How practical is a laser microphone and how to protect against it?

Laser Microphones like any other eavesdropping equipment has pros and cons.

Pros

• Can be invisible if using infra-red (IR) beam
• Will not be detected with typical radio frequency (RF) finders
• Can be turned on/off at will - avoiding further detections

Cons

• The beam (laser or other) needs to be in within line of sight of the target
• Is affected by rain / snow (more explanation below)
• IR beam can be easily detected using the Front-Facing Camera on an iPhone

Recent History

These devices were all the rage in the 90s - you can see this by seeing the style of websites selling them (here and here are examples)

For a while they fell out of favor with the rise of networking/computer eavesdropping technologies which provided more value per dollar.

Recently though, they have been having a semi-resurgence with anti terror units having use for them. Here is an example of one such company marketing it.

Laser Microphone History

The earliest recording of a "Laser" microphone is by an inventor named Léon Theremin.

He created something named the Buran eavesdropping system which essentially used an low powered infra-red beam to detect vibrations made by sound-waves. He demonstrated this device on a pane of glass.

Operating Principles

Laser microphones work by shining a light on an object that vibrates (think: glass, plastic cup etc.) When someone speaks (or any other noise is introduced) it pushes the air around it causing the object that the laser is shining on to vibrate.

When the object vibrates it causes minute differences in the distance traveled by the light as it reflects back to a receiver. These difference are detected using something called Interferometry.

These signals are then translated back into sound which allow you to hear what is going on.

Mechanics - the moving parts

Here are the pieces involved to make this work:

Laser beam

This is the device that will 'shine a light' on the object in the room (or on the window. Modern versions of these systems can work at distances of 500 meters

Reciever

This is the device that picks up the reflection of the beam (typically placed at about a 90 degree angle to the originating beam)

This processes the light signal and sends it to the next piece of electronics for processing.

Demodulator

This is the device that actually turns it into sound for you. This device can be controlled with software to remove noises such as wind etc. to provide a clearer sample.

A Laser Microphone can be pretty effective if used properly and in ideal conditions

But remember that the information you are looking for may be in a digital device and not in voice spoken format.

Countermeasures

Detecting a Laser microphone is relatively easy in many cases (mentioned earlier in the answer)

Vibrating the windows is not always effective as software can (and has) been written to remove the frequency from the vibrating noises.

However, one can create a device that vibrates the windows with frequencies that used typically in human speech. Such a device could if done correctly render the laser microphone useless.

Known cases of use

Though there isn't any official documentation of its use, there are some places that discuss its use during the cold war (here is an example).

There has also been speculation whether government agencies used such a device during the Snowden investigation.

Usage Limitations

In order for this system to work you need line-of-sight to an object that vibrates.

This object obviously has to be within reasonable distance to the source of the noise.

Rain, Snow or any other weather that interferes with an IR beam would affect the ability to eavesdrop.

One last thing to mention is the need for both the shining laser and the receiver to be stationary the whole time and not move. Usually, this is achieved by using a pair of tripods (like in the image posted in the question)

Some interesting links

http://www.instructables.com/id/Laser-Surveillance-System-for-under-%2420/

http://hackaday.com/2010/09/25/laser-mic-makes-eavesdropping-remarkably-simple/

http://lifehacker.com/5961503/build-a-laser-microphone-to-eavesdrop-on-conversations-across-the-street

http://www.lucidscience.com/pro-laser%20spy%20device-1.aspx

https://web.archive.org/web/20180305115138/http://www.williamson-labs.com/laser-mic.htm

AS A USER OF THIS TECHNOLOGY (no, LeRoy Theremin did not use this since lasers were not developed until Theodore Maiman made the first laser operate on 16 May 1960 at the Hughes Research Laboratory in California) I can firmly state that IF the window is firmly installed and not rattling loosely in its frame, that with computer audio filtering, it is very easy to pick out individual voices in a loud cacophony, that includes very loud music. Ive used it to make very high fidelity recordings of concert environments that included conversational elements of the audience.

I am not as experienced as ScottS in the use of this apparatus, but I understand the basic concept of it.

I think that ScottS answers your first question better than I can. As long as there is a firmly mounted glass window, and the internal noise is louder than a whisper, then the device should be able to pick it up.

There are several ways I can think of to protect against this type of surveillance. The thing you mentioned about vibrating windows is correct; you can essentially mount a small vibrating motor or speaker to each window, which will create enough vibration to drown out the movement caused by internal sound waves. Another thing you can do is to have some sort of a wire screen or curtain on the outside of the window, which interferes with the laser beam. The best option, though, is to simply not have windows in sensitive rooms. Even if a laser can't pick up vibrations, a powerful camera may be able to read lips, or other things like that. Windows, in general, are insecure. (Not to mention that it is a lot easier to break into a room that has a window in it.)

As far as the historical use of this goes, I am not an expert in spy history, but it seems that it has been used in real life espionage by both the US and Russia. Apparently the US embassy in Russia was designed with curtain walls to prevent this type of spying. I don't have any specific instances for you, though.

For your last question, it seems that the only requirement is that the laser is able to reflect back to the sensor, with enough strength for the sensor to detect it. So I can see it easily working from over 100 ft away, as long as the laser is powerful enough. The weather shouldn't matter, as long as the sensor is shielded from direct sunlight. (But snow or rain may interfere, as they would occasionally block the beam of the laser. The recording would probably still be intelligible, but would have a lot of static.) As far as target material, I have seen it used on materials other than glass, but the material must be reflective. Otherwise the laser beam never gets back to the sensor. Also, it must be relatively smooth, because any bumps will distort the vibrations.

By the way, I found a very interesting project here that explains how to make a simple device for yourself.