Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Simple streetlight hack could protect astronomy from urban light pollution

space.com

Post reply on HN
Print view View original

81–90 of 159 posts

Re: Simple streetlight hack could protect astronomy from urban light pollution

#81

Post by namibj »

jcalvinowens wrote: post #35
wruza wrote: post #9

Ah, the good old "human eye can't see N Hz" that will make night lights insufferable for those who happen to see it. My grandma has a couple of lightbulbs in her apartment that annoy nobody except me. I feel like laser-blinded when they are on, no matter where I look. It's blindingly bright and visually dark. Idk if it's flicker frequency or light spectrum tbh, but it's that same "hey you shouldn't see it, cause nobo…

I feel the same way. If I'm sitting still I find I don't care much... but if I'm working with my hands and moving around it drives me crazy and gives me headaches, especially if I'm focusing on something spinning. Just for fun, I replaced the LED dimmer in my garage with a homemade board that has a frequency knob in addition to a duty cycle knob. I've found that the cutoff for bothering me is around 10KHz, which is f…

Could it be that you inadvertently smoothed the visual flicker on the electrical side? Consider fast brightness sensor with known (ideally approximately linear) sensitivity to the AC component of brightness, and check with an oscilloscope or so. A soundcard with 96 kHz sample rate should probably have a high-enough filter cutoff to be quite and easily useful there, though I'd see to check the frequency response with an edge generator to confirm that it has linear sensitivity from low-ish frequencies to the >=10 kHz ones of particular interest.

Re: Simple streetlight hack could protect astronomy from urban light pollution

#82

Post by p1mrx »

Evidlo wrote: post #49
galdosdi wrote: post #10

Earlier quoted context omitted.

I'm not sure I'd be so quick to dismiss the feasability, at least not based on your reasoning so far. FM Radio stations have managed to accomplish getting the speaker in your car radio to "blink synchronously" according to a complex signal pattern in the exact same way as millions of other radios simultaneously.... for about a century now. As long as all streetlights are in locations with good radio reception (which,…

Wavelength of FM radio is about 3m, so you only have to be a few centimeters shifted to be seeing an entirely different phase and be out of sync.

If the LEDs blink at 150 Hz, then a single on-off cycle has a "wavelength" of 2000 km.

The wavelength of the control signal doesn't seem relevant.

Re: Simple streetlight hack could protect astronomy from urban light pollution

#83

Post by bagels »

zokier wrote: post #14

> But there is a downside to LEDs: They're much brighter than old-fashioned energy-guzzling light bulbs. Such a weird statement to make. LEDs are not somehow intrisically brighter than other light sources, it is simply a design parameter. If new LED lights are brighter than whatever they replaced, it is because someone purchased lamps with higher light output, not because they are LEDs. You can get plenty bright with…

Just about any place that I've seen sodium lights replaced with LEDs, they have been replaced with vastly brighter LEDs. Maybe sloppy writing, I think the article is referring to the LED installations being brighter than the installations they are replacing.

Re: Simple streetlight hack could protect astronomy from urban light pollution

#84

Post by quickthrower2 »

p1mrx wrote: post #82
Evidlo wrote: post #49

Earlier quoted context omitted.

Wavelength of FM radio is about 3m, so you only have to be a few centimeters shifted to be seeing an entirely different phase and be out of sync.

If the LEDs blink at 150 Hz, then a single on-off cycle has a "wavelength" of 2000 km. The wavelength of the control signal doesn't seem relevant.

Plus you just add a position based delay. This is quite the bikeshed!

Re: Simple streetlight hack could protect astronomy from urban light pollution

#85

Post by p1mrx »

klyrs wrote: post #33
bradley13 wrote: post #5

Right. They're going to get all the streetlights in an entire city to blink in unison. Sure they are. Anyway, in most cases the LEDs don't run at 100%. In our area a, I happen to know that they are between 29% and 40%. That dimming is accomplished by blinking. Which would conflict with the idea of having them blink synchronously. Much simpler solution: ensure the LEDs ghts are directed exclusively downward, and have…

> They're going to get all the streetlights in an entire city to blink in unison. Sure they are. Thre are a super simple and effective distributed algorithms for this, if each lamp can see another few lamps. They don't even need a good clock, just a relatively stable oscillator within a few percent of a standard frequency, a light sensor and an integrator. I agree that it's better to just put hoods on the lamps to di…

I find it difficult to believe that a city-wide distributed synchronization algorithm could actually work. How do you deal with the photo-electronic-photo propagation delay, and ensure that each sensor is paying attention to the correct emitters, possibly including itself?

Even if you somehow got it to work, one guy with a hacked flashlight could throw the whole thing into chaos.

Re: Simple streetlight hack could protect astronomy from urban light pollution

#86

Post by cjensen »

amluto wrote: post #60

No. The article is light on details, but: > A simple device that makes LED lights flicker at a very high frequency that is imperceptible to the human eye > shutter, which needs to be lightweight and agile enough to blink about 150 times per second. Subjecting an entire city to 150 Hz flicker is not even remotely acceptable. IMO this needs to comply with IEEE 1789.

