Low-power laser reveals high-resolution details from a kilometre away

A team from Heriot-Watt University has developed a time-of-flight (ToF) imaging system that can gather high-resolution, 3D information about objects from up to a kilometre away.

The new system is said to work by sweeping a low-power infrared laser beam rapidly over an object. It then records the round-trip flight time of the photons in the beam as they bounce off the object and arrive back at the source. The system can resolve depth on the millimetre scale over long distances using a detector that can ‘count’ individual photons.

Although other approaches can have exceptional depth resolution, the ability of the new system to image objects like items of clothing that do not easily reflect laser pulses makes it useful in a wider variety of field situations, said Heriot-Watt University Research Fellow Aongus McCarthy, the first author of a paper on the research published in Optics Express.

‘Our approach gives a low-power route to the depth imaging of ordinary, small targets at very long range,’ McCarthy said in a statement. ‘Whilst it is possible that other depth-ranging techniques will match or out-perform some characteristics of these measurements, this single-photon counting approach gives a unique trade-off between depth resolution, range, data-acquisition time, and laser-power levels.’

The primary use of the system is likely to be scanning static, man-made targets, such as vehicles. With some modifications to the image-processing software, it could also determine their speed and direction.

One of the key characteristics of the system is the long wavelength of laser light the researchers chose. The light has a wavelength of 1,560nm, meaning it is longer, or ‘redder,’ than visible light, which is around 380-750nm in wavelength. This long-wavelength light travels more easily through the atmosphere, is not drowned out by sunlight, and is safe for eyes at low power. Many previous ToF systems could not detect the extra-long wavelengths that the Scottish team’s device is designed to sense.

The scanner is said to be particularly good at identifying objects hidden behind clutter, such as foliage. However, it cannot render human faces, instead drawing them as dark, featureless areas. This is because at the long wavelength used by the system, human skin does not reflect back a large enough number of photons to obtain a depth measurement. However, the reflectivity of skin can change under different circumstances.

‘Some reports indicate that humans under duress—for example, with perspiring skin—will have significantly greater return signals,’ and thus should produce better images, McCarthy said.

Outside of target identification, photon-counting depth imaging could be used for a number of scientific purposes, including the remote examination of the health and volume of vegetation and the movement of rock faces, to assess potential hazards.

Ultimately, McCarthy said, it could scan and image objects located as far as 10km away. ‘It is clear that the system would have to be miniaturised and ruggedised, but we believe that a lightweight, fully portable scanning depth imager is possible and could be a product in less than five years.’

Next steps for the team include making the scanner work faster. Although the data for the high-resolution depth images can be acquired in a matter of seconds, currently it takes about five to six minutes from the onset of scanning until a depth image is created by the system.

Most of that lag, McCarthy said, is due to the relatively slow processing time of the team’s available computer resources.

‘We are working on reducing this time by using a solid-state drive and a higher specification computer, which could reduce the total time to well under a minute. In the longer term, the use of more dedicated processors will further reduce this time.’