Photographing on the Ground in the distance — a difficult task. To catch enough light from the object at a large distance is not easy. And even then, the atmosphere will introduce distortion that can spoil the; pollution very disturbing, especially in the cities. So any picture from the distance of a few kilometers or so (assuming the camera is fixed high enough to cope with the curvature of the Earth) is very difficult to do.
However, in recent years scientists began to use sensitive photodetectors, which are much better able to cope with this task. These detectors are so sensitive they can detect individual photons and combine them into images at a distance of 10 kilometers.
What is the maximum range the camera?
Obviously physics will never rest on our laurels. Recently Zheng-Ping Li and his colleagues from the University of science and technology of China in Shanghai showed how to photograph objects at distances up to 45 kilometers in smog-struck urban environment. Their method uses detectors of individual photons combined with a unique computational algorithm that allows to obtain images of very high resolution, combining the scattered data points.
A new technique is relatively simple in principle. It is based on the method of lidar — LIDAR (Light Identification Detection and Ranging): “detection, identification and range finding by means of light” — illuminates the object with laser light and then creates an image from reflected light.
The great advantage of this kind of active visualization is that reflected from the object, the photons return to the detector within a certain time window, which depends on the distance. Therefore, any photons arriving outside of this window can be ignored.
Such a filter drastically reduces the noise that is created by unwanted photons from the environment. And this allows the lidar system to be very sensitive and focused on the distance.
To make the new system work even better in an urban environment, Zheng-Ping and his colleagues used infrared laser with a wavelength of 1550 nm, a repetition rate of 100 kHz and a moderate power of 120 milliwatts. This wavelength makes the system safe for the eyes and allows the team to filter out the sun’s photons, which otherwise could overload the detector.
Researchers send and receive the photons through a single optical device — a common astronomical telescope with the lens 280 mm. the Reflected photons are then recorded in a commercial detector of single photons. To create a snapshot, the scientists scan the field of view using Besobrasova mirror which can be rotated up, down, left and right.
Thus, they can create a two-dimensional image. But by controlling the time of arrival of photons they can capture the photons reflected from different distances, to create three-dimensional images.
The last achievement that was achieved is to develop an algorithm that collects the image together, using single-photon data. This kind of computational imaging has come a long way in recent years, allowing scientists to create images of relatively small data sets.
The results speak for themselves. The team installed a new camera on the 20th floor of a building on the island of Chongming in Shanghai and sent it to the building of civil aviation Pudong across the river, 45 kilometers away.
A conventional image obtained through a telescope, did not show anything but noise. But a new technique allows to obtain images with a spatial resolution of 60 cm, on which you can make out the Windows of the building. “This result demonstrates the excellent ability of the LiDAR system single photon near-infrared spectrum to distinguish goals through managed,” say the researchers.
Should imply that the main use of this system is surveillance, recognition and identification purposes. In addition, the device was no larger than box of shoes, is relatively portable.