

LED streetlamp aims to improve public's view of stars - darrhiggs
http://www.bbc.co.uk/news/technology-22292129

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tzs
Lighting engineer discussing this:
[http://www.reddit.com/r/science/comments/1d3hd1/researchers_...](http://www.reddit.com/r/science/comments/1d3hd1/researchers_believe_they_have_come_up_with_a_new/c9mn785)

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lutusp
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.

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dkirkman
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.

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lutusp
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.

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dkirkman
> 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.

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mikestew
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...](http://www.lrc.rpi.edu/programs/NLPIP/lightingAnswers/lightPollution/indicatorDirectUplight.asp)

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anigbrowl
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.

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

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ComputerGuru
Are street lights the biggest contributor to light pollution?

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dkirkman
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

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

