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A laser pointer at 2B FPS [video]

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

#61

Post by m-ee »

nerdsniper wrote: post #56
monster_truck wrote: post #22

Earlier quoted context omitted.

Does anyone have a $50,000 scope they could just give to this dude? He seems like he would make great use of it.

Honestly I think if we each wrote a nice personal letter to Keysight they’d probably gift him one in exchange for the YouTube publicity. Several other electrical engineers on YT get free $20-50k keysight scopes not just for themselves, but once a year or so to give away to their audience members. And yes, this person could make use of it. His videos are among the highest quality science explainers - he’s like the 3B1…

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.

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

#62

Post by kqr »

brcmthrowaway wrote: post #51

How is the image focused and not a big blur?

Shere are so many levels this could be answered at.

All light in a narrow cone extending from the camera gets recorded to one pixel, entirely independently from other pixels. There's no reason this would be blurry. Blur is an artifact created by lenses when multiple pixels interact.

There is a lens in the apparatus, which is used to project the image from the mirror onto the pinhole, but it is configured so the plane of the laser is in focus at the pinhole.

What I don't understand is how the projection remains in focus as the mirror sweeps to the side, but perhaps the change in focus is so small.

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

#63

Post by userbinator »

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…

I believe this technique is known as "equivalent-time sampling".

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

#64

Post by nerdsniper »

m-ee wrote: post #61
nerdsniper wrote: post #56

Earlier quoted context omitted.

Honestly I think if we each wrote a nice personal letter to Keysight they’d probably gift him one in exchange for the YouTube publicity. Several other electrical engineers on YT get free $20-50k keysight scopes not just for themselves, but once a year or so to give away to their audience members. And yes, this person could make use of it. His videos are among the highest quality science explainers - he’s like the 3B1…

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?

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

#65

Post by generuso »

userbinator wrote: post #63
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…

I believe this technique is known as "equivalent-time sampling".

The author uses "real time sampling" to acquire evolution of light intensity for one pixel at 2 GSps rate. The signal is collected for approximately one microsecond at each firing of the laser, and corresponding digital data is sent from the oscilloscope to the computer.

"Equivalent time sampling" is a different technique which involves sliding the sampling point across the signal to rebuild the complete picture over multiple repetitions of the signal.

https://www.tek.com/en/documents/application-note/real-time-...

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

#66

Post by generuso »

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…

The author explained that he originally attempted to pulse the laser at 30 KHz, but for the actual experiment used a slower rate of 3 KHz. The rate at which the digital data can be read out from the oscilloscope to the computer seems to be the main bottleneck limiting the throughput of the system.

Overall, recording one frame took approximately an hour.

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

#67

Post by generuso »

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…

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.

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

#69

Post by generuso »

nerdsniper wrote: post #56
monster_truck wrote: post #22

Earlier quoted context omitted.

Does anyone have a $50,000 scope they could just give to this dude? He seems like he would make great use of it.

Honestly I think if we each wrote a nice personal letter to Keysight they’d probably gift him one in exchange for the YouTube publicity. Several other electrical engineers on YT get free $20-50k keysight scopes not just for themselves, but once a year or so to give away to their audience members. And yes, this person could make use of it. His videos are among the highest quality science explainers - he’s like the 3B1…

What this experiment does is very similar to how an ordinary LIDAR unit operates, except that during a LIDAR scan the laser and the receiver are always pointed in the same direction, while in this demonstration the detector is scanning the room while the laser is stationary and is firing across the room.

But in principle, a LIDAR could be reconfigured for the purposes of such demonstration.

If one wants to build the circuit from scratch, then specifically for such applications there exist very inexpensive time-to-digital converter chips. For example, Texas Instruments TDC7200 costs just a few dollars and has time uncertainty of some tens of picoseconds.

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

#70

Post by generuso »

brcmthrowaway wrote: post #27

Did he actually repeat the experiment 1280x720 times for every pixel?

Yes, the laser was fired at 3 KHz, while the mirrors were slowly scanning across the room.

For each laser pulse, one microsecond of the received signal was digitized with the sample rate of 2 billion samples per second, producing a vector of light intensity indexed by time.

A large number of vectors were stored, each tagged by the pixel XY coordinates which were read out from the mirror position encoders. In post-processing, this accumulated 3D block of numbers was sliced time-wise into 2D frames, making the sequence of frames for the clip.

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