Video: Laser flight path caught on camera for the first time

Pew pew! Researchers have created the first video of a laser bouncing off a mirror.

Watching laser beams fly through the air makes for dramatic battles in sci-fi films, but they’re not so easy to see in real life. In order to observe a laser, or any other light source, photons from it must directly hit your eyes. But since laser photons travel in a tightly-focused beam, all heading in the same direction, you can only see them when the laser hits something that reflects a portion of the light and produces a visible dot.

A tiny proportion of photons scatter off air molecules, but normally these are too faint to see. You can get around this by firing a laser through smoke, giving the photons more molecules to scatter off – but that’s not the effect we see in the movies.


“The challenge was to have a movie of light moving directly in air,” says Genevieve Gariepy of Heriot-Watt University in Edinburgh, UK. “We wanted to look at light without interacting with it, just looking at it passing by.”

To make this work, she and her colleagues constructed a camera sensitive enough to pick up those few scattering photons. It is built from a 32 by 32 grid of detectors that log the time a photon arrives at them with incredible precision, equivalent to snapping around 20 billion frames a second.

Lighting the way

The team arranged the camera to film a side-on view of a green laser firing at an arrangement of mirrors. By firing 2 million pulses over a 10 minute period and subtracting background noise, they were able to build up enough air-scattered photons in the camera to track the laser’s path as it bounced.

“What comes out is a frame by frame of the light moving through our system,” says Gariepy. In their video, this position data is overlaid on a background photograph taken with a regular camera, and coloured green to match the laser’s true colour.

The experiment started as a pure research challenge, but Gariepy thinks their camera could have practical applications. In another experiment, the team filmed a focused laser that ionised air molecules to produce a plasma. Gariepy says a similar setup could help people studying the properties of such plasmas by letting them watch the plasma evolve over time.

Precise timing data could also be used to measure the distance photons have travelled, an effect previously exploited to take pictures around corners. “It takes maybe an hour or so to acquire an image” around a corner, says Gariepy, but the ability to take multiple images rapidly could generate movies from around corners. “With our camera this can be done in seconds.”

Journal reference: Nature Communications, DOI: 10.1038/ncomms7021