A laser pointer at 2B FPS [video]

Original Article ↗ Hacker News Discussion ↗

How the setup works

  • Uses a photomultiplier tube as a 1‑pixel “camera” and a mirror scanning system to select which scene pixel is observed.
  • A pulsed laser fires repeatedly through smoke; scattered photons from each pulse are measured at 2 GS/s by an oscilloscope for about a microsecond.
  • The mirror slowly scans so that each pulse corresponds to a different pixel. The system repeats the scene hundreds of thousands of times, gathering a tiny 1‑pixel time‑series per location.
  • These time‑series are then mosaiced into a 1280×720 video showing light propagating.

“2 Billion FPS” vs pixels‑per‑second

  • Several commenters stress it’s really 2 billion samples per second per pixel, not 2 billion full‑frame images per second.
  • Others argue a “frame” can be any resolution, even 1×1, so calling it 2 billion FPS is defensible if you accept that definition.
  • Consensus: the final clip is a composite of ~900k individually recorded 1‑pixel videos of repeatable events; understanding that distinction matters more than the label.

Oscilloscope triggering and instrumentation

  • Discussion around the clever trigger multiplexing hack to reach 2 GS/s on a low‑cost scope that otherwise limits full‑rate dual‑channel triggering.
  • Some are surprised the scope lacks a dedicated external trigger; others note many budget DSOs do.
  • The approach is likened to (but distinguished from) equivalent‑time sampling: the scope itself is in real‑time mode, while the overall system uses repetition to step through the scene.

Relation to other imaging methods

  • Compared to rapatronic cameras for nuclear tests: those used ultra‑fast shutters and multiple cameras, capturing single, non‑repeatable events; this project instead repeats a controllable event and scans one pixel at a time.
  • Parallels are drawn to LIDAR and time‑of‑flight depth cameras; in principle such hardware could demonstrate similar effects.

Physics and conceptual questions

  • Several comments clarify this does not evade the one‑way speed of light problem; path length and synchronization issues remain fundamental.
  • Requests to apply this to double‑slit experiments are addressed: the scattering needed for imaging is already a measurement, so it won’t reveal hidden “in‑flight” quantum behavior.

Proposed improvements and practical limits

  • Suggestions: use galvo mirrors or spinning mirrors, better master clocks, and time‑averaging for noise reduction.
  • Main bottleneck is data readout from the oscilloscope; full frames already take on the order of an hour to acquire.
  • Clarifications on focus: small angular acceptance per pixel and depth of field keep the image sharp despite mirror motion.