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[DSLF] Digest Number 583
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There are 6 messages in this issue.
Topics in this digest:
1. Re: A few things we should know about light pollution!
From: Steve Davis <w2sgd@juno...>
2. R: R: R: R: A few things we should know about light pollution!
From: "Pierantonio Cinzano" <info@inquinamentoluminoso...>
3. amount of reflected light is not that high
From: Jan Hollan <jhollan@amper....muni.cz>
4. Re: R: R: R: R: A few things we should know about light pollution!
From: Yvan Dutil <yvan.dutil@sympatico...>
5. Fwd: NPR Piece on light pollution
From: IDA <IDA@DARKSKY...ORG>
6. Re: A few things we should know about light pollution!
From: "fabio.falchi@libero..." <fabio.falchi@libero...>
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Message: 1
Date: Fri, 15 Feb 2002 08:41:02 -0500
From: Steve Davis <w2sgd@juno...>
Subject: Re: A few things we should know about light pollution!
... and the legal profession!
David Keith wrote:
> The "difference" is completely the "work" of the editors
> of the Handbook.
Exactly. Either one will work.
> Apparently many jurisdictions feel that they must rewrite
> definitions to suit themselves...
#1 They aren't engineers.
#2 Legal definitions are for the purpose of "The Law", and
they can vary from one law to another law even within the
same jurisdiction. Look at intent.
#3 Even engineers change their minds from time to time
which, BTW, could trigger unwanted changes to the law.
-Steve
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Message: 2
Date: Fri, 15 Feb 2002 19:39:42 +0100
From: "Pierantonio Cinzano" <info@inquinamentoluminoso...>
Subject: R: R: R: R: A few things we should know about light pollution!
<I agree with you that this argument does
<not work for the old cobra lens system with are much to curved anyway. But
<it does apply to modern high efficiency equipment.
Not only manifacturers can easily make shielded curved glass fixtures if
necessary so that the issue is not a problem, but however these discussions
seems to me quite "academic".
If you make a statistic review of all the new installations in the last
three months in Canada, in US or in Italy, you probably will realize that
radiometric efficiency is the LAST preoccupation of lighting designers on
the field.
Usual installations in Italy, in regions where there are no laws against
light pollution, have interdistances less than 3 times the height of poles,
and both radiant efficiences and downward efficiences are very poor. I can
show you e.g. an installation by Philips at the Giustizia Bridge in Mestre
with 11% downward efficiency (sic!) demonstrated by photometric
measurements.
Radiant efficiency became an important issue only when discussing limits to
the upward light emission, curiously it is not an issue anymore when the
problem is to save money for citizens "on the field". The behaviour of the
N.Y. Governor clearly showed that saving money for citizens is not a main
interest for many.
By the way, did you noticed that an horizontal flat glass seems became dirty
less rapidly than a curved glass? How many administrations in US clean
frequently the optics of their fixtures? And we spend time here to discuss
about radiant efficiency?
Cheers,
Pierantonio
-------------------------------------------------------------------------
PIERANTONIO CINZANO
International Dark-Sky Association - Italia
Direttore Scientifico
e-mail: cinzano@inquinamentoluminoso...
web: http://www.inquinamentoluminoso.it
------------------------------------------------------------------------
-----Messaggio originale-----
Da: Yvan Dutil [mailto:yvan.dutil@sympatico...]
Inviato: giovedì 14 febbraio 2002 6.02
A: DarkSky-list@yahoogroups...
Cc: International Dark-Sky Association
Oggetto: Re: R: R: R: [DSLF] A few things we should know about light
pollution!
Pierantonio Cinzano a écrit :
> <I have discuss with lighting engineer about this issue. As I unserstand
it,
> they
> <can actually achieved the correct illumination on the ground with flat
lens
> but
> <the internal loss in the lamp are increased. This is caused by a
> augmentation
> <of the reflection on surface at grazing angle. Curved lens dimish these
> effect.
>
> 1) You do not need flat glass to obtain a fixture with zero emission at 90
> degrees and above. You need only that the body of the fixture screens the
> upward light. So this problem has no interest for us.
I agree with you. However, putting a shield is often not an option due to
mecanical constrains (wind, ice load). If the feature is include from
scratch
in the design, it may have a lesser impact.
> 2) By the way, did you ever checked in common road fixtures the angle of
> incidence of light rays coming from the lamp (sometime elissoidal, i.e.
> large) AND from the reflector on the curved glass of the kind that usually
> you find? The smaller loss due to curved glass in respect of flat glass
> works only for a theoretical pointed source without reflector.
> It works at best on the paper which light polluters typically exibit when
they try to obtain larger limits during a discussion for a Bill. It work
> less well when applyed to a fixture where the source of rays is not
pointed
> but is a system lamp+reflector.
>
> Moreover did you noticed that some curved glasses are really squared with
> angles very reflective?
> Did you noticed that a great part of the surface of some curved glasses
> makes an angle with the rays coming from the lamp which is greater than
the
> angle with a flat glass?
