LED streetlamp aims to improve public's view of stars | Hacker News Reader

LED streetlamp aims to improve public's view of stars

bbc.co.uk

69 pointsdarrhiggs14 comments

Lighting engineer discussing this: http://www.reddit.com/r/science/comments/1d3hd1/researchers_...

There's another issue not covered by the article, apart from the degree to which the light's emission angles are controlled, and that is spectral lines. Many conventional streetlight designs emit their light along narrow spectral lines, a trait resulting from their gas-discharge design, and those spectral lines prevent spectroscopy (a large part of modern astronomy) from being carried out as it should.

LED lights don't have narrow spectral lines, so they represent an advantage for this reason also.

Street lights that emit narrow spectral lines are not a problem at all for spectroscopy, except of course at the line wavelengths. I have a bit of experience here -- a few more than 400 nights observing spectroscopically at Lick Observatory, where we have just a wee bit of light pollution!

All lights, narrow or broad band, present problems for optical broad band imaging.

If we could get all cities to go with low pressure sodium lamps (that only emit at a few wavelengths), astronomers would be very happy. The only problem is that city residents tend to not like the monochromatic look you get from not having a broadband light source. So San Diego, for example, switched to low pressure sodium in the 80's to try to protect the skies for Mt. Palomar. But residents started screaming (it's leading to more crime!) so the city went back to high pressure sodium.

Thanks for posting. I would have thought narrow lines would produce the problem that those specific lines would become unavailable for study, or would be freely mixed with the "real" data in confusing ways. It didn't occur to me that they could simply be subtracted in a deterministic way.

On that basis, I might have argued for high pressure sodium (to smear the lines) instead of low pressure, and I would have been quite wrong.

Given these issues, it's no wonder that a mountaintop in Hawaii, and another one in the Atacama desert, are now the preferred locations for optical astronomy.

> Thanks for posting. I would have thought narrow lines would produce the problem that those specific lines would become unavailable for study, or would be freely mixed with the "real" data in confusing ways. It didn't occur to me that they could simply be subtracted in a deterministic way.

Your original thought is basically correct: we do lose the ability to do any meaningful work at the wavelengths of the lamp emission lines. But the damage is concentrated at a few wavelengths. So we trade having noise over the entire spectrum for a few really, really noisy pieces of spectrum. It's generally a excellent tradeoff.

But even low pressure sodium lamps have some emission spread over a large wavelength range, so it is becoming increasingly difficult to do optical work near major urban areas. In the long run most of those sites will probably transition to mostly IR work.

Narrow spectral lines means astronomers can filter the city glow somewhat easily.

I'm not so sure that all of the claims are true (or maybe just not well-explained) in that there exist "full cutoff" lights that don't direct light upward. The researchers' light may indeed be an improvement, but it was not made clear to me what it would be over existing full cutoff lights. Based on the illustration, maybe it has more to do with the light not scattering all over the place and therefore not reflecting off objects as much.

It also seems to be an awful lot of "on paper" considering they haven't built a prototype.

Finally, as I was searching for "full cutoff" to make sure I wasn't imagining it, I found this informative website on the subject of light pollution and solutions: http://www.lrc.rpi.edu/programs/NLPIP/lightingAnswers/lightP...

I find this story quite strange because there are LED street lights right outside my front door here in Oakland. They went in last year, and it was a bit weird the first time I saw them turned on because the light looked very cold, almost blue compared to the lights that were there previously.

They're not saying they've invented LED streetlights, just a new type of them.

Are street lights the biggest contributor to light pollution?

On Mauna Kea (big island of Hawaii) the biggest sources of light pollution are the airports and harbors.

For the telescopes near big cities, I'm not sure if anybody has worked out the relative contributions of street lights to say billboards or car dealerships or backyard flood lighting. But from standing on the summit of Mt. Hamilton and looking down into the valley it would seem to be mostly street lights

I wouldn't be surprised. I mean, their entire purpose is to make it light outside. A neighborhood without street lights is noticeably much darker.

The only other likely sources are building lights and cars, and I'd think street lights far outnumber either of those in their ubiquity.

It depends on location, but in many places they actually are.

Surprised sidewalks aren't mentioned. The "perfect" design shows that the they would need separate lights.

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