In uk.sci.astronomy message , Fri, 19
Oct 2007 23:40:33, Les Hemmings
posted:
wrote:
The latest locale to join Toronto and Raleigh in the LED City
initiative is none other than the home of the Wolverines, which
recently announced plans to replace about 1,400 street lights with
light-emitting diodes.

...
Is this

going to be good or bad for clear skies ?

They will require new fittings, and it's likely that any modernisation
of fittings will give, for efficiency, better direction of the light.


Well, LED's only work with one polarity (all diodes work like a one way
valve, letting current flow one way but not the other) so on alternating
current they will flicker at the same frequency as the mains (50 Hz in the
UK i think) which means there are 25 periods of darkness per second.

If directly driven with AC, 50 darks per second. But, at the very
least, it seems sensible to use full-wave rectification, so 100 darks
per second.

Being forward diodes, LEDs need to be driven by a current source rather
than a voltage source; that would, at least, narrow the darks.

Normally, AIUI, adjacent houses in a street are driven from different
legs of a 3-phase supply (one can see this where supply is overhead); it
seems likely that street lights would be likewise.

The AC mains could be stepped down to a suitable voltage by a 50Hz
transformer; but to generate high current low voltage DC from mains it's
much better to use a switching regulator IIRC; that switches at high
frequency (probably above audio; but ask a dog) and uses less magnetic
material.

I've done some of that myself, in a non-lighting context.

But one really needs to hear from the actual designers of such fittings
to see what is deemed cost-effective nowadays.

Could this mean we could use mains current as the clock to phase CCD's so
they are only "on" when the LED street lights are off. If whole cities are
LED lit we could, by phasing our light capture periods with the LED nulls,
have a huge improvement in the results we couyld acheive in built up areas.

It seems unlikely that enough flicker at 50 or 100 Hz would be
acceptable to users of the light. At switching frequencies, I'd expect
individual lamp standards to run at similar but different frequencies.


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