New paper on #ArXiv!
"Tracking moving objects through scattering media via speckle correlations"
With @YJaureguiS, and Harry Penketh.

Short(?) thread explaining what it is about.
1/
arxiv.org/abs/2202.10804
Scattering is a major problem for imaging, and it doesn't take very much of it before we can't see essentially anything of what it is happening.
Not surprisingly, imaging in the presence of scattering is a very active field of research.
2/
There isn't a single best way on how to deal with scattering, and the answer depends a LOT on how much scattering we are talking about and its properties.
As a rule of thumb, the most complicated situation is where all light is multiply scattered.
3/
That said, linear scattering can't destroy the information, it "just" scrambles it. The result is that the scattered light forms complex patterns, which might look random but contain a lot of correlations where the information is hidden.
Which gives us some hope.
4/
An approach that has attracted some attention is based on a correlation that links the angle of incidence of the incoming wave, with the output angle of the scattered light, called the "optical memory effect".
5/
To make the long story short, this correlation allows you to measure the convolution between the (unknown) object to be imaged, and the (unknown) speckle pattern generated by the scattering medium.
6/
This by itself is not very useful (too many unknowns) but, as realized by Labeyrie in the '70s, performing an autocorrelation one can separate the information about the object from the information about the speckle.
7/
The only remaining problem is that we are left with the autocorrelation of the object, not an image of the object itself.
This autocorrelation can be numerically inverted to obtain the desired image, but this is a slow and computationally intensive process.
8/
What if things are moving? In principle one can perform an autocorrelation inversion per each frame, but this needs to be done in post-processing (too slow to do otherwise), making real-time tracking complicated.
9/
What if we are more interested in the tracking itself than in the real-time imaging? (i.e. we care more about knowing how the stuff in the scene moved than the precise look of the scene)
In that case we show that there is a simple and computationally inexpensive solution!
10/
Instead of autocorrelating the light collected at each frame, we cross-correlate the light detected at different times, and perform the difference between two such correlations (to subtract all the extraneous signal)
[Showing here a simulation. The experiment is in the paper]
11/
This results in a easy-to-compute video where the moving object is fixed in the middle, and the background moves around it, showing clearly what it is happening.
[Technical note: This allows for tracking well beyond the memory effect range.]

The end 🙂
12/12
Paper accepted! 🥳

• • •

Missing some Tweet in this thread? You can try to force a refresh
 

Keep Current with Jacopo Bertolotti

Jacopo Bertolotti Profile picture

Stay in touch and get notified when new unrolls are available from this author!

Read all threads

This Thread may be Removed Anytime!

PDF

Twitter may remove this content at anytime! Save it as PDF for later use!

Try unrolling a thread yourself!

how to unroll video
  1. Follow @ThreadReaderApp to mention us!

  2. From a Twitter thread mention us with a keyword "unroll"
@threadreaderapp unroll

Practice here first or read more on our help page!

More from @j_bertolotti

Jul 18
#PhysicsFactlet (335)
Yesterday, at a small playground where my son was playing, I saw this Kugel fountain, so here comes a short thread about Kugel fountains and how they work.
🧵 1/

(Alt Text: a Kugel fountain slowly rotating in a sunny day.)
First of all, what is a Kugel fountain?
There are a few variations on the theme, but usually they are big stone spheres, sitting on a hemispherical hole, with water flowing from below. Despite their weight, they can spin with a small push, and keep spinning for a long time.
2/
How does it work?
It can't be buoyancy, as the stone sphere is a a LOT more dense than the water (we all have direct experience of stones sinking when you put them in water, and this one is not any different).
3/
Read 10 tweets
May 13
#PhysicsFactlet (331)
"Anderson localization" is a weird phenomenon that is not well known even among Physicists, but has the habit of popping up essentially everywhere.
An introductory thread 🧵
1/
The idea of "localization" originally came about as an explanation (by P.W. Anderson, hence the name) of why the spins in certain materials did not relax as fast as expected.
nobelprize.org/prizes/physics…
2/
What Anderson realized was that when you have a wave (in this case a quantum mechanical wavefunction) that propagates in a random system, interference can play a major role, and potentially impede propagation completely.
journals.aps.org/pr/abstract/10…
(Paywalled)
3/
Read 19 tweets
Jan 3
#TheLongRoadToLearnSomethingNew
I decided it is high time I learn something about machine learning. I couldn't care less about learning how to use Tensorflow or any other package that do machine learning for you. I "just" want a Physicist's intuition for how and why it works.
1/
A million years ago I asked here for advices on resources. Some were very good advices, some were not. But I am mow armed with a textbook, and will irregularly update here on my progresses.
2/
I am not very far on my path. I've read a few online resources and (so far) the first 30 pages of this book.
I am aware machine learning is a HUGE topic, so I will begin by concentrating on neural networks (and probably a sub-sub-class of neural networks).
3/ Image
Read 11 tweets
Dec 31, 2021
#PhysicsFactlet The ones I am most proud of from 2021 (in chronological order)
A visualization of what an eigenvector is (at least for 2x2 matrices)
Pulse chirping (keeping the pulse duration constant for ease of visualization, although in reality one usually keeps the bandwidth constant)
Read 8 tweets
Dec 10, 2021
#PhysicsFactlet (308)
There are not many problems in Physics that can be solved exactly, so we tend to rely on perturbation theory a lot. One of the problems with perturbation theory is that infinities have the bad habit of popping up everywhere when you use it.
(A thread 1/ )
If you know anything about Physics you are probably thinking about quantum field theory and all the nasty infinities that we need to "renormalize". But quantum field theory is difficult, so let's look at a MUCH simpler problem: the anharmonic oscillator.
2/
Disclaimer: I can't know how much you (the reader) know about this. For some of you this thread will be full of obvious stuff. For others there will be so many missing steps to be hard to follow. I will do my best, but I apologize with everyone in advance.
3/
Read 24 tweets
Nov 23, 2021
In celebration of 10k followers, here is a new edition of "people you should follow, but that (given their follower count) probably you don't".
i.e. people I follow, with <5k followers, non-locked, active, that in my personal opinion you should follow too.
1/
In random order:
@LCademartiriLab food, chemistry, architecture, and beauty in general. Trigger warning: strong opinions.
@VKValev bit of history of Physics + chiral media
@DrBrianPatton social justice in science
@alisonmartin57 weaving and bamboo structures
2/
@VergaraLautaro history of Physics
@PKoppenburg LHCb
@OptoLia optics, entrepreneurship.
@BrunoLevy01 fluid simulations
@lisyarus physics-based graphics coding
@RobJLow mathematics and education
@bruko Photography (and overall great human being)
3/
Read 8 tweets

Did Thread Reader help you today?

Support us! We are indie developers!


This site is made by just two indie developers on a laptop doing marketing, support and development! Read more about the story.

Become a Premium Member ($3/month or $30/year) and get exclusive features!

Become Premium

Don't want to be a Premium member but still want to support us?

Make a small donation by buying us coffee ($5) or help with server cost ($10)

Donate via Paypal

Or Donate anonymously using crypto!

Ethereum

0xfe58350B80634f60Fa6Dc149a72b4DFbc17D341E copy

Bitcoin

3ATGMxNzCUFzxpMCHL5sWSt4DVtS8UqXpi copy

Thank you for your support!

Follow Us on Twitter!

:(