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A laser pointer at 2B FPS [video]

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

#71

Post by Doxin »

gzalo wrote: post #35
skopje wrote: post #21

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!.

Hmm, it's a clever hack, but they would use an oscilloscope with an "External trigger" input, like most of the older Rigols. That would let you use the full sample rate without needing to trigger from CH2

Even modern entry level Rigol scopes have external trigger inputs. I've got like one step up from the cheapest model and it has an external trigger input. I think the idea there is that you'd use a bunch of these scopes for the QA on an assembly line. there's a bunch of pass/fail features I've never once touched too.

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

#72

Post by Tuna-Fish »

bjt12345 wrote: post #68

Could redshift/blueshift explain why the light appeared to move at different velocity when he moved the camera to another position?

No. It's strictly because of travel time from the "observed spot" to the camera. He explains it in the back half of the video, but for his setup (and every camera everywhere, including your eyes!), it doesn't matter just how long light took to get to the point where it scatters, but also how long it takes for it to get from that point to the detector. If the camera is placed far enough to the side, that time delay is close enough to identical for all samples, but if the camera is in line with the laser, there is a substantial time difference for light scattering close to the camera to light scattering near the far wall.

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

#73

Post by Intermernet »

bjt12345 wrote: post #68

Could redshift/blueshift explain why the light appeared to move at different velocity when he moved the camera to another position?

He actually explains it. It's due to the round trip time increasing from further away water droplets when filmed at an oblique angle. Light hitting a droplet 3m away has a 3x times longer round trip than light hitting a droplet 1m away.

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

#74

Post by slow_typist »

kqr wrote: post #62
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…

Depth of field, small aperture

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

#75

Post by franga2000 »

kqr wrote: post #60
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…

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.

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

#76

Post by PatronBernard »

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…

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.

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

#77

Post by mewpmewp2 »

ssl-3 wrote: post #40
gpm wrote: post #39

Earlier quoted context omitted.

> Unless we're severely distorting the definition of a "video frame" to also include "one image in a series of images What is your definition of "video frame" if not this? > that can be as small as one pixel," Why would this be a criteria on the images? If it is, what is the minimum resolution to count as a video frame? Must I have at least two pixels for some reason? Four so that I have a grid? These seem like weird…

We already have a useful and established term with which to describe just one pixel from a video frame. That term is "pixel".

So what if he captured 2 pixels at a time, would that constitute enough to be a definition of a frame?

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

#78

Post by teaearlgraycold »

brcmthrowaway wrote: post #51

How is the image focused and not a big blur?

It’s in focus because only a small angular area from the scene contributes to the light that reaches the sensor. You get blur when sensor elements (individual pixels) receive light from too wide of an area.

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

#79

Post by TheOtherHobbes »

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.

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

#80

Post by db48x »

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…

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.

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