A laser pointer at 2B FPS [video] • Hacker News Board

Re: A laser pointer at 2B FPS [video]

#111

Post by layla5alive » Wed, Oct 22, 2025, 2:32 AM UTC

estimator7292 wrote: post #33

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 p…

Something is off here, 2MSamples/sec wouldn't be 2 billion times per second, that would be 2GSamples/sec

Re: A laser pointer at 2B FPS [video]

#112

Post by layla5alive » Wed, Oct 22, 2025, 2:43 AM UTC

ssl-3 wrote: post #38

tredre3 wrote: post #23

Earlier quoted context omitted.

The video is definitely more interesting than 28 fps but it's also not really 2B fps. It captures two billion pixels per second. Essentially he captures the same scene several times (presumably 921,600 times to form a full 720 picture), watching a single pixel at a time, and composite all the captures together for form frames. I suppose that for entirely deterministic and repeatable scenes, where you also don't care…

Others say that you're wrong, but I think you're describing it approximately perfectly. As you say: It does capture two billion pixels per second. It does watch a single pixel at a time, 921,600 times. And these pixels [each individually recorded at 2B FPS] are ultimately used to create a composition that embodies a 1280x720 video. That's all correct. And your summary is also correct: It definitely does not really ca…

Quite right, he's doing 2B pixels per second @ 1x1, and effectively many minutes per 720p frame, using a rolling shutter.

Re: A laser pointer at 2B FPS [video]

#113

Post by m-ee » Wed, Oct 22, 2025, 4:15 AM UTC

nerdsniper wrote: post #64

m-ee wrote: post #61

Earlier quoted context omitted.

Keysight is not very hobbyist friendly these days. A year or two ago it broke on the Eevblog forums that they were refusing to honor warranties/service contracts unless you had a corporate account with them. If you were just a guy with a scope you would be SOL.

Do you have any better recommendations?

There was a Rohde and Schwarz rep that was pretty active on those forums when I still paid attention to it, might count for something.

Hard to justify going with the legacy brands as hobbyist with what comes out of China these days. Rigol and Sient make very capable hardware although h the UI can be a bit painful if you’re used to something else. Micsig produces very capable probes for a small fraction of the cost of the legacy players even high end specialty stuff like optically isolated probes. Those are still thousands but try pricing the equivalent from tek or Agilent, it would buy you a sedan.

Re: A laser pointer at 2B FPS [video]

#114

Post by ssl-3 » Wed, Oct 22, 2025, 6:34 AM UTC

layla5alive wrote: post #112

ssl-3 wrote: post #38

Earlier quoted context omitted.

Others say that you're wrong, but I think you're describing it approximately perfectly. As you say: It does capture two billion pixels per second. It does watch a single pixel at a time, 921,600 times. And these pixels [each individually recorded at 2B FPS] are ultimately used to create a composition that embodies a 1280x720 video. That's all correct. And your summary is also correct: It definitely does not really ca…

Quite right, he's doing 2B pixels per second @ 1x1, and effectively many minutes per 720p frame, using a rolling shutter.

Well... not quite a rolling shutter.

It takes >900k pixels to shoot one frame of that (amazing) video, and requisitioning each of those pixels required physically moving a mirror along X and Y to align the single-pixel camera properly.

There isn't really a shutter at all, whether mechanical or electrical. And from my understanding, a "rolling shutter" usually refers to things like reading out a CCD array or similar, or maybe some mechanical aspect of a film camera.

But this isn't an array of anything. It's just a pixel, and some very clever work with motors, lenses, and mirrors.

(Up next: Someone will show up to tell me that an array of 1 item is still an array. yawn)

Re: A laser pointer at 2B FPS [video]

#115

Post by cma » Wed, Oct 22, 2025, 8:44 PM UTC

ehsankia wrote: post #55

easygenes wrote: post #50

Earlier quoted context omitted.

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 local…

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 ea…

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