Map a room by shining a laser through the keyhole

[Nipuna]

An eye might not be able to see round corners, but a laser set-up can (Image: Getty)

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Peeking through a keyhole has become even more useful. An imaging technique that measures the path of a laser to build up a three-dimensional picture could now let spies map an entire room through a tiny hole.

In 2012, researchers used a laser to see around corners. The system worked by firing short laser pulses at a nearby wall, bouncing light around a corner to a hidden object, which then bounces some of it back to a camera next to the laser.

The camera only measures light arriving during a very short window of time, and changing the gap between the laser pulse and this interval allows you to measure light that has travelled different distances, building up a 3D image of the hidden object.

Spell it out
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Now Chenfei Jin of the Harbin Institute of Technology in China and colleagues have taken this a step further. They used a laser set-up to measure the 3D shape and position of three cardboard letters, spelling HIT, through a 2 centimetre hole in a nearby wall.

This time, the light returning from the object passed through the hole and on to another wall, which in turn scattered light into the camera. Pointing the camera at different parts of this wall let them scan the otherwise hidden room.

The letters were coated in highly reflective material while the rest of the room was covered in black light-absorbing cloth, so the technique might not work as well in a real-world setting, but Jin says the cheap and portable equipment it requires should make it appealing. "My research can be considered for many applications like fire rescue, battlefield surveillance, anti-terrorism surveillance and so on," he says.

Guy Satat of the Massachusetts Institute of Technology says he likes the work, but points out that if the hole were easily accessible, it would be easier for would-be spies to simply put a camera up against the wall and take a photo of the room. "Their method is capable of working remotely from the wall but requires very specific and delicate geometry inside and outside the room," he says.

Journal reference: Optics Letters, DOI: 10.1364/OL.40.000052