A laser microphone uses a laser beam to bounce sound back to its source. Instead of its main purpose being to amplify voice or sound, like a traditional microphone, a laser microphone is used for surveillance. It is not anything like the microphone most people associate with the word microphone. A traditional microphone uses a sensor that responds to voices or music and transforms the sounds into an audible electrical signal. A laser microphone relies on the transmitted laser beam being able to land on a smooth surface and bounce back to the transmitting receiver where it becomes audible.
When a person wishes to record a verbal interchange without the knowledge of one or more of the speakers, a laser microphone is often used for the job. Many spy movies and detective shows on television feature laser microphones. The microphone is often shown being set up in a room near the stakeout. Its beam is precisely directed through a window and targeted to record the covert interchange without arousing suspicion.
While this undercover tool is usually successful in spying scenarios, the recording is sometimes thwarted by a common counter-surveillance technique. This detection method uses high tech light sensors to detect the laser beam and disable it or trace it back to its source. In some cases, the nefarious characters only converse in rooms with rippled glass windows and doors, as this type of glass prevents successful laser beam penetration and spoils the transmission.
Using a light beam for recording sound from a secluded location is thought to have first been successful for Leon Theremin who, around 1947, invented an eavesdropping device in the Soviet Union. His beam was infrared instead of laser, but it worked well enough for the head of the secret service, the KGB, to use it for spying on Moscow's British, French, and American embassies during World War II. It is believed that the laser microphone currently on the market is regularly used by the US National Security Agency.
Until recently, laser microphones were mainly devices utilized by undercover agents in movies, books, and television shows or by actual government agencies. In the real world, they were readily available only to those in the surveillance business. All this changed in October 2009, when a popular electronics magazine published a how-to article with directions anyone could use to build their own laser microphone.
The latest development in laser microphones utilizes smoke or vapor in conjunction with the laser beam. It does not require the beam to bounce off a surface, but rather uses the vapor or smoke stream to detect the waves in open air. A US patent for this new laser microphone was issued in August 2009.