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A laser pointer at 2B FPS [video]

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615 points thunderbong | 3 comments | 19 Oct 25 06:42 UTC | HN request time: 0.022s | source
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estimator7292 ◴[21 Oct 25 01:15 UTC] No.45651413[source]▶
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 pixel (even though the mirror is rotating continuously). So he runs the experiment 30,000 times per second, each one recording a single pixel at 2B FPS for a few microseconds. Each pixel-sized video is then tiled into a cohesive image

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userbinator ◴[21 Oct 25 05:28 UTC] No.45652745[source]▶
I believe this technique is known as "equivalent-time sampling".
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1. generuso ◴[21 Oct 25 05:47 UTC] No.45652860[source]▶
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-...

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2. easygenes ◴[21 Oct 25 10:42 UTC] No.45654369[source]▶
I think parent meant that the image construction technique is analogous to equivalent time sampling. You’re correct in the mode of the oscilloscope’s use. However, the mode of the larger system is using a repetitive signal and sliding sampling points across it.
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3. ◴[21 Oct 25 17:29 UTC] No.45658678[source]▶