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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.
Introduction
Invisibility has been on humanity's wish list at least since Amon-Ra, a deity who could
disappear and reappear at will, joined the Egyptian pantheon in 2008 BC. With recent
advances in optics and computing and with the advent of flexible electronics such as a
flexible liquid crystal display, that would allow the background image to be displayed on
the material itself, however, this elusive goal is no longer purely imaginary.
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 the cloth using an external projector.
They can even reflect images when the material is wrinkled. The same year Time
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magazine named it the coolest invention of 2003. It is an interesting application of optical
camouflage and is called the Invisibility Cloak. Through the clever application of some
dirt-cheap technology, the Japanese inventor has brought personal invisibility a step
closer to reality.
Their 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 key development
of the cloak, however, was the development of a new material called retroreflectum.
Professor Tachi says that this material allows you to see a three-dimensional image. The
computer feeds the image into an external projector which projects the image onto a
person wearing a special retro reflective 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. That was only one invention created in this field and
researches are still being carried out in order to implement it using nanotechnology.
Optical Camouflage: An Overview
Optical camouflage is a kind of active camouflage which completely envelopes the
wearer. It displays an image of the scene on the side opposite the viewer on it, so that the
viewer can "see through" the wearer, rendering the wearer invisible. The idea is relatively
straightforward: to create the illusion of invisibility by covering an object with something
that projects the scene directly behind that object. If you project background image onto
the masked object, you can observe the masked object just as if it were virtually
transparent.
Optical camouflage can be applied for a real scene. In the case of a real scene, a
photograph of the scene is taken from the operator’s viewpoint, and this photograph is
projected to exactly the same place as the original. Actually, applying HMP-based optical
camouflage to a real scene requires image-based rendering techniques.
As for camouflage, it means to blend with the surroundings. 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
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the French word 'camoufler' meaning 'to disguise'. The camouflage technique of disguise
is not as common as coloration, but can be found throughout nature as well. Animals may
disguise themselves as something uninteresting in the hopes that their predators will
ignore them or as something dangerous so that predators will avoid them. And so had
humans the desire to disguise themselves just as some animals could do. 19th century
armies tended to use bright colors and bold, impressive designs. These were intended to
daunt the enemy, attract recruits, foster unit cohesion, or allow easier identification of
units in the fog of war. The transfer of camouflage patterns from battle to exclusively
civilian uses is a recent phenomenon. The concept of camouflage - to conceal and distort
shapes - is also a popular artistic tool.
Technology Focus
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. Optics (appearance or look in ancient Greek) is a branch of physics that
describes the behavior and properties of light and the interaction of light with matter.
Optics explains optical phenomena. The pure science aspects of the field are often called
optical science or optical physics.
This technology is currently only in a very primitive stage of development. 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.
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
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realistic illusion possible. This would likely require Phase Array Optics, which would
project light of a specific amplitude and phase and therefore provide even greater levels
of invisibility. We may end up finding optical camouflage to be most useful in the
environment of space, where any given background is generally less complex than
earthly backdrops and therefore easier to record, process, and project.
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. Augmented reality (AR) is a field of computer research which deals
with the combination of real world and computer generated data.
Display of GPS (which is an augmented reality system)
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The above is an example of how it looks like when viewed through the display of
augmented reality system.
At present, most AR research is concerned with the use of live video imagery which is
digitally processed and "augmented" by the addition of computer generated graphics.
Advanced research includes the use of motion tracking data, fiducial marker recognition
using machine vision, and the construction of controlled environments containing any
number of sensors and actuators.
Relation of different environments
The real world and a totally virtual environment are at the two ends of this continuum
with the middle region called Mixed Reality. Augmented reality lies near the real world
end of the line with the predominate perception being the real world augmented by
computer generated data. Augmented virtuality is a term created by Milgram(Milgram
and Kishino 1994; Milgram, Takemura et al. 1994) to identify systems which are mostly
synthetic with some real world imagery added such as texture mapping video onto virtual
objects. This is a distinction that will fade as the technology improves and the virtual
elements in the scene become less distinguishable from the real ones.
Monitor Based Augmented Reality
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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.
