As the technology of camouflage has advanced over the past hundred years, so has the technology of seeing through camouflage. These days, military forces can use thermal imaging to "see" the heat emitted by a person or piece of equipment. Additionally, they may use radar, image enhancement, satellite photography and sophisticated listening devices to detect the enemy.
To hide from this scanning technology, military forces have to think past visual concealment. In modern warfare, camouflage for equipment and soldiers may be made of material that keeps excess heat from escaping, so their thermal "signature" does not show in thermal imaging. In ships, the major heat source is the engine exhaust. To reduce this thermal emission, some modern ships cool the exhaust by passing it through sea water before it is expelled. Some tanks have a similar cooling system to mask the heat of their exhaust.
To counteract image enhancement -- the amplification of tiny amounts of light (including low-frequency infrared light) -- some armies are developing sophisticated smoke screens. A heavy cloud of smoke blocks the path of light, imparting a sort of invisibility to whatever is behind the smoke screen. According to one report, the United States is working on a smoke screen that would be impenetrable to night vision technologies while still allowing advanced U.S. thermal imagers to function correctly. On a larger scale, the British shipbuilder Vosper Thorneycroft has developed a system that uses a series of water nozzles to produce a constant fog all around a ship, obscuring it from view.
Stealth technology allows militaries to hide equipment from radar. In stealth equipment, the surface of a vehicle is made up of many flat planes, interconnected at odd angles. These planes serve to deflect the radar radio waves so they don't bounce straight back to the radar station, but instead bounce off at an angle and travel in another direction. Equipment may also be coated with a layer of "radar-absorbent" material. When a radio wave hits an object, the electrons in that object are excited to some degree, so the wave has passed on some of its energy. In a good conductor, such as a metal radio antenna, the electrons move very easily, so the radio wave doesn't lose much energy in getting those electrons excited. Radar-absorbent material, on the other hand, is a very poor conductor, so there is greater resistance to moving the electrons. Because of this resistance, the radio wave loses more energy, which is emitted as heat. This reduces the overall reflected radio signal.
Decoy technology has also advanced in response to modern detection systems. The U.S. Army and other military forces have developed easily-transported, inflatable dummies that not only resemble tanks and other equipment visually, but also replicate the thermal or radar signature of that equipment. To radar and other long-range scanners, these dummies are virtually indistinguishable from real equipment. A less precise decoy strategy is to flood an area with all sorts of objects that show up on radar, thermal-imaging and listening devices, making it harder for the enemy to focus in on any particular piece of equipment.
As detection and spy equipment continues to advance, military engineers will have to come up with more sophisticated camouflage technologies. One interesting idea that is already in the works is "smart camouflage" -- outer coverings that alter themselves based on computer analysis of changing surroundings. No matter how advanced camouflage gets, the basic strategy will still be the approach used by the first human hunters: Figure out how your enemy sees you, and then mask all of the elements that make you stand out.
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