On 11/11/2015 15:26, MartinR wrote:
On Wednesday, 11 November 2015 13:29:07 UTC, Martin Brown wrote:

[snip]
The ones moving from low pressure sodium to anything else are in for a
big shock. At upto 200 lumens per W the big orange yellow sodium Dline
based streetlamps are still the most efficient light source known.

White LEDs are getting closer to half that but at a much higher cost.

Big problem for astronomers is that sodium light could be more easily
filtered out at the telescope whereas broadband white light cannot.

LPS lighting is mandated where absolutely essential in the vicinity +/-
50km around major optical observatories (they prefer none at all).


Councils will continue to install them as the lux value at the road surface
is similar to sodium, and people prefer white light to orange or
peachy-pink.

Possibly true but their electricity use will double unless they also
switch them off midnight through 5am (which some councils are doing).

Also efficiency will increase and upfront and life cycle costs will
reduce.

They will inevitably get better with time. I suspect early adopters will
get burnt with expensive installation and little or no saving on
electricity perhaps even an increase in usage depending on the mix.

As more of the light from LEDs is directed at the surface and not up into the sky,
wouldn't astronomers prefer that?

Astronomers do prefer full cutoff lighting fixtures where no light goes
above the horizontal, but they also prefer monochromatic light too.

But a tarmac road has an albedo of around 10% and concrete pavements
more like 30% so the light ends up backscattered to the sky anyway.

Historically LPS lamps were so efficient that nobody optimised them to
put light onto the ground and 30% went skywards immediately. Look down
on a city from an aircraft and you will see the difference. Belgium have
full cutoff LPS fixtures but they are very rare in the UK.

Cobra head HPS and flat LED are the current favourites.

--
Regards,
Martin Brown