Thursday, October 19, 2006 | 12:09 PM ET
CBC News
http://www.cbc.ca/technology/story/2006/10/19/invisibility-cloak.html

A team of British and U.S. scientists has demonstrated the first working "invisibility cloak," although don’t expect it to appear in the Halloween costumes aisle just yet.

The team, led by Professor Sir John Pendry of Imperial College in London, built the prototype at Duke University in North Carolina and reported its findings Thursday in Science Express, the advance online publication of the journal Science.

Little more than 12 centimetres across, the small device can redirect microwave beams so they flow around a "hidden" object inside with little distortion, making it appear almost as if nothing were there at all.

Like light, microwaves bounce off objects, making them visible and creating a "shadow," although it has to be detected with instruments.

The new work could be a baby step to an improved version that would make the Klingons and Harry Potter jealous by hiding people and objects from visible light.

Like 'water flowing around a smooth rock'

In the experiment, the scientists used microwaves to try to detect a copper cylinder "hidden" by the cloak, which is made from metamaterials — or engineered mixtures of metal and circuit board materials, which could include ceramic, Teflon or fibre composite materials.

"The waves' movement is similar to river water flowing around a smooth rock,” said cloak designer David Schurig, a research associate in Duke's electrical and computer engineering department.

The test came five months after the team published a theory that such a device was possible to design.

"By incorporating complex material properties, our cloak allows a concealed volume, plus the cloak, to appear to have properties similar to free space when viewed externally," said David Smith, a professor of electrical and computer engineering at Duke, in a release Thursday.

"The cloak reduces both an object's reflection and its shadow, either of which would enable its detection."

Wireless, radar applications

Cloaking differs from stealth technology, which doesn't make an aircraft invisible but reduces the cross-section available to radar, making it hard to track. Cloaking simply passes the radar or other waves around the object as if it weren't there.

Cloaks that render objects essentially invisible to microwaves could have a variety of wireless communications or radar applications, the researchers said.

The scientists said their cloak represents the most comprehensive approach to invisibility yet realized, with the potential to hide objects of any size or material property.

Earlier scientific approaches to achieving "invisibility" often relied on limiting the reflection of electromagnetic waves, they added. 

Cloaking system using optoelectronically controlled camouflage
US Patent
Publication number: US5307162
Publication date: 1994-04-26
Inventor: SCHOWENGERDT RICHARD N (US)
Applicant: SCHOWENGERDT RICHARD N (US)

Lockheed Martin's F-117A Nighthawk stealth fighter was the star of the Gulf War, flying behind enemy lines to hit Baghdad targets with pinpoint accuracy. At night, the F-117 was unstoppable. But by day, the black jet stayed on the ground.

The F-117 isn't fast enough to outrun missiles or agile enough to dodge them, and it can't fight back because it's armed only with bombs. These limitations don't matter at night, because the F-117's stealthy shape enables the aircraft to avoid detection by enemy radar. But in the light of day, the enemy can see the black plane against the sky, and can take aim without the help of radar.

F-117 pilots train almost exclusively for night missions, and the darker it gets the happier they are. But this is a compromise at best. In the summer, a night-only fighter can fly only one sortie per day. And the darkness that hides the F-117 also hides its targets.

Air Force generals would love nothing more than to have a stealth aircraft that would be invulnerable during daylight hours, as well as at night. And as Popular Science has learned, military engineers are already hard at work on the technologies needed to build such a plane. Special lights, coatings, and other technologies under investigation could not only make future fighters disappear from radar screens, but could also make them almost completely invisible to the human eye. By the early 2000s, stealth may be practical in broad daylight.

Today's experiments exploit a principle that was demonstrated half a century ago, in a secret project codenamed Yehudi . In that project, engineers mounted lights on an antisubmarine aircraft to make it harder to spot against a bright sky. Similar technology was used in the Vietnam War to shorten the distance at which the F-4 Phantom could be detected.

Lighting systems were available when Lockheed's Skunk Works was awarded the contract to build Have Blue, the world's first stealth aircraft, in 1976. The breakthrough that made Have Blue possible was the ability to reduce an airplane's radar reflectivity to less than one-hundredth of what was considered normal in the 1960s, slashing the effective range of enemy radar. Reducing the radar reflectivity so dramatically meant that the designers of Have Blue also had to reduce the visual and infrared signatures of the plane, according to a rule of thumb known as "balanced observables."

