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A laser microphone is a surveillance device that uses a laser beam to detect sound vibrations in a distant object. This technology can be used to eavesdrop with minimal chance of exposure. The object is typically inside a room where a conversation is taking place, and can be anything that can vibrate (for example, a picture on a wall) in response to the pressure waves created by noises present in the room. The object preferably has a smooth surface. The laser beam is directed into the room through a window, reflects off the object and returns to a receiver (for example a solar panel) that converts the beam to an audio signal. The beam may also be bounced off the window itself. The minute differences in the distance traveled by the light as it reflects from the vibrating object are detected interferometrically. The interferometer converts the variations to intensity variations, and electronics are used to convert these variations to signals that can be converted back to sound. However, countermeasures exist in the form of specialized light sensors that can detect the light from the beam. Rippled glass can be used as a defense, as it provides a poor surface for a laser microphone. However, by simple extrapolation it should be easily possible to adapt this technique to a maser or focused microwave beam in order to fire the beam through ordinary building materials and bounce it off of internal metal objects such as heating ducts or filing cabinets. At this point the only acceptable defense is electromagnetically and acoustically isolating the interior of the structure. ==History== The technique of using a light beam to remotely record sound probably originated with Léon Theremin in the Soviet Union at or before 1947, when he developed and used the ''Buran'' eavesdropping system.〔Galeyev〕 This worked by using a low power infrared beam (not a laser) from a distance to detect the sound vibrations in the glass windows.〔〔Glinsky 261〕 Lavrentiy Beria, head of the KGB, had used this Buran device to spy on the U.S., British, and French embassies in Moscow.〔 It has been reported that the National Security Agency makes use of laser microphones. On 25 August 2009, U.S. patent 7,580,533 was issued for a device that uses a laser beam and smoke or vapor to detect sound vibrations in free air ("Particulate Flow Detection Microphone based on a laser-photocell pair with a moving stream of smoke or vapor in the laser beam's path"). Sound pressure waves cause disturbances in the smoke that in turn cause variations in the amount of laser light reaching the photo detector. A prototype of the device was demonstrated at the 127th Audio Engineering Society convention in New York City from 9 through 12 October 2009. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Laser microphone」の詳細全文を読む スポンサード リンク
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