In augmented reality, the scene is viewed by an imaging device, which in this case is
depicted as a video camera. The camera performs a perspective projection of the 3D
world onto a 2D image plane. The intrinsic(focal length and lens distortion) and
extrinsic(position and pose)parameters of the device determine exactly what is projected
onto its image plane. The generation of the virtual image is done with a standard
computer graphics system. The virtual objects are modeled in an object reference frame.
The graphics system requires information about the imaging of the real scene so that it
can correctly render these objects. This data will control the synthetic camera that is used
to generate the image of the virtual objects. This image is then merged with the image of
the real scene to form the augmented reality image.
Components of an Augmented Reality System
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Retro Reflectivity
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 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.
A retro reflector is a device that sends light or other radiation back where it came from
regardless of the angle of incidence, unlike a mirror, which does that only if the mirror is
exactly perpendicular to the light beam. Retro reflectors are clearly visible in a pair of
bicycle shoes. Light source is a flash a few centimeters above camera lens.
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Surface Reflectivity (of various kinds of surfaces)
This effect can be commonly obtained in two ways:
1.) With reflecting and refracting optical elements arranged so that the focal surface of
the refractive element coincides with the reflective surface, typically a transparent sphere
and a spherical mirror - this same effect may be achieved with a single transparent sphere
provided that the refractive index of the material is exactly 2 times the refractive index of
the medium from which the radiation is incident. In that case, the sphere surface behaves
as a concave spherical mirror with the required curvature for retro reflection. This is
conventionally known as a cat's eye retro reflector in either configuration.
2.) With a set of three mutually perpendicular mirrors which form a corner (a corner
reflector or corner cube)
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Reflectivity of a retro reflective material
Corner retro reflectors occur in two varieties. In the more common form, the corner is
literally the truncated corner of a cube of transparent material such as conventional
optical glass. In this structure, the reflection is achieved either by total internal reflection
or silvering of the outer cube surfaces. The second form uses mutually perpendicular flat
mirrors bracketing an air space. These two types have similar optical properties.
A retro reflector may consist of many very small versions of these structures incorporated
in a thin sheet or in paint. In the case of paint containing glass beads, the paint glues the
beads to the surface where retro reflection is required, and the beads protrude, their
diameter being about twice the thickness of the paint.
Video Camera and Projector
Video Camera
Professional video camera (often called a Television camera even though the use has
spread) is a high-end device for recording electronic moving images (as opposed to a
movie camera that records the images on film). Originally developed for use in television
studios, they are now commonly used for corporate and educational videos, music videos,
direct-to-video movies, etc. Less advanced video cameras used by consumers are often
referred to as camcorders.
There are two types of professional video cameras: High end portable, recording cameras
(which are, confusingly, called camcorders too) used for ENG image acquisition, and
studio cameras which lack the recording capability of a camcorder, and are often fixed on
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studio pedestals. It is common for professional cameras to split the incoming light into
the three primary colors that humans are able to see, feeding each color into a separate
pickup tube (in older cameras) or charge-coupled device (CCD). Some high-end
consumer cameras also do this, producing a higher-quality image than is normally
possible with just a single video pickup.
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.
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 thecloak) can be located any distance from the projector.
In optics, a diaphragm is a thin opaque structure with an opening (aperture) at its centre.
The role of the diaphragm is to stop the passage of light, except for the light passing
through the aperture. Thus it is also called a stop (an aperture stop, if it limits the
brightness of light reacting the focal plane, or a field stop or flare stop for other uses of
diaphragms in lenses). The diaphragm is placed in the light path of a lens or objective,
and the size of the aperture regulates the amount of light that passes through the lens. The
centre of the diaphragm's aperture coincides with the optical axis of the lens system.
Most modern cameras use a type of adjustable diaphragm known as an iris diaphragm,
and often referred to simply as an iris.
The number of blades in an iris diaphragm has a direct relation with the appearance of the
blurred out-of-focus areas in an image, also called Bokeh. The more blades a diaphragm
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has, the rounder and less polygon-shaped the opening will be. This results in softer and
more gradually blurred out-of-focus areas.
Computer and Combiner
Computer
A computer is a machine for manipulating data according to a list of instructions. 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.