This rule says that a stealth aircraft should be designed so that every detection system arrayed against it has roughly the same range. There is no point in building an airplane that is invisible to radar at five miles if optical sensors can see it at ten miles.

Have Blue was the prototype for an aircraft that would make its attack run at a moderate altitude of 10,000 to 15,000 feet-close enough to designate the target accurately, but high enough to elude medium-caliber gunfire. At the time, the designers' goal was an aircraft that would be as stealthy in daylight as at night.

The designers realized that visual detection depends on a number of factors, including the position of the observer, his angle of view, the position of the sun, and the presence of haze or clouds. Altitude is extremely important. A jetliner at its cruising height always appears brightly lit in the sky, because dust and moisture in the air beneath the aircraft scatter light onto its underside. There are relatively few particles of dust and water in the thin air above the airplane. So the higher the plane flies, the more light is scattered onto it, and the darker the sky behind it.

A dark color that absorbs as much light as possible provides the best camouflage for a high-flying airplane. But even the jet-black Blackbird and U-2 spyplanes look brighter than the sky when seen from below as they cruise at 80,000 feet. At lower altitudes, there is less light-scattering atmosphere below the aircraft, so lighter colors provide the least contrast.

For Have Blue, Lockheed devised a scheme of graduated grays, lighter on the bottom and darker on top. The aircraft's designers also planned to test light apertures, which would be installed on the sides and undersurfaces of the airplane, about two feet apart. (Seen from a distance, the individual lights would blur into a single image.) The apertures would be connected to a central light source by fiber optic lines, and their output would be controlled by light sensors on the upper side of the aircraft. The sensors would "read" the background light and adjust the skin's luminance to mirror it.

This system never flew on Have Blue, possibly because the first aircraft was lost in an accident. Work on visual stealth continued, however.

In 1980, the Air Force tested a small aircraft, probably unmanned, under a project known as IMCRS (what the acronym stands for is not known). The aircraft's lower wing skins incorporated slit-like Fresnel lenses to beam light ahead of and below the aircraft, in the direction of the most likely threats.

The IMCRS experiment may have been related to a Defense Advanced Research Project Agency program known as Active Camouflage. Under that program, a small powered drone was fitted with fluorescent lamps and tested at the White Sands Missile Range with so much success that the project has since been reclassified as Top Secret.

Neither of these lighting systems were adopted for stealth aircraft in the 1980s. They were complex to install and design. Their effects were hard to predict and difficult to test. Carefully designed conventional camouflage worked well enough under most circumstances to ensure that an aircraft would not be visible before a radar could detect it.

So why were the first F-117s painted soot black instead of a toned gray scheme that would provide better camouflage? One Lockheed engineer recalls that the commander of Tactical Air Command "didn't believe that real fighter pilots flew pastel-colored airplanes." An Air Force source close to the program says that some senior officers doubted the F-117 could survive in daylight, and wanted to ensure that nobody would try it.

Black is one of the least stealthy colors for daytime flying at medium altitudes. In fact, the British Royal Air Force is painting its trainers black to make them more visible and reduce the risk of collisions. Black isn't much good at night either, because there is nearly always some light from the moon. That's why the latest F-117s have been seen in a more sensible gray color.

The B-2 stealth bomber's underside is a very dark gray. Many people think that it is designed to attack only at night, like the F-117. This is unlikely, because the B-2 was designed to bomb Russia, and the most direct route from the center of the United States to central Russia lies smack across the Arctic Circle, where the sun shines 24 hours a day for a large part of the year.

The B-2's underside is dark because it cruises at altitudes as high as 50,000 feet, where a dark gray blends into the sky. It does not use an "active camouflage" lighting system, but it may have an upward-facing light sensor that tells the pilot when to increase or reduce altitude slightly to match the changing luminance of the sky.

It appears likely that active camouflage will make a comeback in the 2000s. Improvements in radar stealth have reached a point where visual and infrared signatures are dominant. One sign of increasing interest in the non-radar aspects of stealth is that the Air Force has commissioned a new flying laboratory called FISTA II (Flying Infrared Signature Technology Aircraft), to replace a vehicle that has been used since the early 1960s to measure the heat signatures of airplanes.