I really dont see how this can happen. Curved surface will have smaller
incident
angles than a flat surface. This is certain for direct rays emitted by the
lamp
and
this is true in average also for reflected rays comings from the reflector.
> Do you belive to all things that lighting engineers say you? Good luck!!
>
> Pierantonio
I must say that I do not agree with you on this issue. With or without
reflector,
a lamp fixture has to emit significant light and angle up to 75 degres from
the vertical. This means angle of 15 degree to an horizonthal surface. At
this
angle you loose about 25% by reflection on each interface air-glass. If
using
a curved surface you reduce this angle to 60 degres the loss goes down to
~10% per interface. That where the 20% gain in radiometric efficiency comes
from. This effect will happen what ever is your optical design however is
more critical for flatter lens system. I agree with you that this argument
does
not work for the old cobra lens system with are much to curved anyway. But
it does apply to modern high efficiency equipment.
Specificaly, this analysis as been done on a lamp that was the most
efficient
(Lumen per watts) in the world a few years ago. It leaked a little amount of
energy above horizon however. And we asked for a zero emission above
horizon.
This could be achived simply by replacing the curved bottom glass by a flat
glass,
which for us was more practical than having shield installed.
However, this resulted in a loss of 20% in optical efficiency. For what I
know
this comes from standard optical analysis tools. By the way, the compagny
which
the the calculation is a strong supporter of the dark sky. It is a lifetime
member
of IDA and provided many support on the issue in Quebec. They are far from
your typical "Joe security lamp" provider. If we can settle this technical
issue
with them, I think we are in trouble.
Yvan Dutil
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Message: 3
Date: Sat, 16 Feb 2002 00:50:15 +0100 (CET)
From: Jan Hollan <jhollan@amper....muni.cz>
Subject: amount of reflected light is not that high
The quote from ``Doug P'' is a clear nonsense, isn't it?
> But the reflected light exiting the fixture at 75 degrees is reduced
> by a huge amount because most of the light reflects back into the
> fixture off the first surface of the lens. We often speak of these
> beams being "in
I have such a simple programme (I did not check it very much, but I think
that it computes correctly):
Transmis [#] [n#]
Per cent of light
which gets through a non-absorbing layer of glass with n=1.540
at a given angle of incidence (not considering multiple reflections).
angles transmitted reflected
/ 1 degree electric vector at each
outside inside perp. along average surface
0.0 0.0 91.4 91.4 91.4 4.4
5.0 3.2 91.4 91.3 91.4 4.4
10.0 6.5 91.7 91.0 91.3 4.4
15.0 9.7 92.1 90.6 91.3 4.4
20.0 12.8 92.7 89.9 91.3 4.4
25.0 15.9 93.4 89.0 91.2 4.5
30.0 18.9 94.4 87.9 91.1 4.5
30.0 18.9 94.4 87.9 91.1 4.5
35.0 21.9 95.5 86.3 90.9 4.7
40.0 24.7 96.7 84.2 90.5 4.9
45.0 27.3 98.0 81.6 89.8 5.3
50.0 29.8 99.1 78.1 88.6 5.9
55.0 32.1 99.9 73.7 86.8 6.8
60.0 34.2 99.7 68.0 83.9 8.4
65.0 36.1 97.7 60.9 79.3 11.0
70.0 37.6 92.2 52.0 72.1 15.1
75.0 38.8 81.1 41.2 61.2 21.8
80.0 39.8 62.1 28.7 45.4 32.6
85.0 40.3 34.1 14.8 24.5 50.6
So, for 75 degrees and some non-absorbing glass with index of refraction
amounting to 1.54, the first surface reflects 22 per cent of light, surely
not most of the light. Most of the light gets through the glass, in
reality.
Common iron-containing glass is not at all non-absorbing, but still the
transmitted part is over 55 per cent at 75 degrees.
For an extreme refr. index of 1.7 (as used for strong eyeglasses to
be able to make them thin) the table reads:
Per cent of light
which gets through a non-absorbing layer of glass with n=1.700
at a given angle of incidence (not considering multiple reflections).
angles transmitted reflected
/ 1 degree electric vector at each
outside inside perp. along average surface
0.0 0.0 87.4 87.4 87.4 6.5
5.0 2.9 87.5 87.3 87.4 6.5
10.0 5.9 87.8 87.0 87.4 6.5
15.0 8.8 88.4 86.4 87.4 6.5
20.0 11.6 89.1 85.6 87.4 6.5
25.0 14.4 90.1 84.5 87.3 6.6
30.0 17.1 91.3 83.0 87.2 6.6
35.0 19.7 92.8 81.1 87.0 6.7
40.0 22.2 94.4 78.7 86.6 7.0
45.0 24.6 96.2 75.7 85.9 7.3
50.0 26.8 98.0 71.8 84.9 7.9
55.0 28.8 99.4 67.1 83.2 8.8
60.0 30.6 100.0 61.2 80.6 10.2
65.0 32.2 98.7 54.1 76.4 12.6
70.0 33.6 93.9 45.6 69.7 16.5
75.0 34.6 83.2 35.8 59.5 22.9
80.0 35.4 64.0 24.7 44.4 33.4
85.0 35.9 35.4 12.6 24.0 51.0
Such a glass is surely not used for outdoor luminaires (n=1.70 holds
perhaps not even for violet light, I have no tables at hand). Even so,
more than half of the light gets through at 75 degrees. Eyeglasses from
such material are always coated to suppress reflection.
