In message on Mon, 18 Apr 2011 10:20:38 +0100
Andy Wade wrote:

On 18/04/2011 08:30, The Medway Handyman wrote:

Using a dimmer switch on your lights at home doesn't save very much in
electricity AFAIK, e.g. dimming to half the light doesn't reduce the
power consumption by 50% - is it different on street lights?

Yes, because you're dimming a sodium discharge lamp, not a tungsten
filament. With tungsten the lumens per watt efficacy drops dramatically
when you under-run the lamp. With a discharge lamp it may still fall
off, but to a much lesser extent.

Dimming is not new. The street lights in the road where I live were
replaced a couple of years ago. After midnight they dim to 75% output,
or so the letter from the council said.

'Intelligent' street light control seems to be becoming a large
industry, e.g. http://www.e-streetlight.com/

Our lights don't dim but they were all replaced about ten years or so ago with
more efficient lamps that don't waste light illuminating the sky.

They have a complex set of prisms built into the glass that distribute the
light evenly over a wide area. Instead of bright spots with dark areas in
between, the roads and footpaths are now very evenly illuminated but at a much
lower level.

The prisms seem to be designed to throw some light back to the footpath as well
as covering the footpath opposite without shining into the windows on either
side of the road, so it doesn't matter if several posts are on the same side of
the road rather than the usual alternating pattern.

--

Terry