Light Pollution Filter Experiment

Hi,

Martin Brown recently wrote about light pollution:
It is modulated with the second harmonic whatever frequency your
local
power grid runs at 50/60Hz. But you can't do much using that.

That gave me an idea. In Sweden we have 50 Hz power grid. In my mind I
imagined that the sky was fluctuating like a stroboscope at 100 Hz. If
I could block the light when it was bright then I had made a light
pollution filter. I mounted a rotating shutter on a synchronous motor
with 1500 rpm. The shutter was a circle with a small angle opening at
every 90 degree. (Optimum is 3000 rpm motor with opening at every 180
degree) I tested the device indoors under white fluorescent light. I
saw a yellow cross! That was the light minimas! Then I tried the
device outdoors aiming it at the sky. Negative, I could not visually
detect any minimas on the sky glow or the distant lamps. Maybe it will
work photographically but I'm not going to try.

Roger Persson

Roger Persson wrote:

That gave me an idea. In Sweden we have 50 Hz power grid. In my mind I
imagined that the sky was fluctuating like a stroboscope at 100 Hz. If
I could block the light when it was bright then I had made a light
pollution filter. I mounted a rotating shutter on a synchronous motor
with 1500 rpm. The shutter was a circle with a small angle opening at
every 90 degree. (Optimum is 3000 rpm motor with opening at every 180
degree) I tested the device indoors under white fluorescent light. I
saw a yellow cross! That was the light minimas! Then I tried the
device outdoors aiming it at the sky. Negative, I could not visually
detect any minimas on the sky glow or the distant lamps. Maybe it will
work photographically but I'm not going to try.

Roger Persson

Hi Roger,

Do not forget that different areas are connected to different phases,
so over a large area you will probably get the light for all three
phases, separated at 120 degree - i.e. quite uniform light intensity
where the "dark" periods for one phase will be nicely filled in by
the other two.

- Alex

On 29 Sep 2003 17:05:22 -0700, (Roger Persson) wrote:

That gave me an idea. In Sweden we have 50 Hz power grid. In my mind I
imagined that the sky was fluctuating like a stroboscope at 100 Hz. If
I could block the light when it was bright then I had made a light
pollution filter. I mounted a rotating shutter on a synchronous motor
with 1500 rpm. The shutter was a circle with a small angle opening at
every 90 degree. (Optimum is 3000 rpm motor with opening at every 180
degree) I tested the device indoors under white fluorescent light. I
saw a yellow cross! That was the light minimas! Then I tried the
device outdoors aiming it at the sky. Negative, I could not visually
detect any minimas on the sky glow or the distant lamps. Maybe it will
work photographically but I'm not going to try.

Street lights are thermal sources: the gas inside is electrically heated. They
don't flicker at the line frequency. This is different from fluorescent lamps,
which are cold sources and do usually flicker at 2X the line freq (moderated in
some cases by the decay time of the phosphor). So the sky doesn't flicker.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

In message , Chris L
Peterson writes
On 29 Sep 2003 17:05:22 -0700, (Roger Persson) wrote:

That gave me an idea. In Sweden we have 50 Hz power grid. In my mind I
imagined that the sky was fluctuating like a stroboscope at 100 Hz. If
I could block the light when it was bright then I had made a light
pollution filter. I mounted a rotating shutter on a synchronous motor

It will be easier to see an effect if you can tune your strobe to be
exactly offset by 1Hz from the target mains frequency. The eye is *much*
more sensitive to a changing signal than to a dc level.

Street lights are thermal sources: the gas inside is electrically heated. They
don't flicker at the line frequency.

Most of them are gaseous line emission devices and respond to ac mains
voltage to a significant extent. Even HPS which is the nearest to
thermal shows plenty of ac output modulation. They run hot and under
pressure but it is collision broadening of line emission and not a
thermal source. Metal halide is slower and incandescents slower still.
But they are seldom used for bulk street lighting anywhere now.

Mercury and low pressure sodium light outputs are very responsive to
line frequency. And represent a fair proportion of street lighting.

This is different from fluorescent lamps,
which are cold sources and do usually flicker at 2X the line freq (moderated in
some cases by the decay time of the phosphor). So the sky doesn't flicker.

But it does. And is the basis for several clear sky detector designs.

Regards,
--
Martin Brown

In message , Roger
Persson writes
Hi,

Martin Brown recently wrote about light pollution:
It is modulated with the second harmonic whatever frequency your
local
power grid runs at 50/60Hz. But you can't do much using that.

That gave me an idea. In Sweden we have 50 Hz power grid. In my mind I
imagined that the sky was fluctuating like a stroboscope at 100 Hz. If
I could block the light when it was bright then I had made a light
pollution filter. I mounted a rotating shutter on a synchronous motor
with 1500 rpm. The shutter was a circle with a small angle opening at
every 90 degree. (Optimum is 3000 rpm motor with opening at every 180
degree) I tested the device indoors under white fluorescent light. I
saw a yellow cross! That was the light minimas! Then I tried the
device outdoors aiming it at the sky. Negative, I could not visually
detect any minimas on the sky glow or the distant lamps. Maybe it will
work photographically but I'm not going to try.

If Sweden uses UK style 3 phase mains power distribution then it will
sadly never work at all. There is still an AC component but it is mainly
at 6x line frequency and superimposed on a large DC light background of
roughly peak output. It is probably not enough for the eye to notice
much.

Street lamps are invariably on all phases of the three phase system
using different phases for load balancing - and less flicker.

Even in 2 phase countries like the USA where you could isolate the
minima cleanly it wouldn't be very striking visually, but it might help
photographically. The eye is a pretty odd non-linear detector.

Regards,.
--
Martin Brown

On Tue, 30 Sep 2003 09:04:11 +0100, Martin Brown
wrote:

Most of them are gaseous line emission devices and respond to ac mains
voltage to a significant extent.

They are, but they still operate quite hot. If you completely cut the power to
either HPS or LPS lights, they decay in brightness over a very perceptible
interval. All you have to do is look at one while wiggling your eye a bit (by
pressing on the eyelid) and you'll see they look pretty continuous. Do that with
a cold fluorescent and the flicker is obvious.

In any case, as noted elsewhere, streetlights run on all three phases of the
available power, meaning that in aggregate the light shining up into the sky is
nearly unmodulated.


But it does. And is the basis for several clear sky detector designs.

Any references? For the reasons above, I can't see how such a device could
possible be practical.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Tue, 30 Sep 2003 12:05:31 +0100, Martin Brown
wrote:

Even in 2 phase countries like the USA where you could isolate the
minima cleanly it wouldn't be very striking visually, but it might help
photographically. The eye is a pretty odd non-linear detector.

Mains power in the US is nearly always 3 phase distribution. Outside of
industrial users, this is generally transformed to single or dual phase, but
across a region, lightning will certainly be modulated by all three distribution
phases.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Thank you Alex and Chris,
That explains why I didn't have any success with the "filter" outdoors.

Roger Persson

No, but it was a good and creative attempt anyway!

Clear Skies

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/

"Roger Persson" wrote in message
om...
Thank you Alex and Chris,
That explains why I didn't have any success with the "filter" outdoors.

Roger Persson