Getting lights to blink in unison is a neat idea, and technically not hard nor too expensive for new lighting. Personally I think the biggest problem with it is that cities aren’t going to agree to overhaul all existing outdoor lighting only for the sake of people using telescopes. Even if there was widespread buy-in this is a project that could easy take 50 years or more, and cities might be reluctant to commit to this idea right when technology is rapidly changing.
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, since they're all on streets and are tall, is likely), it would not be that hard to implement in principle a central FM or other radio signal that oscillates according to the expected blinking. Lights that are engaging in dimming could multiply another signal by the central signal to diminish it even more.
Could you make a dark period long enough to mask the multipath variance among emitters and observers? Or would this dark period be so long as to cause annoying flicker?
The wavelength of the control signal doesn't seem relevant.
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.
Um am I really really tired and not reading correctly, or is this confusing linear area with square area? London doesn't stretch from New york to Florida.
186000 miles/second * pulse of .003 = 558 miles ~= 900 km
Either way, propagation delay is a huge problem with synchronizing systems. It's not something you usually think about
Better solution is less lights, or turn them off at night or any of the other solutions we've been talking about for 30 years. Remember when street lights were killing sea turtles?
The physics of radio wave propagation are surprisingly complex. Buildings and objects in the path between transmitter and receiver can cause various interference effects that basically slow down the signal. I'm glossing over a lot, but it would take significant effort to precisely measure the exact delay between any one receiver and the transmitter. It's much more complicated than just the distance, and each one has to be measured individually.
I'm not an expert in RF, but I think this concept of a single synchronizing transmitter just doesn't work in the real world. You'd probably build a cell network with many transmitters covering small areas. Each transmitter has the hardware to synchronize with thr others, and the lights just have a simple circuit slaved to the transmitter clock.
There's probably more clever ways to synchronize signals over long distances, but RF is black magic to me.
That is a way better idea than the one in the article, for sure. So many benefits. Would need the motion sensing technology to be top notch tho. Need to illuminate for pedestrians as well. Or perhaps illuminated sidewalks, or smaller lights for pedestrians.
Motion sensing, I find, similar to LED technology, is often used more for increasing lighting levels rather than decreasing. My pet peeve are people who have motion sensors that activate their 20k lumen floodlights based on motion on the public sidewalk. Cities should not allow those.
Isn't that just a matter of perspective [whether the presence of motion is used to increase lighting or the absence of it is used to decrease]?
I believe relatively dim, warm coloured lights are a much better idea.
The problem with warm lights is that they are much harder to see in the dark. Maybe dim lights around 505nm would be a better idea.
That's only if they're moving within the sensors that switch on the light.
> The problem with warm lights is that they are much harder to see in the dark.
Not sure what "in the dark" means here. Light by definition is easier to see in the dark than it is in light as it has less light to compete with.
In any case, I find warm lights much easier. Cold lights are too much sensorial information for me. In any case, street light at night shouldn't be meant to allow you to see everything like it's a sunny day. It's supposed to give you enough light that you can see your surroundings and potential threats around you. For that, warm lights should be more than enough. If you need more light clarity, light your own phone torch or something.
And that is the other reason. Those would still make nights look like nights, they should look sort of moonlight like with reasonable brightness.
Dim, but cool coloured lights? That's certainly better than bright cool ones but I still find them very annoying and overstimulating at night.
The only thing I can't do with dim, warm lighting is read small text from something other than a backlit screen. For everything else, it's perfectly fine. And if I do see myself having to do that, I'll just add a spot of light to what I'm trying to read, problem solved.
A few years ago I lived in a neighbourhood where all the street lights had a dark yellow tint, very warm coloured. They weren't all that bright either. I could see a person a block away with ease.
Again, that was far from ideal to read a book, but why would I do that out on the streets at night?
Good point! In this case of nighttime lighting infrastructure it has become so vastly overbuilt as if to save people “the trouble” of these simple targeted solutions.
I agree with you, though: lights blinking off and on all the time are not great. I drove somewhere in Europe where they did that with streetlights on major roads, and found it startling, long past the point at which I'd have assumed I'd become used to it. It wasn't the ones that were responding to me, it was the ones that responded to traffic on the opposite side of the road - they'd flash on in my peripheral vision and make me jump. Did not like.
Amen, but alas we're not there.
In any case, threats don't always come from purposeful attackers. It's useful to see animals, vehicles and bikes with no light, etc.
What is troublesome is that many places, that many of us probably hail from, “were there” not so long ago, and it was destroyed by duplicitous, malicious political actors.
Having “safe streets” is not a pipe dream. Indeed not having them is a symptom of people trying to destroy our societies.
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 direct the light to only where it's needed, but distributed synchronization is a solved problem.
Even if you somehow got it to work, one guy with a hacked flashlight could throw the whole thing into chaos.
The article states they use GPS for synchronization. I wish they went into it further, but a number of affordable chipset can do emit PWM at the proposed frequencies, globally aligned in time to PPS(every receiver computes the same top of second), then the telescope exposes during the blanking time.
On a related note I have a 1MHz GPS time source in my office that is synced to a 10MHz OCXO(more common for test gear) so my equipment is phase synchronized to GPS. It's really spooky to sync the oscilloscope to one time source and probe another time source with the scope and watch how their phases don't perceptibly drift from each other.
The mayor then got complaints that the street lights are not as bright as mandated by some regulation (when measured with an empty road in their dimmed state ...).
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 only as much illumination as needed.
Even better: turn the darned things off when they are not needed - why illuminate empty streets?