LED street lights

[jason.p]

I've just seen a BBC news report about a county replacing their sodium street lights with LEDs which apart from saving money will reduce light pollution. Is this good news for us, or is the white light more difficult to filter out?

[euphony]

I saw the same thing, Sheffield City Council I think it was. I was under the impression that it would be worse to filter out. The report said it would reduce sky glow citing the fact that you will no longer be able to see the light pollution from Sheffield in the Peak district. This may be the case, but I can't see it reducing light pollution when you are sat underneath the lights!

From a distance yes, but certainly not if you're sat within the said lights, which I'd imagine for our purposes will be worse.

I could be wrong though.

[sologuitarist61]

A bit of both I think. We have the LED's and they direct the light downward and so stop a lot of the spill into the night sky, but where you do get the LP filters don't work as well.

[Keiran]

Hopefully it will cut the sky glow, but if you are in direct line of sight to the light, its not possible to filter it. I think the only time you will really know how much it affects you is when they are installed around your area.

Keiran

[lukebl]

GOOD - if you have a dark site, or live in the countryside

VERY BAD - If, like me, you live in suburbia, and can currently filter out the light pollution

[notaclue]

If they make the lamp housing deeper then any light is makes is directed down. In my town centre they have some LED lighting. It does not seem to do much unless they turn off the sodium lights and even then it is not the best of lights. As you can see they are more likely to just produce light while their LED color changing lights just do nothing.

Even though the lights are nice to look at, astronomically it is a nightmare. Producing white light is nice. It helps you to see. But that is a astronomers downfall. We see in white light from the stars and those filters you bought are now useless. I guess you could complain that you bought these and they are now useless to your local council who will just laugh and patronise you for a bit and laugh.

Maybe a mile outside of a city if the lights are reduced in power and brightness is welcome. But, and this is one of the things we need to ask ourselves, will it make our viewing more rewarding or do we have to be pedantic as astronomers were in the late 60's when they lights were changed from soft white and hooded to bright yellow to save on energy?

[ronin]

I suspect worse overall.

Owing to cost many places are now switching off their lights after 11:00 or 12:00 which they may well cease when they install LED lamps.

The "white" light cannot be filtered out, those old sodium lights were easy.

If less goes up then I guess more will go down and we will sit in a pool of light that is more intense, so our night vision is lessened.

The other factor is that although more will go down there will then be a large amount of reflection of that downward directed light and that will go up. So light pollution may be reduced but do not think that it will be eliminated to a great extent.

Wonder when the first post of "I wish they had left the old sodium lamp in place" will be ?

BBC has just covered Sheffield replacing all their lights with LED ones.

Will take 5 years to complete and the "slogan" is:

This will make Sheffield a lighter, brighter place to live.

An interesting slogan.

[JamesF]

Experiences I've read here seem to be mixed. Some of those who have had their streetlights replaced with LED lamps seem quite pleased despite the initial worry about filtering LP whilst others haven't been. I think it depends on where the lights are. If they're easily screened or don't spread light in your area the effect may be quite different from having one shining right into your garden.

James

[jnb]

Are white LED lights genuinely white light? As I understood it LEDs work by exploiting particular transitions in a semiconductor which should mean that they produce a single wavelength of light and so should be easily filtered. So how do white LEDs work?

Is it a good idea. On the whole yes. Yes there is an issue with reflected light but that is only a fraction of the light that would be directly sent into the sky from an unshrouded light, yes there is an issue with dark adaptation if you happen to be under one of those lights but ours is a minority interest and so much as we would wish otherwise is not a reason for an urban population to change their behaviour. My concerns would be do they really significantly reduce light sent directly into unwanted areas (ie the sky) and that they still reinforce the general opinion that more light is good.

[Cath]

White LED's contain 3 individual led's to make up the white light (you can see the 3 individual led's when you look at one close up). If the light is broadband then you cannot filter it without also removing most of the light from the stars etc.

Our only hope is that the light from the white LED's does not find it's way back up into the sky too much (some of it will), but that depends on the ground they are shining onto. If the ground is quite reflective then a lot of the light will be bounced back up into the atmosphere

[The Sailor]

I have both types of light close to my observatory, the CC have been good enough to shade the sodium light a few metres away and it has made a lot of difference, the LED lamp is shaded by a couple of evergreen bushes in my garden and I cannot see any glow from the observatory so happy here.

