a practical

invisibility method need not allow light of all frequencies to pass all the time, so there may be

ways around this limitation. For example, if the wearer of a perfect invisibility device had

goggles that allowed him or she to perceive infrared light while the invisibility device only

diverted visible light, the wearer would be effectively invisible to the human eye while still being

able to see heat sources.

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Bibliography

1. Invisibility cloak a step closer as scientists bend light 'the wrong way',

dailymail.co.uk, 11th August 2008.

2. themoneytimes.com,Scientists Turn Fiction Into Reality, Closer to Make Objects

"Invisible"

3. mirror.co.uk, Secrets of invisibility discovered

4. Wikipedia.com

5. www.Latestechnologies.com

6. Knott, Eugene; Shaeffer, John, and Tuley, Michael (1993).Radar Cross Section, 2nd

ed. Artech House, Inc., 231. ISBN 0-89006-618-3.

7. Sequential Monte Carlo Methods in Practice, by A Doucette, N de Freitas and N

Gordon. Published by Springer.

8. Countering stealth

9. How "stealth" is achieved on F-117A

10.Ufimtsev, Pyotr Ya., "Method of edge waves in the physical theory of diffraction,"

Moscow, Russia:Izd-vo. Sov. Radio [Soviet Radio Publishing], 1962, pages 1-243.

Documents

Report on Invisibility