https://spectrum.ieee.org/non-line-of-sight-infrared
Just because an object is around a corner doesn't mean it has to be hidden. Non-line-of-sight imaging can peek around corners and spot those objects, but it has so far been limited to a narrow band of frequencies. Now, a new sensor can help extend this technique from working with visible light to infrared. This advance could help make autonomous vehicles safer, among other potential applications.
Non-line-of-sight imaging relies on the faint signals of light beams that have reflected off surfaces in order to reconstruct images. The ability to see around corners may prove useful for machine vision—for instance, helping autonomous vehicles foresee hidden dangers to better predict how to respond to them, says Xiaolong Hu, the senior author of the study and a professor at Tianjin University in Tianjin, China. It may also improve endoscopes that help doctors peer inside the body.
The light that non-line-of-sight imaging depends on is typically very dim, and until now, the detectors that were efficient and sensitive enough for non-line-of-sight imaging could only detect either visible or near-infrared light. Moving to longer wavelengths might have several advantages, such as dealing with less interference from sunshine, and the possibility of using lasers that are safe around eyes, Hu says.
Now Hu and his colleagues have for the first time performed non-line-of-sight imaging using 1,560- and 1,997-nanometer infrared wavelengths. "This extension in spectrum paves the way for more practical applications," Hu says.
In the new study, the researchers experimented with superconducting nanowire single-photon detectors. In each device, a 40-nanometer-wide niobium titanium nitride wire was cooled to about 2 kelvins (about –271 °C), rendering the wire superconductive. A single photon could disrupt this fragile state, generating electrical pulses that enabled the efficient detection of individual photons.
The scientists contorted the nanowire in each device into a fractal pattern that took on similar shapes at various magnifications. This let the sensor detect photons of all polarizations, boosting its efficiency.
The new detector was up to nearly three times as efficient as other single-photon detectors at sensing near- and mid-infrared light. This let the researchers perform non-line-of-sight imaging, achieving a spatial resolution of roughly 1.3 to 1.5 centimeters.
Journal Reference:
Yifan Feng, Xingyu Cui, Yun Meng, Xiangjun Yin, Kai Zou, Zifan Hao, Jingyu Yang, and Xiaolong Hu, "Non-line-of-sight imaging at infrared wavelengths using a superconducting nanowire single-photon detector," Opt. Express 31, 42240-42254 (2023) https://doi.org/10.1364/OE.497802
(Score: 3, Interesting) by VLM on Friday February 09, @06:52PM
Probably your best single web page demonstration of NLOS would be
https://www.cs.princeton.edu/~fheide/steadystatenlos [princeton.edu]
And there's a pretty decent "Nature Reviews Physics" journal article from a couple years ago on the topic.
The Princeton one boils down to something like they're not tracking the traffic signs so much as the retroreflector stuff in the paint that makes the signs reflect causes speckles that you can reverse engineer into individual 3-d spots. It doesn't reverse engineer a perfect first surface mirror very well, but it will reverse engineer the likely locations and colors of a couple hundred twinkling multi-colored sand grains. Which was originally deployed on road signs to make them brighter at night when illuminated by headlights, but turns out to make them pretty easy to see with NLOS.
The interesting side effect is you can read a red stop sign but in IR I imagine it would just be a peculiarly sized and shaped octagon so going IR might not be super helpful.
A not terribly bad analogy for optical NLOS would be post-80s era over the horizon bistatic radar. Another not entirely awful analogy would be synthetic aperture radar but passive. There are some pretty cool papers on passive SAR which are kind of related-ish to this optical system. There's an IEEE paper about using PSAR to watch the echo of the sun off the floor of a desert and they localized the position of the sun pretty well by looking at the weird speckle reflections off the desert floor around 300 MHz IIRC, this optical NLOS is roughly the same end goal and vaguely the same idea but somewhat higher frequency LOL.