Jim

[jnb]

Our only hope is that the light from the white LED's does not find it's way back up into the sky too much (some of it will), but that depends on the ground they are shining onto. If the ground is quite reflective then a lot of the light will be bounced back up into the atmosphere

Tarmac has an albedo of 5-10% so on a completely theoretical and untested basis I would rather have well directed bright LED lights than poorly shrouded non LED lights

[jason.p]

Quite by coincidence the small village where I'm working has had their one sodium light changed to LED in the last few days. I believe the policy is to change them when they need attention and not really as a rolling programme. As far as I can see, the light is directed down to a "pool" of light and very little going up. If you're in direrct line however it's blinding I suppose if it reduces that "city glare" it can only be a good thing. If you've got one at the end of your garden it could be a problem. At the moment I've got one sodium light but I've managed to rig up a sort of sight screen which should still be OK if they ever get round to changing it to LED.

[Mr Spock]

I've just been to the village hall to look at a Sheffield CC roadshow. Over the next year they are resufacing our roads and... fitting LED streetlights...

[The Sailor]

Just took a photo from the security camera at the observatory, the light on the right is the shaded sodium and the one left is the LED. The lighter sky at the top of the photo is the camera weather shade.

Jim

[dwc]

I'd be interested to see the sight screen Jason.p, as I'm thinking of a similar approach for my garden. And also to see what kind of shield your CC fitted, Jim, as I'm trying to get mine to do the same.

[whooshbang]

On a positive note, the council will likely dim these LED street lamps down late at night, it won't help much with filtering but it will at least result in less light being reflected up.

My concerns are with security conscious neighbours that will upgrade to LED flood lights and newbies who will now see lighting their gardens up as an energy efficient security measure.

[The Sailor]

And also to see what kind of shield your CC fitted, Jim, as I'm trying to get mine to do the same.

Dave, here is a link to the page the shield photo is on:

http://stargazerslou...-a-trial-basis/

It looks like that plastic lawn edging stuff.

Meant to say but if you look at the photo above you will see a bit of trellis on the right, I put this up on the observatory overhang to help with shielding the light, have a rose bush attached from below, is meant to grow all over it lol.

Jim

[dwc]

Thanks Jim

[rustyjames]

Have LEDs in my close,they are at the front and there seems to be less of a glow when we stand at the rear,they were fitted over the summer so I have seen a slight difference since last winter,BUT we still have an old style lamp at the rear which is partially obscured by my hedge

[Mr Bond]

Any light that only points down, regardless of output must be better than the current lamps we have, surely??

I've flown from southern UK, out over France and the difference is startling. The only light going up in France was dim, and reflected from a dark grey surface (the road), while the UK was awash with direct light from every angle.

Getting rid of this in the UK has to be progress!

[The Sailor]

The shaded sodium light in the photo is in the centre of my arc so was a bit of a nuisance, as the lane at the back leads to OAP accommodation they are not switched off at night. Contacting the CC Lighting Dept I had a nice chat with the engineer and found out that the lights were in fact local council owned but as they had no money the CC looked after their maintenance, he then told me he would look into shielding it. The job was done quite quickly the next day and I only realised when I was in the observatory that night and thought the light was broke, looking out saw the shade, sent them a thank you next day .

Jim

[DaveS]

RE: LEDs

Most (If not all) white LEDs use a blue LED and a phosphor. If you look at the spectum there is usually a sharp peak in the blue around 450 nm, then a gap in the cyan from 470-500 nm or so, then a broad continuum from 500-ish down to 650-700 nm.

There is a confusion here between the lamp and its housing, I don't see why they cannot install monochromatically filtered low-pressure sodium lamps in total cut-off housings. This would be both very eficiant as well as easy to filter.

[The Sailor]

Sorry Dave S but I got lost after the first sentence, I don't normally carry a light analyser around in my back pocket, to me a light, is a light, is a light. To me LP is LP no matter where it comes from, don't worry myself with it's spectrum .

Jim

[RikM]

Sorry Dave S but I got lost after the first sentence, I don't normally carry a light analyser around in my back pocket, to me a light, is a light, is a light. To me LP is LP no matter where it comes from, don't worry myself with it's spectrum .

Jim

But if you have to have some LP, better LP you can filter out, which you can with sodium and mercury lamps but not with white LEDs. Looks like they also impinge on 656nm H-alpha, so even narrow band imagers are going to be just as stuffed as visual astronomers