The UK government has warned the Guardian newspaper that foreign agents could use laser technology to eavesdrop on them, in the wake of recent surveillance leaks. What are laser listening devices and are they effective?
When we speak, our voices produce sound waves. Sound waves travel in all directions and can hit anything in the room around you. If the sound waves hit something solid, they bounce off.
Laser listening devices are designed to pick up the vibrations produced by sound waves as they hit resonant surfaces - ones that are good for picking up vibrations - such as a window or a plastic cup.
When the laser bounces back to a receiver, the vibrations can be decoded and the conversation reconstructed.
These sophisticated pieces of equipment often look similar to cameras. A box - called a cache case, which has the technology inside - is mounted on a tripod while another box for the receiver captures the returning laser signal.
For the laser to work, there must be a direct line of sight between the operator and the target, and the laser must be within about 500m (165ft) of its target, in order to pick up the vibrations.
The technology is small enough that one person can use it.
"The technology behind laser listening devices has been around for some time," said Jeremy Marks, the director of the spy equipment retailer Spycatcher Online.
"I have heard about the product being used for about five to 10 years."
But, Mr Marks said, high-end laser listening devices could be expensive, and are difficult to acquire for commercial use.
One Ukraine-based manufacturer called Argo-A Security said it only sold its devices to law enforcement and government agencies, without exception.
The equipment could be tailored to a customer's needs, and would cost between $17,000-$40,000 (£11,000-£26,000) to buy, a spokesman for the firm told the BBC.
While there are measures to combat this sort of eavesdropping, it is very hard to ensure your conversations will not be heard, one counter-surveillance specialist said.
"It is extremely difficult to protect any room with a window," said James Williams, the operations director of QCC Interscan, which provides counter-eavesdropping protection services to the government and private clients.
"If a government wants to spy on you, there is very little you can do."
The installation of counter-intelligence window film is one effective protection measure, he added. Adding heavy curtains to windows could also absorb some of the sound vibrations in a room.
But he added that lasers are trained to pick up certain frequencies and could still be recalibrated to get around those hurdles.
"Any sensitive meeting room with windows would not be a good starting point," Mr Williams said.
Secure rooms are usually located in the middle of the building, with no windows and a range of other security measures in place.
But in Spycatcher Online's experience, laser listening devices are usually only used in "extreme cases".
"It wouldn't be my first choice for sure," Mr Marks said. "If you've exhausted other methods, then yes, you would."
Security experts say that more effective eavesdropping options would involve placing some kind of bug inside the target room.
The biggest threat comes from GSM and 3G mobile signal eavesdropping technologies, Mr Williams said, agreeing that laser devices are "pretty niche".
But he added that there are other listening measures, such as using "modulated light", which would involve changing one of the light bulbs in the target room.