A modified tanker aircraft, FISTA II carries not only ultra-sensitive infrared imagers but also a visual imaging system, an indication that the Pentagon is becoming serious about visual stealth.

Modern follow-ons to Yehudi are both more effective and easier to install. Instead of individual lights, the Pentagon has tested thin fluorescent panels of the type already used on military aircraft for nighttime formation flying. A civilian technician working at the isolated Tonopah Test Range airstrip in Nevada says he witnessed a test of an F-15 Eagle with a prototype system. According to the technician, the fighter virtually disappeared as it lifted off the runway.

"We had no problem acquiring the aircraft from about a mile away," the technician recalls, "but at distances over two miles it became harder and harder to spot. Although it was a crude system, it was pretty impressive. Trying to pick out the aircraft against a clear, blue sky was next to impossible. The only time we could easily spot the aircraft was when it produced an unexpected contrail." (Contrails form when the water vapor in aircraft exhaust freezes. On the B-2 and F-117, anti-contrail systems inject chemicals into the exhaust stream to break water into droplets too small to be seen.)

An even more experimental active-camouflage system uses thin sheets of light-emitting polymer that glow and change color when charged. Different voltages cause the sheets to glow blue, gray, white, or whatever shade is needed to match the sky. As an added advantage, the thin sheets are easy to apply to existing aircraft.

One such "electrochromic" polymer has been developed at the University of Florida, and the Air Force is studying it as a way of applying a variable tint to the cockpit canopy of a fighter aircraft. In theory, such a coating could also be used over a white-painted skin to vary its color.

But what about concealing an aircraft from an enemy flying above it? Defense contractors have told Popular Science that an even more exotic invisibility system is being tested on two new stealth aircraft at the high-security Groom Lake air base in Nevada. The skin is derived from an electromagnetically conductive polyaniline-based radar-absorbent composite material. It is optically transparent except when electrically charged, much like the LCD panels used in laptop computers.

What makes this new material attractive is that it can change brightness and color instantaneously. Photosensitive receptors, mounted on all sides of the plane, read the ambient light and color of the sky and ground. An onboard computer adjusts the brightness, hue, and texture of the skin to match the sky above the plane or the terrain below it.

The system is also claimed to make the aircraft even stealthier. The electrically charged skin dissipates radar waves, reducing the range at which an air defense radar can track the aircraft by as much as 50 percent.

Such systems do not have to be perfect. The goal is not to build an invisible airplane, but to delay the visual acquisition of an aircraft for as long as possible. In fact, the most effective way of fooling either the eye or a missile may be to present it with an image that is difficult to interpret.

Using fast-changing electrochromic panels, the military is experimenting with "flickering skins" that could prevent missiles from locking onto their targets. In demonstrations at Groom Lake, engineers have turned the entire skin of an aircraft into a missile jammer by applying a special coating that flickers in intensity in both the visible and infrared spectrum.

A flickering skin could help aircraft hide from a new generation of missiles that use visual and infrared sensors to build an image of a target. Older heat-seeking missiles could be lured away from aircraft by decoys-hot flares ejected during flight. But the newer missiles use visual sensors to "see" the edges of an aircraft and distinguish its shape from that of a decoy. A shimmering skin, which looks something like a desert mirage, confuses the missile's sensors by displacing or distorting the aircraft's image.

Engineers have also taken steps to reduce the heat signatures of military aircraft. In the 1970s, infrared sensors had a much greater range than visual imaging systems-video cameras with telephoto lenses that were mainly used to track or identify targets that had already been detected. Infrared accordingly became the stealth designers' second priority, after radar.

Infrared sensors detect hot spots, such as engine exhaust or the leading edges of the wing, which are heated by air friction. At closer ranges, infrared sensors detect solar radiation glinting off curved surfaces or scattering from the skin.

Designers countered infrared sensors in several ways. The exhaust nozzles were flattened into slits, because a flat nozzle has a longer perimeter than a round plume, and the exhaust mixes more quickly with the cool air.

Designers also developed paints containing compounds such as zinc sulfide, to suppress reflections from the airplane's skin. Paint cannot eliminate the heat generated by skin friction, but special coatings can change the "emissivity" of the surface-that is, the efficiency with which it transforms heat into infrared radiation. Only certain wavelengths of infrared radiation travel efficiently through the atmosphere, so the goal is to concentrate infrared radiation outside those bands and let the atmosphere soak it up.

