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Comment by theodorethomas | Hacker News Reader

I wondered if a city-wide synchronised Pulse-Width-Modulation of LED lights could allow astrophotography (or even naked-eye astronomy) to be possible again, with a filter synchronised to the off-cycle.

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Using PWM to dim lights should be banned wholesale. Many people, me included, get crazy headaches from the worst lights and mild discomfort from the best ones. It's also completely unnecessary as non-flickering bulbs do exist and are readily available for the same price.

Those are probably 50-60 hz flickering from the mains frequency rather than PWM. (Or 100-120 hz due to the rectifier). Actual PWM ICs drive LEDs over 10,000 hz, which is truly imperceptible to the human eye.

Surely a PWM of 10,000Hz wouldn't allow for night sky photography. With full exposure it can already take minutes of hours to take a night sky a photo.

And it's usually recognized that 1,000hz is the bare minimum to spare sensitive individuals from symptoms, with 2,500 usually used for safety margin. Then you have to account for some jitter in the synchronized network so you're looking at at best a 1/5,000 exposure.

Is that sufficient to be worth the trouble?

You can still get a long exposure by accumulating many 1/5,000 exposure shots together. Just like a regular long exposure, this will still cancel out random noise and improve SNR, and also still has the tradeoff of motion blur. This kind of accumulation is commonly done in photography-adjacent technologies like spectroscopy.

Regardless, I agree with your larger point that the proposed system would be entirely impractical!

> Actual PWM ICs drive LEDs over 10,000 hz, which is truly imperceptible to the human eye.

But then you can get capacitors or inductors emitting a very annoying 10 kHz whine. There are higher end LEDs for various commercial uses that use PWM at over 20 kHz to avoid that.

That's unrectified LEDs flashing at mains frequency, not PWM.

Los Angeles has the Griffith Observatory. To allow the city to have street lighting while not interfering with the observatory, the lights are sodium vapor lights that emit a specific wavelength that the observatory can filter out. Since LEDs also use specific wavelengths, this could have been done.

I was disappointed that the article didn't mention this, especially because they did mention sodium, but only as a pretty color.

I quite like the 2 wavelength light that sodium lamps give out. When I was in school, my university gradually went from all sodium to all LED, and it was definitely a loss. The complete loss of hue, combined with the soft buzz of a dying sodium lamp had a certain otherworldly feel to it. I found it quite relaxing.

No, because the phosphors glow for much longer than human persistence of vision.

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