A cloak of invisibility is a method of rendering physical objects undetectable or invisible. Researchers from the United States and England have developed a prototype for a cloaking device that can hide an object approximately 30 centimeters (1 foot ) in diameter. The device works by producing a mirage, in a manner similar to the way a layer of heated air causes reflection or refraction of light to create an illusion that can conceal objects on the earth's surface when viewed from a distance at a grazing angle.
In the microwave portion of the radio-frequency (RF) spectrum, where most radar systems operate, cloaking differs from stealth technology. A stealth aircraft evades radar detection by presenting a minimal profile for reflecting microwave electromagnetic fields. Cloaking actually diverts electromagnetic waves around the object. This can completely eliminate its image at a specific wavelength. At visible-light wavelengths, such a process may cause distortion of images directly behind the cloaked object. In an ideal visual cloaking system, such distortion would have to be minimized so evidence of the cloaking would be difficult or impossible to notice.
Cloaking technology is of interest to the military for obvious reasons. It may also have industrial and civilian applications. For example, it might be used to minimize electromagnetic interference (EMI) to sensitive medical devices or cellular telephones. Because of technical limitations, it will probably be at least several decades before a device is developed that can completely cloak a human being or other large physical object at visible-light wavelengths, particularly if that object and a would-be observer are in relative motion.