Why do you think 150Hz is unacceptable? People used to watch 50Hz TV, and weren't bothered too much. And San Jose used to use 60Hz flickering streetlights (sodium? not sure) to reduce light pollution for Lick Observatory.

Re: Simple streetlight hack could protect astronomy from urban light pollution

#87

Post by calamari4065 »

galdosdi wrote: post #10
bradley13 wrote: post #5

Right. They're going to get all the streetlights in an entire city to blink in unison. Sure they are. Anyway, in most cases the LEDs don't run at 100%. In our area a, I happen to know that they are between 29% and 40%. That dimming is accomplished by blinking. Which would conflict with the idea of having them blink synchronously. Much simpler solution: ensure the LEDs ghts are directed exclusively downward, and have…

I'm not sure I'd be so quick to dismiss the feasability, at least not based on your reasoning so far. FM Radio stations have managed to accomplish getting the speaker in your car radio to "blink synchronously" according to a complex signal pattern in the exact same way as millions of other radios simultaneously.... for about a century now. As long as all streetlights are in locations with good radio reception (which,…

Okay, let's do some math. Very rough napkin numbers, but it'll get us close.

A 150Hz blink rate means a period of 1/150s, or ~6.7ms.

The speed of radio signals through air near the surface is a bit lower than c, let's call it 299,250 km/s. [0]

When you multiply those numbers you get ~1500km. That means each 'on' and 'off' signal is 1500km long, moving near c. More specifically, the signals are concentric rings around the transmitter 1500km in width.

Let's look at London. I picked it at random, but it just happens to have a land area around 1500km^2. Assume the transmitter is in the center of the perfectly circular city, that means it takes 3.3ms for each 'on' and 'off' signal to cover the entire city. Each on and off pulse is just about exactly long enough to cover the entire city.

All that together means that at any given moment 50% of the lights in the city are on, and the other 50% are off. That's about as far from synchronized as you can get, really.

Also, light is slightly faster than radio in these conditions[1]. The light reaching the center of the city won't be pulsing in the same phase as the signal you're sending out. It will also be blurred and muddied due to the difference in signal speed. You probably wouldn't see much flicker in the light at all, as 50% of the lights are always on. On average, you receive 50% of the light you would otherwise, but that value will never reach 0. There won't be any periods of total darkness where you can make an observation, as the article suggests.

Moral of the story: light (and radio) is much, much slower than most people realize.

[0] https://ieeexplore.ieee.org/document/170108 [1] I couldn't find a confident number in my 16 seconds of searching. Seems like just a few percent faster, but the real number doesn't matter here.

Bonus: you can synchronize the streetlights exactly, but it's much more expensive and complex than a single radio transmitter. You need to have every single light contain a receiver with an accurate clock that synchronizes with a geosynchronous satellite with an atomic reference clock. Search for "GPS disciplined oscillator" if you're curious.

Re: Simple streetlight hack could protect astronomy from urban light pollution

#88

Post by p1mrx »

quickthrower2 wrote: post #84
p1mrx wrote: post #82

Earlier quoted context omitted.

If the LEDs blink at 150 Hz, then a single on-off cycle has a "wavelength" of 2000 km. The wavelength of the control signal doesn't seem relevant.

Plus you just add a position based delay. This is quite the bikeshed!

How does the LED know its own position? At that point you might as well abandon FM and use GPS.

Re: Simple streetlight hack could protect astronomy from urban light pollution

#89

Post by mumblemumble »

op00to wrote: post #62
amluto wrote: post #60

No. The article is light on details, but: > A simple device that makes LED lights flicker at a very high frequency that is imperceptible to the human eye > shutter, which needs to be lightweight and agile enough to blink about 150 times per second. Subjecting an entire city to 150 Hz flicker is not even remotely acceptable. IMO this needs to comply with IEEE 1789.

Why not?

Even flicker that isn't perceptible to humans is still associated with increases in all sorts of things including eyestrain, headaches, anxiety, migraines, and seizures.

150Hz is well below the human perceptual threshold but still within the range that can cause problems.

Re: Simple streetlight hack could protect astronomy from urban light pollution

#90

Post by mumblemumble »

cjensen wrote: post #86
amluto wrote: post #60

No. The article is light on details, but: > A simple device that makes LED lights flicker at a very high frequency that is imperceptible to the human eye > shutter, which needs to be lightweight and agile enough to blink about 150 times per second. Subjecting an entire city to 150 Hz flicker is not even remotely acceptable. IMO this needs to comply with IEEE 1789.

Why do you think 150Hz is unacceptable? People used to watch 50Hz TV, and weren't bothered too much. And San Jose used to use 60Hz flickering streetlights (sodium? not sure) to reduce light pollution for Lick Observatory.

IEEE 1789 lays a lot of that out. In a nutshell, LEDs are harsher.
Post reply on HN