A new camera can photograph you from 45 kilometers away
A new
camera can photograph you from 45 kilometers away
Developed in China, the
lidar-based system can cut through city smog to resolve human-sized features at
vast distances.
by Emerging Technology from the arXiv May 3,
2019
Long-distance photography on Earth is a tricky challenge.
Capturing enough light from a subject at great distances is not easy. And even
then, the atmosphere introduces distortions that can ruin the image; so does
pollution, which is a particular problem in cities. That makes it hard to get
any kind of image beyond a distance of a few kilometers or so (assuming the camera
is mounted high enough off the ground to cope with Earth’s curvature).
But in recent years, researchers have begun to exploit
sensitive photodetectors to do much better. These detectors are so sensitive
they can pick up single photons and use them to piece together images of
subjects up to 10 kilometers (six miles) away.
Nevertheless, physicists would love to improve even more.
And today, Zheng-Ping Li and colleagues from the University of Science and
Technology of China in Shanghai show how to photograph subjects up to 45 km (28
miles) away in a smog-plagued urban environment. Their technique uses
single-photon detectors combined with a unique computational imaging algorithm
that achieves super-high-resolution images by knitting together the sparsest of
data points.
The new technique is relatively
straightforward in principle. It is based on laser ranging and detection, or
lidar—illuminating the subject with laser light and then creating an image from
reflected light.
The big advantage of this kind of active imaging is that
the photons reflected from the subject return to the detector within a specific
time window that depends on the distance. So any photons that arrive outside
this window can be ignored.
This “gating” dramatically reduces the noise created by
unwanted photons from elsewhere in the environment. And it allows lidar systems
to be highly sensitive and distance specific.
To make the new system even better in urban environments,
Zheng-Ping and co use an infrared laser with a wavelength of 1550 nanometers, a
repetition rate of 100 kilohertz, and a modest power of 120 milliwatts.
This wavelength makes the system eye-safe and allows the team to filter out
solar photons that would otherwise overwhelm the detector.
The researchers send and receive these photons through the
same optical apparatus—an ordinary astronomical telescope with an aperture of
280 mm. The reflected photons are then detected by a commercial single-photon
detector. To create an image, the researchers scan the field of view using a
piezo-controlled mirror that can tilt up, down, and side to side.
In this way, they can create two-dimensional images. But by
changing the gating timings, they can pick up photons reflected from different
distances to build a 3D image.
The final advance the team has made is to develop an
algorithm that knits an image together using the single-photon data. This kind of computational imaging
has advanced in leaps and bounds in recent years, allowing researchers
to create images from relatively small sets of data.
The results speak for themselves. The team set up the new
camera on the 20th floor of a building on Chongming Island in Shanghai and
pointed it at the Pudong Civil Aviation Building across the river, some 45 km
away.
That’s also significantly better than the conventional
diffraction limit of 1 meter at 45 km, and certainly better than other recently
developed algorithms. The image here shows the potential of the technique in images
taken in daylight from a distance of 21 km. ”Our results open a new venue for
high-resolution, fast, low-power 3D optical imaging over ultralong ranges,” say
Zheng-Ping and co.
That’s interesting work that has a wide range of
applications. The team mention remote sensing, airborne surveillance, and
target recognition and identification. Indeed, the entire device is about the
size of a large shoebox and so is relatively portable.
And Zheng-Ping and co say it can be significantly improved.
“Our system is feasible for imaging at a few hundreds of kilometers by refining
the setup, and thus represents a significant milestone towards rapid,
low-power, and high-resolution LiDAR over extra-long ranges,” they say.
So keep smiling—they may be watching.
Ref: arxiv.org/abs/1904.10341 :
Single-Photon Computational 3D Imaging at 45 km
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