Image-based rendering techniques are used. Actually, applying HMP-based optical
camouflage to a real scene requires image-based rendering techniques.
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 (thesilvered 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.
Sequence
Sequence of events once a person puts on the cloak with the retro-reflective material:
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The person wearing the cloak appears invisible because the background scene is beingdisplayed 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 seems as if an invisible person exists
in an otherwise normal-looking world.
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The complete system of an Invisibility Cloak
Head Mounted Display
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.
A head mounted display
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A head-mounted display (HMD) is a display device that a person wears on the head to
have video information directly displayed in front of the eyes.
Short for head-mounted display, a headset used with virtual reality systems. An HMD
can be a pair of goggles or a full helmet. In front of each eye is a tiny monitor. Because
there are two monitors, images appear as three-dimensional. In addition, most HMDs
include a head tracker so that the system can respond to head movements. For example, if
you move your head left, the images in the monitors will change to make it seem as if
you're actually looking at a different part of the virtual reality.
An HMD has either one or two small CRT, LCD, LCoS (Liquid Crystal on Silicon), or
OLED displays with magnifying lenses embedded in a helmet, glasses or visor. With two
displays, the technology can be used to show stereoscopic images by displaying an offset
image to each eye. Lenses are used to give the perception that the images are coming
from a greater distance, to prevent eye strain. One company, Sensics, makes an HMD
with 24 OLED displays, with the lenses designed to combine 12 displays into a seamless
image for each eye. Head-mounted displays may also be coupled with head-movement
tracking devices to allow the user to "look around" a virtual reality environment naturally
by moving the head without the need for a separate controller. Performing this update
quickly enough to make the experience immersive requires a great amount of computer
image processing. If six axis position sensing (direction and position) is used then thewearer may physically move about and have their movement translated into movement in
the virtual environment.
Some head-mounted or wearable glasses may be also be used to view a see through
image imposed upon a real world view, creating what is called augmented reality. This is
done by reflecting the video images through partially reflective mirrors. The real world
view is seen through the mirrors' reflective surface. Experimental systems have been used
for gaming, where virtual opponents may peek from real windows as a player moves
about. This type of system has applications in the maintenance of complex systems, as it
can give a technician what is effectively "x-ray vision" by combining computer graphics
rendering of hidden elements with the technician's natural vision. Additionally, technical
data and schematic diagrams may be delivered to this same equipment, eliminating the
need to obtain and carry bulky paper documents.
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Military, police and firefighters use HMDs to display relevant tactical information such
as maps or thermal imaging data. Engineers and scientists use HMDs to provide
stereoscopic views of CAD schematics, simulations or remote sensing applications. And
owing to advancements in computer graphics and the continuing miniaturization of video
displays and other equipment, consumer HMD devices are also available for use with 3d
games and entertainment.
Real World Application
While an invisibility cloak is an interesting application of optical camouflage, there are
also some other practical ways the technology might be applied:
1. AUGMENTED STEREOSCOPIC VISION IN SURGERY
It allows the combination of radiographic data (CAT scans and MRI imaging) with the
surgeon's vision. Doctors performing surgery could use optical camouflage to see through
their hands and instruments to the underlying tissue, thereby making the complicated
surgeries a bit better. Surgeons may not need to make large incisions if they wear gloves
that project what's on the inside of a patient using a CAT scan or MRI data.
2. COCKPIT FLOORS
Pilots landing a plane could use this technology to make cockpit floors transparent with
micro reflectors. This would enable them to see the runway and the landing gear simply
by glancing down. Hard landings would be a thing of the past if pilots could gauge how
far they are above the ground just by looking at an image of the outside terrain projected
on the floor. This allows them to avoid many obstacles on the path below and be aware of
the floor below them thereby creating a complete awareness.
3. TRANSPARENT REAR HATCH
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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4. WINDOWLESS ROOMS
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.
5. STEALTH TECHNOLOGY
Stealth means ‘low observable’. The very basic idea of Stealth Technology in the military
is to ‘blend’ in with the background. The applications of stealth technology are mainly
military oriented.