Low-infrared paints and coatings are now widely used on many aircraft. Lockheed Martin even coated a 747, reducing its infrared signature tenfold.

After years of research focused on the suppression of infrared and radar signatures, aircraft designers now appear to be giving more attention to visual stealth. There are still some basic physical problems to be solved. For example, even a very efficient lighting system requires a lot of energy to match the brightness of the sky, equivalent to several times the power absorbed by the fighter's radar. Experts in the field of electrochromic materials caution that there are major technical hurdles that have not yet been cleared in the unclassified world, and not for lack of interest: The building industry would love to see a practical, large-area electrochromic film, because it could greatly reduce the energy needed to heat and cool buildings.

Electrochromic materials must not only be able to change color, but also to withstand sunlight and extreme weather, and continue operating through many switching cycles. The problems are compounded for a stealth aircraft, because the material must also be compatible with existing radar and infrared stealth technologies. This may well be the reason why, for now, visual stealth measures are confined to a few experimental aircraft-and may stay that way for some time to come.Lockheed Martin's F-117A Nighthawk stealth fighter was the star of the Gulf War, flying behind enemy lines to hit Baghdad targets with pinpoint accuracy. At night, the F-117 was unstoppable. But by day, the black jet stayed on the ground.

The F-117 isn't fast enough to outrun missiles or agile enough to dodge them, and it can't fight back because it's armed only with bombs. These limitations don't matter at night, because the F-117's stealthy shape enables the aircraft to avoid detection by enemy radar. But in the light of day, the enemy can see the black plane against the sky, and can take aim without the help of radar.

F-117 pilots train almost exclusively for night missions, and the darker it gets the happier they are. But this is a compromise at best. In the summer, a night-only fighter can fly only one sortie per day. And the darkness that hides the F-117 also hides its targets.

Air Force generals would love nothing more than to have a stealth aircraft that would be invulnerable during daylight hours, as well as at night. And as Popular Science has learned, military engineers are already hard at work on the technologies needed to build such a plane. Special lights, coatings, and other technologies under investigation could not only make future fighters disappear from radar screens, but could also make them almost completely invisible to the human eye. By the early 2000s, stealth may be practical in broad daylight.

Today's experiments exploit a principle that was demonstrated half a century ago, in a secret project codenamed Yehudi . In that project, engineers mounted lights on an antisubmarine aircraft to make it harder to spot against a bright sky. Similar technology was used in the Vietnam War to shorten the distance at which the F-4 Phantom could be detected.

Lighting systems were available when Lockheed's Skunk Works was awarded the contract to build Have Blue, the world's first stealth aircraft, in 1976. The breakthrough that made Have Blue possible was the ability to reduce an airplane's radar reflectivity to less than one-hundredth of what was considered normal in the 1960s, slashing the effective range of enemy radar. Reducing the radar reflectivity so dramatically meant that the designers of Have Blue also had to reduce the visual and infrared signatures of the plane, according to a rule of thumb known as "balanced observables."

This rule says that a stealth aircraft should be designed so that every detection system arrayed against it has roughly the same range. There is no point in building an airplane that is invisible to radar at five miles if optical sensors can see it at ten miles.

Have Blue was the prototype for an aircraft that would make its attack run at a moderate altitude of 10,000 to 15,000 feet-close enough to designate the target accurately, but high enough to elude medium-caliber gunfire. At the time, the designers' goal was an aircraft that would be as stealthy in daylight as at night.

The designers realized that visual detection depends on a number of factors, including the position of the observer, his angle of view, the position of the sun, and the presence of haze or clouds. Altitude is extremely important. A jetliner at its cruising height always appears brightly lit in the sky, because dust and moisture in the air beneath the aircraft scatter light onto its underside. There are relatively few particles of dust and water in the thin air above the airplane. So the higher the plane flies, the more light is scattered onto it, and the darker the sky behind it.

A dark color that absorbs as much light as possible provides the best camouflage for a high-flying airplane. But even the jet-black Blackbird and U-2 spyplanes look brighter than the sky when seen from below as they cruise at 80,000 feet. At lower altitudes, there is less light-scattering atmosphere below the aircraft, so lighter colors provide the least contrast.

