Laser microphone

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Laser microphones are microphones with a laser beam. They detect vibrations with a laser and convert it to a digital signal. Lasers are usually bounced off a window, or off any object near to the conversation monitored. Any object which can resonate/vibrate (for example, a picture on a wall) will do so in response to the pressure waves created by noises present in a room. The minute differences in the distance travelled by the light to pick up this resonance is detected interferometrically. Light that is subject to the varying distance is mixed with light that travels a constant distance. The interferometer converts the variations in distance to intensity variations and electronics are used to convert these variations to digital signals that can be interpreted as sound.

This technology can be used to secretly eavesdrop on people with minimal chance of exposure. However, specialized light sensors may be used to detect the light from the beam. It was designed by Ethal Hussenburn in the 1970s during the Cold War. During the making the technology was advanced by the NSA and CIA making it smaller and compact. The Shelby laser Microphone was then invented.