Stealth Technology is used in the construction of mobile military systems such as
aircrafts and ships to significantly reduce their detection by enemy, primarily by an
enemy RADAR. The way most airplane identification works is by constantly bombarding
airspace with a RADAR signal. When a plane flies into the path of the RADAR, a signal
bounces back to a sensor that determines the size and location of the plane. Other
methods focus on measuring acoustic (sound) disturbances, visual contact, and infrared
signatures. The Stealth technology works by reducing or eliminating these telltale signals.
Panels on planes are angled so that radar is scattered, so no signal returns.
The idea is for the radar antenna to send out a burst of radio energy, which is thenreflected back by any object it happens to encounter. The radar antenna measures the
time it takes for the reflection to arrive, and with that information can tell how far away
the object is. The metal body of an airplane is very good at reflecting radar signals, and
this makes it easy to find and track airplanes with radar equipment.
The goal of stealth technology is to make an airplane invisible to radar. There are two
different ways to create invisibility:
• The airplane can be shaped so that any radar signals it reflects are reflected away from
the radar equipment.
• The airplane can be covered in materials that absorb radar signals.
6. MUTUAL TELEXISTENCE
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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. Real-time video of two or more distance separated individuals is projected
onto surrogate robotic participants via sophisticated communication technology.
Telexistence
1. Human user A is at one location while his telexistence robot A is at another location
with human user B.
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2. Human user B is at one location while his telexistence robot B is at another location
with human user A.
3. Both telexistence robots are covered in retroreflective material so that they act like
screens.
4. 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.
5. This gives each human the perception that he is working with another human instead of
a robot.
Drawbacks
• Large amount of external hardware required –
For the invisibility cloak to work properly, we need a number of components such as a
video camera (which sits behind the person wearing the cloak and captures the
background image.), a computer (which takes 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), a projector (which
takes the modified image produced by the computer and shines it onto the garment,
which acts like a movie screen), an iris diaphragm (The projector sends the light through
the iris diaphragm, which is actually a small opening), a combiner (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), and most importantly a retro reflective cloak (which has
special reflecting properties) to cover the object which needs to be made invisible.
• The illusion is only convincing when viewed from a certain angle -
The Invisibility cloak that we have in hand at present appears to be invisible only from
one point of view. But a real invisibility cloak, if it's going to dupe anyone who might see
it, needs to represent the scene behind its wearer accurately from any angle. Moreover,
since any number of people might be looking through it at any given moment, it has to
reproduce the background from all angles at once. That is, it has to project a separate
image of its surroundings for every possible perspective.
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Future Enhancements
There are many technology gaps to bridge to reach true invisibility. Our eyes are only the
raw photo sensors that deliver basic electrochemical signals to our brain, which then
processes these low-level cues into higher cognition notions. Thus, it might be possible to
think of invisibility at the human brain level. This is called cognitive blindness which
could be individually selective compared with real world, physics based absolute
invisibility. We see using the persistence of vision property. Light is first accumulated in
retinal photo sensors (cones and rods) before propagating the impulses into
electrochemical reactions. Thus, we average light, and this causes various scene aliasing
effects. Thus vibration and light averaging might also be a future direction for finding
other invisibility tricks. Adaptive camouflage technology could one day allow soldiers to
take a picture of their surroundings and digitally transfer the image using a handheld
computer to the surface of their clothing.
Adaptive Camouflage
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An insight into the future - a future combat aircraft with both stealth and active camouflage technology.
Conclusion
Invisibility Today...
In Susumu Tachi's cloaking system, a camera behind the wearer feeds background
images through a computer to a projector, which paints them on a jacket as though it
were a movie screen. The wearer appears mysteriously translucent - as long as observers
are facing the projection head-on and the background isn't too bright.
The present system of invisibility
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…And Tomorrow
To achieve true invisibility, optical camouflage must capture the background from all
angles and display it from all perspectives simultaneously. This requires a minimum of
six stereoscopic camera pairs, allowing the computer to model the surroundings and
synthesize the scene from every point of view. To display this imagery, the fabric is
covered with hyper pixels, each consisting of a 180 x 180 LED array behind a
hemispherical lens.
The future of invisibility
The cameras will transmit images to a dense array of display elements, each capable of
aiming thousands of light beams on their own individual trajectories. These elements
project a virtual scene derived from the cameras' views, making it possible to synthesize
various perspectives.
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