For Have Blue, Lockheed devised a scheme of graduated grays, lighter on the bottom and darker on top. The aircraft's designers also planned to test light apertures, which would be installed on the sides and undersurfaces of the airplane, about two feet apart. (Seen from a distance, the individual lights would blur into a single image.) The apertures would be connected to a central light source by fiber optic lines, and their output would be controlled by light sensors on the upper side of the aircraft. The sensors would "read" the background light and adjust the skin's luminance to mirror it.

This system never flew on Have Blue, possibly because the first aircraft was lost in an accident. Work on visual stealth continued, however.

In 1980, the Air Force tested a small aircraft, probably unmanned, under a project known as IMCRS (what the acronym stands for is not known). The aircraft's lower wing skins incorporated slit-like Fresnel lenses to beam light ahead of and below the aircraft, in the direction of the most likely threats.

The IMCRS experiment may have been related to a Defense Advanced Research Project Agency program known as Active Camouflage. Under that program, a small powered drone was fitted with fluorescent lamps and tested at the White Sands Missile Range with so much success that the project has since been reclassified as Top Secret.

Neither of these lighting systems were adopted for stealth aircraft in the 1980s. They were complex to install and design. Their effects were hard to predict and difficult to test. Carefully designed conventional camouflage worked well enough under most circumstances to ensure that an aircraft would not be visible before a radar could detect it.

So why were the first F-117s painted soot black instead of a toned gray scheme that would provide better camouflage? One Lockheed engineer recalls that the commander of Tactical Air Command "didn't believe that real fighter pilots flew pastel-colored airplanes." An Air Force source close to the program says that some senior officers doubted the F-117 could survive in daylight, and wanted to ensure that nobody would try it.

Black is one of the least stealthy colors for daytime flying at medium altitudes. In fact, the British Royal Air Force is painting its trainers black to make them more visible and reduce the risk of collisions. Black isn't much good at night either, because there is nearly always some light from the moon. That's why the latest F-117s have been seen in a more sensible gray color.

The B-2 stealth bomber's underside is a very dark gray. Many people think that it is designed to attack only at night, like the F-117. This is unlikely, because the B-2 was designed to bomb Russia, and the most direct route from the center of the United States to central Russia lies smack across the Arctic Circle, where the sun shines 24 hours a day for a large part of the year.

The B-2's underside is dark because it cruises at altitudes as high as 50,000 feet, where a dark gray blends into the sky. It does not use an "active camouflage" lighting system, but it may have an upward-facing light sensor that tells the pilot when to increase or reduce altitude slightly to match the changing luminance of the sky.

It appears likely that active camouflage will make a comeback in the 2000s. Improvements in radar stealth have reached a point where visual and infrared signatures are dominant. One sign of increasing interest in the non-radar aspects of stealth is that the Air Force has commissioned a new flying laboratory called FISTA II (Flying Infrared Signature Technology Aircraft), to replace a vehicle that has been used since the early 1960s to measure the heat signatures of airplanes.

A modified tanker aircraft, FISTA II carries not only ultra-sensitive infrared imagers but also a visual imaging system, an indication that the Pentagon is becoming serious about visual stealth.

Modern follow-ons to Yehudi are both more effective and easier to install. Instead of individual lights, the Pentagon has tested thin fluorescent panels of the type already used on military aircraft for nighttime formation flying. A civilian technician working at the isolated Tonopah Test Range airstrip in Nevada says he witnessed a test of an F-15 Eagle with a prototype system. According to the technician, the fighter virtually disappeared as it lifted off the runway.

"We had no problem acquiring the aircraft from about a mile away," the technician recalls, "but at distances over two miles it became harder and harder to spot. Although it was a crude system, it was pretty impressive. Trying to pick out the aircraft against a clear, blue sky was next to impossible. The only time we could easily spot the aircraft was when it produced an unexpected contrail." (Contrails form when the water vapor in aircraft exhaust freezes. On the B-2 and F-117, anti-contrail systems inject chemicals into the exhaust stream to break water into droplets too small to be seen.)

An even more experimental active-camouflage system uses thin sheets of light-emitting polymer that glow and change color when charged. Different voltages cause the sheets to glow blue, gray, white, or whatever shade is needed to match the sky. As an added advantage, the thin sheets are easy to apply to existing aircraft.

