Re: fco and uplight (and an idea for Christmas)

[Jan Hollan <jhollan@amper....muni.cz>]

>    From: Patrice Scattolin <scattol@videotron...>
>
> >Still: uplight and skyglow of cloudless sky have little in common, being
> >not at all proportional to each other. Uplight is no quantity to bother
> >about when the visibility of stars or Milky Way is concerned.
> >Astronomers have no professional reason to care about the total amount of
> >uplight, if they do, they are demonstrating their incompetency.
> >
> >
> That's an intriguing statement. Even in cloudless sky, light going up
> towards the sky, be it uplight from a luminaire or light reflected off
> the ground as serious negative impact on seeing the night sky.

Of course all light which is going upwards is potentially contributing to
skyglow as it is dispersed on its way, becoming skyglow if the dispersion
aims it downwards. However, the amount of the contribution to skyglow is
much dependent on the initial direction of the light.

There is no way how to assess the manmade contribution (to skyglow of a
clear sky) from the single _quantity_: amount of uplight. The other
needed information is the angular distribution of that uplight (and the
distances of emission and observation sites).

Almost horizontal light contributes five times more to general skyglow
than steeply emitted one (say, from a street surrounded by a continuous
barrier of trees or houses), and three times more than light going in all
directions (in amounts proportional to cos(zenith_distance)) from a flat
lit terrain devoid of any obstacles. If the almost horizontal component
emitted from luminaires (and billboards) would contribute one fourth to the
total amount of uplight, it would still cause at least one half of manmade
skyglow of cloudless sky.

Ten kilometres or more from the light sources, the influence of almost
horizontal emissions from luminaires is much more dramatic: if it would
make just one per cent of uplight, it would still contribute one half of
manmade skyglow! One lumen of uplight from lit terrain corresponds to one
cent here, whereas one lumen of direct light from curved-lens cobra
fixtures corresponds to one dollar (or euro), as its impact to heaven's
glow budget is concerned.

This is the reason why fully shielded fixtures producing no direct uplight
are the most important tool to reduce skyglow, if outdoor lighting is
unavoidable. Amounts of light hitting the ground, from which then the only
uplight would be produced, become the important quantity just at this
situation (still, skyglow may be difficult to compute from them, if the
terrain is not flat, e.g. because of many houses and trees).

So, after all, even those who want to admire the heavens should be
concerned about the simple quantity ``amount of uplight''
  -- but solely in the appropriate case that
  no direct light from luminaires goes away from the ground

Just those designs using different FS luminaires can and should be
compared using this quantity, advocated by David Keith. But luminaires
emitting more than 0.1 per cent of their light above horizontal plane and
close to it have to be excluded from such comparisons
 (David's samp0479.ies has 0.0 per cent uplight, so it's a valid match for
FCO in terms of minimising skyglow, unlike the other two non-FS lights,
see
 http://amper.ped.muni.cz/light/ies2/d_keith/below20/all.tab).

That's the main point. To visualise the importance of the upward angles,
let's me remember my old illustration,
http://amper/light/drafts/graphics/scatt_en.88.png, within
http://amper/light/drafts/graphics/ (where some short additional
commentary is offered).


The other, new point concerns ``skyglow from downlight''.

You mentioned clouds being as bright as aurora (clouds are much brighter
here in Brno, even the clear sky is brighter than but the most prominent
and short-lived parts of aurora). Thank to your remark, I realized the
existence of another effect, which I never noticed: sky with occasional
clouds brightly lit from below is to manifest more skyglow in its clear
parts, due to these clouds.  It's similar to the sky being brighter around
Moon (or Sun) then elsewhere: the light disperses mostly into just a bit
differing directions. For ``shining'' cloud bottoms, the effect is less
pronounced, as the light from them does not travel through all thickness
of atmosphere when going back to earth. However, as aerosols tend to be
just in the lowest kilometres of troposphere, the effect could be
sometimes perceptible and measurable. I should inspect my archive of
fish-eye quantitative images to find it. I guess that clouds may
occasionally increase the relative influence of terrain-reemitted light on
the luminance of the remaining clear sky, but not substantially.

There can be also direct downlight which contributes to skyglow:  that
from distant lamps which are at larger heights then the observer, whenever
they shine on the air above the observer. Of course, direct glare from
lamps is worse, but even if it is blocked by a hand, an elbow or e.g. by a
wall screening the garden from streetlamps, the dispersed light remains,
becoming a demonstration of the predominantly forward-scattering
properties of the air. One more reason for screening the light emissions
to unneeded directions inside the luminaires (as Friedel and his
colleagues have developed in Belgium) or just outside the luminaires, by
added outer shields.

(I remembered one image on the web, showing skyglow from a nearby lamp,
which is hidden behind my head: see 51.jpg or 51.png within
 http://amper.ped.muni.cz/noc/krnap/vit_spml05/ -- the
 http://amper.ped.muni.cz/noc/krnap/vit_spml05/51.png  gives directly
the luminances, middle magenta representing 0.01 cd/m2.  Zenith luminance
is 2 mcd/m2, then the magenta strips toward my head are 4, 6, 10 and 16
mcd/m2, or millinits, due to the lamp. On the opposite side from the lamp,
the influence of town Vrchlabi is apparent, affecting slightly larger part
of the sky and producing luminances of 25 and 40 millinits too. Black
spots indicate more than some 95 millinits. The landscape was covered with
snow, reflecting the lamp's light into my face.)


The third point is short:

optimised geometry of the emissions from luminaires is an important tool
for the future, needing some effort and expertise, but easy to verify by
laymen.

However, reducing the amount of light emissions is a simple tool which
can be implemented within a week:
 - switching off lights late at night at least,
 - and perhaps changing the bulbs and ballasts so that the evening light
levels are not over those recommended by the most sound standards (very
often they are, by a factor of 2, 10...).

Any electrician can do such things, saving a lot of money and greenhouse
gasses as well, with no or almost no investments. A better sleep for many
people and more beautiful Christmas to everybody would be an additional
benefit.

A Christmas evening with thousands of natural stars, if the weather allows
(Moon does, being in the last quarter), would be the best gift to many,
many children... I'm sure that a much appreciated one, too. What about
trying it in your municipality? Couldn't the mayor himself/herself turn a
switch off, for half an hour at least, to enjoy the splendour of heavens
with all the other citizens?


(Sorry for another long writing, I'd like to be able to express my
thoughts more briefly...;
again, for later access and quotes, it will be available at
http://amper.ped.muni.cz/jenik/letters/public/, should become msg00210.html
)

jenik

PS. in the above-mentioned
 http://amper.ped.muni.cz/noc/krnap/vit_spml05/51.jpg, the bright stars
are Capella, Saturn with Castor and Pollux, and then Jupiter and Arcturus
in the glowing high-pressure sodium sky above Vrchlabi.