[HN Gopher] A laser pointer at 2B FPS [video]
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A laser pointer at 2B FPS [video]
Author : thunderbong
Score : 114 points
Date : 2025-10-19 06:42 UTC (1 days ago)
(HTM) web link (www.youtube.com)
(TXT) w3m dump (www.youtube.com)
| mjmas wrote:
| 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.
| Neywiny wrote:
| 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.
| dist-epoch wrote:
| Techniques like this are/were used to film nuclear explosions
| (but with a single explosion).
| 01HNNWZ0MV43FF wrote:
| Scanning a single pixel over an image? How does that work with
| an explosion? The laser pointer is reproducible
| bob1029 wrote:
| https://en.wikipedia.org/wiki/Rapatronic_camera
| snitty wrote:
| But that bears no relation to what happened in the video.
| masfuerte wrote:
| The rapatronic camera had an incredibly fast electronic
| shutter. To record a video they needed one camera per
| frame. Rather like "bullet time" in the movies. The
| technique is the youtube video is completely different.
| throw327489 wrote:
| Who detonated 2073600 bombs?
| jfengel wrote:
| Ah, two billion. The first several times I saw this it looked
| like "twenty eight", which didn't seem terribly interesting.
| MostlyStable wrote:
| 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
| snitty wrote:
| >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.
|
| It is not different phases, but it is a composite! On his
| second channel he describes the process[0]. Basically, it's a
| photomultiplier tube (PMT) attached to a precise motion control
| rig and a 2B sample/second oscilloscope. So he ends up
| capturing the actual signal from the PMT over that timespan at
| a resolution of 2B samples/s, and then repeating the experiment
| for the next pixel over. Then after some DSP and mosaicing, you
| get the video.
|
| >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?
|
| The point here isn't to measure the speed of light, and my
| general response when someone asks "can I get around physics
| with this trick" by answer is no. But I'd be lying if I said I
| totally understood your question.
|
| [0] https://www.youtube.com/watch?v=-KOFbvW2A-o
| avidiax wrote:
| 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".
| ahofmann wrote:
| But I think he does capture an entire line quite quickly. As I
| understood it, he "scans" a line of pixels in seconds.
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