Street Light Problem

[DBCloggy]

We are lucky where I live as they now turn 95% of street lights off at midnight, so at least you get some much darker hours. Now to get our neighbour to switch off some of his garden lighting!

[acey]

Can't comment on OP regarding individual streetlight, but regarding  LED, the point to note is that for astronomy a relevant lighting parameter is the "S/P ratio". Light meters are calibrated to measure "photopic" units, but for deep-sky astronomy we want to use "scotopic" (low light) vision. You could have two white lights which both give the same reading on a light meter, but the one with the higher S/P ratio will look brighter to dark-adapted eyes.

LEDs have a higher S/P ratio than other types of streetlight (including low- and high-pressure sodium lights). Daylight has a very high S/P ratio, so LEDs that attempt to replicate daylight (or moonlight) have very high S/P ratio. This means (all other things being equal) that for the same amount of electricity you get a brighter-looking light at night. The obvious answer would be to use less electricity to get equally bright-looking lights. But don't count on it.

Cut-off lights reduce glare coming directly to the eye from distant lights, but a lot of skyglow is from light reflected off the ground, buildings etc, and up into space. So if you see cut-off LEDs going up in your neighbourhood don't start celebrating too soon. The only thing that will help your deep-sky astronomy is if those lights are dimmed or switched off at certain times of night.

[DaveS]

We are lucky where I live as they now turn 95% of street lights off at midnight, so at least you get some much darker hours. Now to get our neighbour to switch off some of his garden lighting!

Oh, gods, don't get me started on garden lighting. If anything was more useless than street lights it's garden lights. Now that's something I *would* ban outright.

[DaveS]

The problem I see with "white" LEDs is that they use an intense blue LED and a broad-band phosphor. If there is a reletivly small amout of phosphor then a lot of the blue gets through giving a high colour temperature, as well, of course as a horrible unfilterable continuum, which doesn't extend far into the red.

A better way (To my mind) would be to have tri or quad colour LEDs somewhere around 450, 550 and 625 nm, with possibly a 700 or 720 nm deep red fourth LED. Then, A) LP filters would still be of use, B ) the light would be "white", and C) late at night only the 720 nm LED could be lit, giving a deep red night-vision-preserving illumination.

But, of course, that would be *far* too sensible.

[acey]

A better way (To my mind) would be to have tri or quad colour LEDs somewhere around 450, 550 and 625 nm, with possibly a 700 or 720 nm deep red fourth LED. Then, A) LP filters would still be of use, B ) the light would be "white", and C) late at night only the 720 nm LED could be lit, giving a deep red night-vision-preserving illumination.

That idea sounds great, but you have to wonder how objects would appear under this light. In general they would differ from their appearance under white light, and the real issue for lighting engineering is not what the light looks like when you stare at it, but how the environment looks under that light, i.e. reflecting it. My guess is that things would look pretty strange, and everyone except astronomers would soon be clammering for the lights to be replaced.

[DaveS]

Yes, I think a basic tri or quad colour LED might have a pretty low CRI but not as low as sodium, and maybe comparable to a CFL fixture. It should be noted that "white" LEDs have a significant dip in the cyan around 470-500 nm, the Stokes Gap.

For pure efficiancy, the LP sodium is still hard to beat, though being nearly monochromatic has a CRI near zero.

[laser_jock99]

White LED street lights starting to go up apace round here (Warks). They look like full cut-off luminaires at low angle on existing height columns- all good. Can see one from the back garden. 

Will be interesting to see if :

a) The Full Cut Off reduces LP

 b ) The wide spectrum ruins everything

c) The council stick with 12:00 am switch offs or decide they are cheap enough to run full pelt all night..........

[BlueAstra]

This was my adventure with street lights: http://stargazerslounge.com/topic/191067-streetlight-success-story/

I still think they are too bright, and I think the LED type nullify my LPFs. The BAA were very helpful.