Even Yvan's values seem to be too high.
An easy way to see the amount of transmitted light is moving a window to
look through it at different angles. It does not at all cease to be
transparent at 80 degrees, even if a double-pane one.
(I wonder, with current common vacuum chambers, would it be expensive to
coat even n=1.5 glass to raise transmission at 75 degrees?)
Where the simple programme transmis is available is apparent giving a
-? as a parameter:
hollan@hollan:~$ transmis -?
Transmis [#] [n#]
Per cent of light
which gets through a non-absorbing layer of glass
at a given angle of incidence (not considering multiple reflections).
A single angle (/1 degree) may be given as a parameter,
otherwise a series of angles from 5 to 85 degrees are computed.
N# gives another index or refraction than 1.54
( (C) Jan Hollan, N.Copernicus Observatory and Planetarium in Brno, 1999;
subject to the GNU General Public License, http://www.gnu.org/copyleft;
source code available at http://astro.sci.muni.cz/pub/hollan/programmes)
jenik hollan
PS.
the ``clean air act'' _including_ an anti-LP amendment did pass the Czech
Parliament in Thursday. The representatives voted with an overwhelming
majority over the Senate recommendation to strike this and other parts of
the law. So, if Vaclav Havel signs it, light pollution will be mentioned
in a second-order law on the environment (the first-order one is the ``law
on environment'' itself).
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Message: 4
Date: Fri, 15 Feb 2002 20:58:54 -0500
From: Yvan Dutil <yvan.dutil@sympatico...>
Subject: Re: R: R: R: R: A few things we should know about light pollution!
Pierantonio Cinzano a écrit :
> <I agree with you that this argument does
> <not work for the old cobra lens system with are much to curved anyway. But
> <it does apply to modern high efficiency equipment.
>
>
> By the way, did you noticed that an horizontal flat glass seems became dirty
> less rapidly than a curved glass? How many administrations in US clean
> frequently the optics of their fixtures? And we spend time here to discuss
> about radiant efficiency?
Effectively, I do not know any city, which does the preventive cleaning of there
lamp, neither one that remove the lamp before they burn as recomended by
the norm. However, the engineer do have to take account of the depreciation
factor in their design. Poor conception is mainly a city issue since often they
simply do not have the ressource to do the job properly.
Yvan Dutil
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Message: 5
Date: Fri, 15 Feb 2002 16:06:20 -0700
From: IDA <IDA@DARKSKY...ORG>
Subject: Fwd: NPR Piece on light pollution
>Please distribute the attached to your respective groups
>
>
>National Public Radio piece
>http://www.glrc.org/
>
>
>Appreciating the Night Sky
>by Ed Janus
>Includes IDA members David Liebl (WI) & Tine Thevenin (MN)
>
> Lights observed in the U.S. by a satellite
>in the Department of Defense's Defense Meteorological Satellite
>Program (DMSP). Photo courtesy of the National Geophysical Data Center.
> The invention of electric lights at the end
>of the 19th Century ended the ancient tyranny of darkness
>over our lives. Turning on the lights at night has allowed us
>to make every hour count. But while nighttime lighting has
>given us unprecedented security and uncountable opportunities,
>we may be reaching the point where we have too much of a good thing.
>The Great Lakes Radio Consortium's Ed Janus reports on two people
>involved in an international effort to turn the lights down a little
>and take back the night:
>
>Release Date: February 11, 2002
>Running Time: 4:36
[Non-text portions of this message have been removed]
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Message: 6
Date: Sat, 16 Feb 2002 10:00:23 +0100
From: "fabio.falchi@libero..." <fabio.falchi@libero...>
Subject: Re: A few things we should know about light pollution!
> Now, I personally believe that IDA is doing themselves a disservice by
> requireing flat lenses. Photometric labs in the USA DO NOT give Full
> Cutoff
> designations to anything but flat lenses. I don't care what the Itali
an
> gentlemen says.
Evidently, US designers can't think about slightly curved glass
combined with a proper small shield.
> So now you've got Full Cutoff fixtures that only perform well on a 5X5
> SMH
> and the lighting designer uses more of them to achieve proper
> uniformity.
In US you're very fortunate to spek of 6x6 or 5x5 SMH. In Europe the
uniformity requirements are far higher. We rarely have 4x4 SMH.
> More fixtures reflecting light off the ground and into the heavens.
It's notorous that not all rays excaping in the sky pollute in the same
way.
> Another side story for WHITE light. Did you know that Asphalt and
> Concrete
> reflect more HPS light than it does Metal halide light? Dr. Adrian
> presented this research to the Roadway committee last year.
> doug
Good news, in luminance designing this means that we can use lower
wattage using HPS than metal halide (aside from the intrinsecally more
lm/W of the HPS vs. MH).
Fabio Falchi
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