One such "electrochromic" polymer has been developed at the University of Florida, and the Air Force is studying it as a way of applying a variable tint to the cockpit canopy of a fighter aircraft. In theory, such a coating could also be used over a white-painted skin to vary its color.

But what about concealing an aircraft from an enemy flying above it? Defense contractors have told Popular Science that an even more exotic invisibility system is being tested on two new stealth aircraft at the high-security Groom Lake air base in Nevada. The skin is derived from an electromagnetically conductive polyaniline-based radar-absorbent composite material. It is optically transparent except when electrically charged, much like the LCD panels used in laptop computers.

What makes this new material attractive is that it can change brightness and color instantaneously. Photosensitive receptors, mounted on all sides of the plane, read the ambient light and color of the sky and ground. An onboard computer adjusts the brightness, hue, and texture of the skin to match the sky above the plane or the terrain below it.

The system is also claimed to make the aircraft even stealthier. The electrically charged skin dissipates radar waves, reducing the range at which an air defense radar can track the aircraft by as much as 50 percent.

Such systems do not have to be perfect. The goal is not to build an invisible airplane, but to delay the visual acquisition of an aircraft for as long as possible. In fact, the most effective way of fooling either the eye or a missile may be to present it with an image that is difficult to interpret.

Using fast-changing electrochromic panels, the military is experimenting with "flickering skins" that could prevent missiles from locking onto their targets. In demonstrations at Groom Lake, engineers have turned the entire skin of an aircraft into a missile jammer by applying a special coating that flickers in intensity in both the visible and infrared spectrum.

A flickering skin could help aircraft hide from a new generation of missiles that use visual and infrared sensors to build an image of a target. Older heat-seeking missiles could be lured away from aircraft by decoys-hot flares ejected during flight. But the newer missiles use visual sensors to "see" the edges of an aircraft and distinguish its shape from that of a decoy. A shimmering skin, which looks something like a desert mirage, confuses the missile's sensors by displacing or distorting the aircraft's image.

Engineers have also taken steps to reduce the heat signatures of military aircraft. In the 1970s, infrared sensors had a much greater range than visual imaging systems-video cameras with telephoto lenses that were mainly used to track or identify targets that had already been detected. Infrared accordingly became the stealth designers' second priority, after radar.

Infrared sensors detect hot spots, such as engine exhaust or the leading edges of the wing, which are heated by air friction. At closer ranges, infrared sensors detect solar radiation glinting off curved surfaces or scattering from the skin.

Designers countered infrared sensors in several ways. The exhaust nozzles were flattened into slits, because a flat nozzle has a longer perimeter than a round plume, and the exhaust mixes more quickly with the cool air.

Designers also developed paints containing compounds such as zinc sulfide, to suppress reflections from the airplane's skin. Paint cannot eliminate the heat generated by skin friction, but special coatings can change the "emissivity" of the surface-that is, the efficiency with which it transforms heat into infrared radiation. Only certain wavelengths of infrared radiation travel efficiently through the atmosphere, so the goal is to concentrate infrared radiation outside those bands and let the atmosphere soak it up.

Low-infrared paints and coatings are now widely used on many aircraft. Lockheed Martin even coated a 747, reducing its infrared signature tenfold.

After years of research focused on the suppression of infrared and radar signatures, aircraft designers now appear to be giving more attention to visual stealth. There are still some basic physical problems to be solved. For example, even a very efficient lighting system requires a lot of energy to match the brightness of the sky, equivalent to several times the power absorbed by the fighter's radar. Experts in the field of electrochromic materials caution that there are major technical hurdles that have not yet been cleared in the unclassified world, and not for lack of interest: The building industry would love to see a practical, large-area electrochromic film, because it could greatly reduce the energy needed to heat and cool buildings.

Electrochromic materials must not only be able to change color, but also to withstand sunlight and extreme weather, and continue operating through many switching cycles. The problems are compounded for a stealth aircraft, because the material must also be compatible with existing radar and infrared stealth technologies. This may well be the reason why, for now, visual stealth measures are confined to a few experimental aircraft-and may stay that way for some time to come.

Original Article: Popular Science; unknown date
Reposted at: http://forum.keypublishing.co.uk/showthread.php?t=4800