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Comment by teraflop | Hacker News Reader

While idly thinking about light pollution a while ago, I thought of a ridiculously impractical solution:

Mandate the replacement of all outdoor night-time illumination with LEDs that are pulse-width modulated at a low duty cycle. Synchronize them all to an accurate global clock (e.g. from a GPS receiver), so that for instance, all of the lights are simultaneously turned on for the first tenth of each UTC millisecond. Then an image sensor with a sufficiently fast global shutter could disable itself during every brief pulse of light, so that it picks up 90% of the incoming starlight, but almost none of the light pollution.

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I think it's time to introduce fines for office buildings that are unoccupied overnight, but have lights on. Certainly in high rise towers this would be tens of thousands of lights.

More stars, less energy consumed. Win, win.

Slovenia is one of the few countries that has legislation against light pollution. For example, all outdoor lights must only emit towards the ground. It helps a lot with dark skies, but enforcing this kind of rules can be a problem in many cases.

http://artificiallightatnight.weebly.com/uploads/3/7/0/5/370...

THIS so much - Most of the public lights emit in +-all directions, making them not only uneffective but constant light polluters at the same time. And it would be so simple - reflect to the ground, say with 170° angle.

this would make the usual 'light cloud' over towns and villages practically disappear. I guess this is a remainder from times where the more light there was, the better (ie look at our mighty civilization and its advances, illuminating whole valleys and soon whole earth!). Now it makes it pretty hard to find unpolluted places in western Europe even in the Alps.

I can be in the Italian side of the alps, and I still get orange blobs from Switzerland or France, even across continuous wide range of frikkin' 4000m peaks!

Reflecting to the ground is helpful, but it definitely wouldn't make the light pollution disappear. Plenty would bounce off lightly-colored pavement and back into the sky.

Interesting. My hometown in Austria recently installed new lampposts; and all of them are downward-facing. I don't know if this was a conscious decision to reduce light pollution, but it worked wonders. The roads are better lit than before, while there is almost no glare from afar and the night sky is much more visible.

The problem is so much bigger than that. Here in Edmonton, we have regular folks who campaign and crowd-source funding to install lights on bridges, buildings, just about anything for the sake of making things look 'nicer'. Which is unfortunate for those of us who want to see the sky.

Anyway, how's the light pollution been on your Africa trip? You must have been through some seriously dark remote areas by now.

In my small city we are a few people interested in night observation. We are pondering about lobbying the city hall to have a "dark night" once a month, maybe something from 1am to 3am, where public lighting would be off.

Considering the mayor comes at every smallish local events just to remind people of his name, he may be interested in something that would make a bunch of people praise him for little effort.

"Then an image sensor with a sufficiently fast global shutter could disable itself during every brief pulse of light"

all that light scattered in the atmosphere and reflecting off other things would still be around when the LEDs are out.

Hmm... is that really true? Light travels pretty fast!

The poster proposed a 90% duty-cycle for capturing the image (i.e. shutter open for 90% of the time, shutter closed for 10% of the time).

Since I'm based in the UK, let's assume the simplest synchronization mechanism - the 50Hz mains signal. (I appreciate this probably isn't perfectly synchronized.)

That means that the shutter is open for 18ms and closed for 2ms, during which we want to have our light pulse.

Let's assume that we have a 1ms flash and we allow 0.5ms for synchronization error and 0.5ms for light from the flash to dissipate.

In 0.5ms, light can travel approximately 150 km. Most of Earth's atmosphere (and hence most of the scattering) is within ~15km of the surface. As I understand it, reflection takes very little time (less than 1us), and each will cause energy loss (and hence less brightness).

I think there are engineering details to work out, but I'm not convinced that reflections or scattering are a significant problem.

I think the bigger problem is that it just doesn't seem worthwhile - how much would it cost to do this, and how much do people care?

As an aside, I wonder what the effect (on humans, wildlife, etc.) of regular 1ms-long pulse of 20x the average brightness would be - maybe it's fine, maybe not?

Speaking from experience, this solution would cause massive headaches for many people, myself included.

I had this idea too but...

This article uses 20 seconds exposure, so it wouldnt be helped by PWMing the lights.

Also, you'd have to do it much faster than 100Hz. Wave Christmas LED lights around and you'll see a strobe effect.

People claim to feel (not see) 100-120 Hz flicker (I wouldn't dismiss them off hand).

Finally, you'd have to have a powerful, lighting, LED able to toggle on/off at fast enough speeds. That isn't obviously available, if for no other reason large internal capacitances.

As for synchronizing it, that's not too bad. I don't think you'd need a GPS clock. Use an accurate crystal clock and the mains as a 60*3 = 180 Hz reference. Turn the light off every 0, 120 and 240 degrees to cover all three phases.

Fun! Solving the wrong problem though - problem with light pollution is the effect on the environment and human psychology, not on astrophotography.

Or we could simply go back to sodium vapor lamps for streetlights, because there are simple sodium vapor filters for astronomy that can entirely filter it out without losing any of the light required for astronomy.

This entire issue is already solved, and works fine.

It is an interesting idea, but most of the lighting industry is interested in reducing flicker, not increasing possible sources of it.

You'd be fighting the current.

It's somewhat of a known problem space. Tackling light pollution boils down to easy steps, you just have to be mindful in design...

- Only be on when needed

- Only light the area that needs it

- Be no brighter than necessary

- Minimize blue light emissions

- Be fully shielded (pointing downward)

Light pollution is (IMO) driven by lack of knowledge of the problem & solutions, a perception that lights = wealth/luxury, and the low cost of electricity.

This might be enough for photography, but the speed of light would probably push this above 1 millisecond for the area that needs to be dark for large telescopes.

But engineering the light sources to be astronomy-friendly and nature-friendly in general seems like a good idea. Basically limit things to the red-orange part of the spectrum. It worked for sodium lights.

Fix the duty cycle to a particular slice of the phase of AC voltage (e.g. the first 0-pi/2) and you accomplish the same thing much more practically, at least within a single power network.

Not a power engineer, but can't most of the power conditioning devices used in buildings mess with the phase of the incoming AC signal? GPS clock synchronization is guaranteed to be globally accurate.

They may cause harmonics, but the underlying sine wave is solid like a rock (well, at least concerning the ENTSO-E over here in Europe, other grids may be more fragile).

It's going to be a bit trickier than that, because power is usually distributed as 3-phase, not single-phase.

So that's 3 different offsets, depending on which phase any given light is wired into.

Plus that's probably going to be slow enough to cause visible flicker.

Have it trigger on a voltage threshold. 3 phase means you're hitting 0V 6x per cycle so that'd bring the pwm up to 720Hz.

I heard once that it is easier to filter out (of the photos) light that comes from LED, but I can't find a good resource on it. Anyone know if this is true?

Having large LEDs turn completely off and back on quickly may prove too costly/difficult. Having a notch of light frequency missing from every outdoor LED might be a better solution. You could even wear special filtering glasses that would filter out all artificial light.

People actually used to do this. Back when most street lighting came from sodium lamps, you could eliminate light pollution almost completely with a pair of glasses intended for soda-lime glass blowing.

That's not true. Old mercury and sodium lights are much easier to filter since their emission lines are narrow. LED often has a very broad and spiky spectrum, which makes it almost impossible to filter. (Source: astrophotographer). You can buy reliable light suppression filters for sodium/mercury (google CLS or UHC filter), but not LED

Easier to filter out than incandescents, not easier than the old sodium lamps. Also a single color LED is not too bad. But a (good) white LED has many primary color LEDs and phosphors to broaden the color spectrum as much as possible.

But IMHO, wrt to incandescents they still suck.

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