by estimator7292
7 subcomments
- Tl:dw for how this works:
He scans one line at a time with a mirror into a photomultiplier tube which can detect single photon events. This is captured continually at 2MSample/s (2 billion times per second: 2B FPS) with an oscilloscope and a clever hack.
The laser is actually pulsing at 30KHz, and the oscilloscope capture is synchronized to the laser pulse.
So we consider each 30KHz pulse a single event in a single pixel (even though the mirror is rotating continuously). So he runs the experiment 30,000 times per second, each one recording a single pixel at 2B FPS for a few microseconds. Each pixel-sized video is then tiled into a cohesive image
- This video has brough warm and fuzzy memories from my other life. When I was a scientist back in USSR my research subject required measuring ridiculously low amounts of light and I used photomultiplier tube in photon counting mode for that. I needed current preamp that can amplify nanosecond long pulses and have concocted one out of arsenide-gallium logic elements pushed to work in a linear mode. The tube was cooled by Peltier elements and data fed to a remote Soviet relative of Wang computer [0].
OMG this was back in 1979-1981.
0. - https://ru.wikipedia.org/wiki/%D0%AD%D0%BB%D0%B5%D0%BA%D1%82...
- The original MIT video from 2011: "Visualizing video at the speed of light — one trillion frames per second" https://youtu.be/EtsXgODHMWk (project site: https://web.media.mit.edu/~raskar/trillionfps/)
He mentions this as the inspiration in his previous video (https://youtu.be/IaXdSGkh8Ww).
by TheOtherHobbes
1 subcomments
- Insanely clever and impressive.
Some possible improvements.
1. Replace the big heavy mirror with a pair of laser galvos. They're literally designed for this and will be much faster and more precise.
Example:
https://miyalaser.com/products/miya-40k-high-performance-las...
2. Increase the precision of the master clock. There's some time smearing along the beam. It's not that hard to make clocks with nanosecond resolution, and picosecond resolution is possible, although it's a bit of a project.
3. As others have said, time-averaging multiple runs would reduce the background noise.
- The triggering scheme is completely brilliant. One of those cases where not knowing too much made it possible, because someone who does analog debug would never do that (because they would have a 50k$ scope!.
- The view from one end of a laser going between two mirrors (timestamp 1:37) is a fairly good demonstration of the camera having to wait for light to get to it.
- Ah, two billion. The first several times I saw this it looked like "twenty eight", which didn't seem terribly interesting.
- I thought his method of multiplexing the single channel was very smart. I guess it's more common on 2 channel or high end 4 channel scopes to have a dedicated trigger input, which I've checked this one doesn't have. That said, there're digital inputs that could've been used. Presumably from whatever was controlling the laser.
by MostlyStable
5 subcomments
- As I understand it, this is sort of simulating what it would be like to capture this, by recreating the laser pulse and capturing different phases of it each time, then assembling them; so what is represented in the final composite is not a single pulse of the laser beam.
Would an upgraded version of this that was actually capable of capturing the progress of a single laser pulse through the smoke be a way of getting around the one-way speed of light limitation [0]? It seems like if you could measure the pulse's propagation in one direction, and the other (as measured by when it scatters of the smoke at various positions in both directions), this seems like it would get around it?
But it's been a while since I read an explanation for why we have the one-way limitation in the first place, so I could be forgetting something.
[0] https://en.wikipedia.org/wiki/One-way_speed_of_light
- I'd like to see this with the double slit experiment
by brcmthrowaway
2 subcomments
- Did he actually repeat the experiment 1280x720 times for every pixel?
- Could redshift/blueshift explain why the light appeared to move at different velocity when he moved the camera to another position?
- light doesn't move at a fixed rate
light moves AT the speed of causality in that frame of time
causality appears to have a MAXIMUM limit in this universe in an "empty" void
but every time you hear a story of how they "slowed down light" what they actually did is make causality more complex in a dense medium, so slower
by ky_vulnerable
0 subcomment
- Cameras really have come a long way since the old 15 FPS movies.
by brcmthrowaway
0 subcomment
- How did he determine the principal point to swing around?
by brcmthrowaway
2 subcomments
- How is the image focused and not a big blur?
- [dead]
by dist-epoch
2 subcomments
- Techniques like this are/were used to film nuclear explosions (but with a single explosion).
- He did a good job on his setup, but I have to think that adding a spinning mirror would have made everything much faster and easier.
He could then capture an entire line quite quickly, and would only need a 1 dimensional janky mirror setup to handle the other axis. And his resolution in the rotating axis is limited only by how quickly he can pulse the laser.
Of course, his janky mirror setup could have been 2 off-the-shelf galvos, but I guess that isn't as much "content".
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