R: double scatter and pdf

["Pierantonio Cinzano" <cinzano@pd....it>]

Dear Jan,

Our works take in accounts a double scattering. Discussion on neglecting the
third scattering can be found in our paper on MNRAS 318, 641-657 (2000) and
in the cited papers of Garstang.

<Was I completely wrong, or can
<the unused near-horizontal light from bad fixtures sometimes prevail even
<at not very large towns (say, with some types of haze)?

Inside few km the near-horizontal light is still an important source of the
artificial night sky brightness. Moreover for small cities the contribution
from the light coming from the rest of the country can be an important
fraction.

About the increase of haze: Forward-directed scattering by aerosol is
geometrically poorly efficient in producing sky brightness with
near-horizontal light emissions, so an increase of haze in cities likely
increases the effects of the light reflected at high-angles by surfaces due
to backscattering and decreases effects of near-horizontal lights due to
extinction.

I prefer to leave that paper in Postscript, sorry, but future papers will be
always in PDF. Thank you for suggestions.

A simulation accounting for the application of best laws like the Lumbardy
law in Italy (max intensity 0cd/klm over 90 degrees) is already planned.
However we like to obtain correct results and not make "exercises". So we
need, before, to improve some details of our method and to study with
accuracy the actual upward emission functions of areas in Europe. Then we
need to study the typical upward emission function of a mix of surfaces. As
you read in our papers, Garstang assumes F as given only by spill light from
fixtures but we consider it only a shape parameter. In our opinion F=0.01
unlikely correspond to a city with full-cut-off fixtures because surface
reflexion is not really Lambertian. So some work must be done before obtain
correct results.

Best regards,

Pierantonio

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PIERANTONIO CINZANO
Dipartimento di Astronomia, Universita` di Padova
vicolo dell'Osservatorio 5, I-35122 Padova, Italy
e-mail: cinzano@pd....it
web: http://www.pd.astro.it/cinzano/
         http://www.inquinamentoluminoso.it
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-----Messaggio originale-----
Da: Jan Hollan [mailto:jhollan@amper....muni.cz]
Inviato: venerdi 5 gennaio 2001 17.43
A: cinzano@pd....it
Oggetto: double scatter and pdf

Dear Piearantonio,

I've just read two of your excellent papers on the sky luminance, and I
have one question and two wishes.

The question and first wish concerns the article
 The Artificial Sky Luminance And The Emission Angles Of The
 Upward Light Flux
as available at http://debora.pd.astro.it/cinzano/papers.html

I'd like to get a crude estimate, how much does double scaterring enhance
the role of light emitted at low elevations inside large towns -- the
article takes into consideration just single scaterring.

You see, I've always said that scattered light from fixtures themselves
contributes always more to the sky luminance than that from the
illuminated terrain, even inside cities. It seems I was wrong for towns
less than 10 km across. I guessed that double scattering on aerosols
favouring small scatter angles does it. Was I completely wrong, or can
the unused near-horizontal light from bad fixtures sometimes prevail even
at not very large towns (say, with some types of haze)?

The wish is:
 please use pdflatex or pdftex and offer a pdf version of the article (and
another articles with just a dvips output) as well. xpdf or acoread
produce much easier-to-read screens than gv (and Acroread is far more
common for non-unix users). I think this is important for so very
interesting articles.
 As far as the figures are concerned, epstopdf utility converts eps to pdf
with correct Media Boxes (from BBoxes, unlike ps2pdf) and if just standard
Adobe fonts are used in eps figures, the output contains no bitmapped
fonts. Outside eps figures, cm or ec fonts are contained as outlines in
the pdftex output, of course.
 For bitmap figures, png format can serve as an alternative input for the
\includegraphics command.
 (I can make those pdflatex outputs within minutes, if you would send me
the LaTeX sources. This is an advantage of using an up-to-date Linux
installation...)

And the second wish:
 Could you please, sometimes in future, make a sky-V-radiance map of
Europe assuming F being some 0.01 or so (i.e., all fixtures being fully
shielded)? It would be nice to see, how the situation would change with
even the present high levels of ground illumination (let's hope they will
cease to rise at least, if they won't decline) when just excellent
fixtures would be in use, producing almost no light over 70 degrees
from nadir.
 Such a map could be a strong motivation that we are able to bring
the stars back to the nature.

Your ev. answer (perhaps with my question) might go rather to
 DarkSky-list@egroups... to be useful to everybody,
I've started to read it a couple of days ago.

with best regards,
Jenik

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                                Jan Hollan
              N. Copernicus Observatory and Planetarium in Brno
Kraví hora 2, CZ - 616 00 Brno                        +420 (5) 41 32 12 87

                                   home:
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e-mail: hollan@ped....cz             http://astro.sci.muni.cz/pub/hollan
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