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Comment by easygenes | Hacker News Reader

Good explanation. One detail though: it is one pixel at a time, not one line at a time. Basically does the whole sequence for one pixel, adjusts mirror to next one, and does it again. The explanation is around the 8 minutes mark.

Just want to make it clear that in any one instant, only one pixel is being recorded. The mirror moves continuously across a horizontal sweep and a certain arc of the mirror's sweep is localized to a pixel in the video encoding sequence. A new laser pulse is triggered when one pixel of arc has been swept, recording a whole new complete mirror bounce sequence for each pixel sequentially. He has an additional video explaining the timing / triggering / synchronization circuit in more depth: https://youtu.be/WLJuC0q84IQ

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One piece I'd like to see more clarification on is, is he doing multiple samples per pixel (like with ray tracing?). For his 1280x720 resolution video, that's around 900k pixels, so at 30Khz, it would take around 30s to record one of these videos if he were to doing one sample per pixel. But in theory he could run this for much longer and get a less noisy image.

I find it interesting that a project like this would easily be a PhD paper, but nowadays Youtubers do it just for the fun of it.

You should check the other channel by the same person, where he goes into more details about the system: https://www.youtube.com/@BetaPhoenixChannel

From what I remember, recording one frame took about an hour.

It's humbling how well-rounded Brian (and other Youtubers such as Applied Science and StuffMadeHere, HuygensOptics) is on top of clearly being a skillful physicist: electronics, coding, manufacturing, ... and the guy is _young_ compared to the seasoned professionals I mentioned in the parentheses.

Yea, he’s recording several thousand samples per pixel. That’s how it becomes a video instead of a snapshot.

Check out his previous video <https://www.youtube.com/watch?v=IaXdSGkh8Ww> for more details about that part.

In one of the appendix videos he mentions that would improve the noise, the issue is the data rate exporting from the scope is a bottleneck and it would slow things down even more.

If he randomized the position with blue noise or something he could use compressed sensing or ai denoising for many less samples. The raw image wouldn't be as good but by the time it is compressed it should much better for the same sample count. It might not be as easy to move the mirror in both axes between each sample though.

edit: saw below he is using a continuous scan so randomizing it probably wouldn't be workable

And the reason it matters that this is a single pixel at two billion times per second is that we can hypothetically stack many of these assemblies on top of each other and get video of a single event that is not repeatable.

What you've invented there is a camera sensor :) Silicon photomultipliers do exist and are used in some LIDAR applications. The bigger problem would be creating the 921600-channel oscilloscope to capture all this raw data.

Yes and no. The mirror is continually scanning horizontally because the mechanics simply can't position that accurately. This is fine, really because the amount the mirror moves in 33uS is essentially zero. He's still capturing single pixels, just while the mirror is moving. Any artifacts from the motion are probably entirely drowned out by the noise in the rest of the system

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