"The key to this plan is the giant laser. It was invented by the noted Cambridge physicist Dr. Parsons. Therefore, we shall call it the Alan Parsons Project."
Dr. Evil
Austin Powers

Here's something that will blow your mind and make you paranoid at the same time. Someone can listen to your conversations in your house or office from hundreds of feet away using light. The "light" is a "laser," and it's bounced off a window pane to detect sound vibrations. It's hard not to imagine Dr. Evil, played by Mike Meyers, air quoting "laser" when we mention that word. The theory was first proposed in the 1940s, but had to wait until lasers were actually invented in the 1960s to gain traction. By the 80s, the Cold War had us and the Soviets spying on each other using "lasers."

How viable is this form of espionage? It's usually a last resort because of the expense, logistics, and precise calibration needed to yield acceptable results. But it's still a fascinating technology that you can experiment with at home using easy to obtain items. Got a laser pointer you cat plays with? A nightlight with a photoelectric sensor? With a few other small components and connectors, you're in business – or maybe we should say the "spy business." The quality of the audio will not be up to par with what the three letter government agencies use, but it will work.

How does it work? In very simple terms, by bouncing a laser beam off a window pane and back into a photoelectric sensor that reacts to the changes in the light pattern caused by the glass vibrating from sound. The sensor converts those changes into electrical signals which then become audio signals. But it's not that simple in operation, as everything must work together in exacting detail. The laser beam and photoelectric sensor must be in perfect alignment, in this case very close to each other. The laser and sensor must not vibrate excessively due to poor mounting, street traffic, or even building sway. There must be no obstructions between the laser and window. And the window pane must be flexible enough to easily vibrate the human voice.

This last criteria is one of the most interesting, as it can be the focus of anti-espionage efforts. A secret "war room" or other place for safe conversation would usually be located in an inner room or subterranean space with no windows. When that's not possible, suspicious spy saboteurs employ heavy drapes or special window film to deaden vibrations. You don't have to be a double-naught spy to play the spy game. Hop over to Amazon and buy an electronic device that attaches to a pane of glass to defeat laser mic surveillance. It emits a sound in the human voice range to mask conversation, jamming the audio frequencies the laser mic is listening for.

Paranoid yet? I've got the perfect "no-tech" idea for laser surveillance countermeasures the next time you're here at the studio and tell me your secret. I'll just yell, "La, la, la, la, la, la, la, la, la, la, la, la, la,la, la, la, la, la, la, la, la, la."

You're welcome.

explore more

• Lucid Science shows you how to build a laser spy device.
• This guy on Lifehacker built and tested a laser mic in a YouTube video.
• The British government warned The Guardian that spies may be using laser mics to eavesdrop on them.
• Speaking of The Guardian, they wonder if laser spying is practical.
• Chris Bryant of the Financial Times in Britain digs a little deeper into the possibilities of laser spying.
• Wikipedia's take on defining a laser mic.
• Not laser mic technology, but a cool article on how Léon Theremin (inventor of that wonderful and creepy sounding Theremin music instrument) invented a passive bug that the Soviets used to spy on the U.S. Ambassador to the U.S.S.R.