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\begin{document}
\title{Three reasons why full shielding is indispensable}
\author{Jan Hollan}
\maketitle

\tableofcontents

\section*{Introduction}%
\addcontentsline{toc}{section}{Introduction}

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Let's distinguish three situations when fully shielded luminaires perform
much better than the non-complying ones, i.e.,
those with non-zero ($\geq 0.5$~cd/klm) horizontal emissions.

\begin{enumerate}
\item you are above them
\item far from them
\item concerned with skyglow
\end{enumerate}

I remind that ``full cutoff'' category differs from a fully shielded one
just by adding a very loose limit of 100~cd/klm at 80 degrees and above.
``CIE cutoff'' category demands 30~cd/klm there. Luminaires, which are both
``Fully shielded'' and ``CIE cutoff'' in the same time, promise to be
low-disturbing ones.

Further I'll consider just the basic $<0.5$~cd/klm limit demanded by many
laws for any horizontal or higher-aimed beam (specific luminous
intensity: luminous intensity divided by the luminous flux produced by
the bulb; an equvalent mcd/lm would be a more SI-complying unit; a bare
lengthy light source like a candle flame has up to some 100~mcd/lm).

\section{when the luminaire is lower than you are}

Looking from a window which is above a fully shielded luminaire, no
direct light is visible. This is a tremendous health (for bedrooms) and
aesthetic improvement. Life quality of hundreds of millions of people
living  in cities could be improved this really simple way. There is no
excuse for using other than fully shielded luminaires in areas with
$>$2-floor houses or slopes.


\section{when it is above you, but far enough}

If the distance is over ten times the height difference (you see light at
84 degrees from luminaire nadir), the luminous opening of the luminaire
becomes just a line segment (its width is one tenth of the original circle or
rectangle). The burner itself is invisible, just its diffuse or specular
reflection in the opposite side of the luminaire cavity can be seen.

In principle, there could be still a lot of light with special
configurations, say 400~cd/klm without glass (flat glass reduces the
amount of light four times at this angle). However, no such luminaires are
made, for symmetric lighting: light going over 80 degrees is avoided. Even
David Keith's extreme example, see
\h{amper.ped.muni.cz/light/ies2/d_keith/tab/samp0479.txt} (missing the
0.5~cd/klm just slightly, having 0.7 at one isolated direction) has
``just'' 50~cd/klm at 85 degrees.

My guess is that maximum specific luminous intensity is 70~cd/klm at
84 degrees from bi-directional fully shielded luminares.\footnote{To 
be accurate, for unidirectional streets, ski slopes and sport
facilities, fully shielded beamers can be employed, which have over
200~cd/klm at 85 degrees (e.g. by using inclined glass inside the
fixture). Such special installations should be subject to some special
permit, to avoid light imissions to distant human or wildlife habitats,
to another streets etc.}
  A thin outer
shield could be added to such a luminaire, blocking any harmful almost
horizontal beam, if needed.

Vast majority of fully shielded luminaires have however less then 10
cd/klm, 3~cd/klm is a typical upper limit. At 3~cd/klm, 30~m from you and
with a 100W HPS (roughly 10klm) source, this translates to 0.03~lx
vertical illuminance. At 100~m it would be 3~mlx, what can be tolerated in
most cases (even if being 30\mtimes stronger than the brilliant Venus as
Evening star).

Quick reduction of luminous intensity as the beam approaches 90 degrees
helps 3D orientation at night tremendously: it's obvious which lights are
far from you (say, over a crossing) and which are close. This is not the
case with non-FS lights.


\section{when you are concerned with the clear sky luminance}

Total amount of uplight is relevant just in the case of overcast sky,
when the cloud layer returns most of the light back to the ground.
Minimising this quantity can help to protect healthy sleep and to
protect wildlife. Some luminaires just missing the fully shielded limit
may be as good as the best fully shielded ones for this purpose (or even
slightly better, on the cost of more glare etc.), if minimizing
installation costs is an aim too. If such luminaires with zero or very
low upward emissions are used, the key to minimizing uplight is
minimizing total emissions, i.e., avoiding any light outside target
areas and any intensities over the demanded ones (incl. late night
demand; this calls for continuous dimming technology).

For a \emph{cloudless sky}, however, \emph{total amount of uplight is an irrelevant
quantity}.  Sky luminance is given mostly by that light, which propagates
just a bit upwards, and which becomes completely dispersed after some 100
miles of atmospheric path. Steeply going light makes little harm, as it
escapes to the space from 90 per cent.

This is the reason why there is MUCH less skyglow over areas with no other
lights than fully shielded ones. Apart from observations, it's an easy
physics to compute the effect, as Pierantonio Cinzano has done
convincingly (use e.g. his Roadpollution software to compute some examples
yourselves), and as my ies2tab programme does too. For reducing skyglow
over EU or US, 0.5~cd/klm is a vital limit. 1~cd/klm cannot be tolerated,
if it would concern most azimuths.


\section*{Basic and additional shielding}%
\addcontentsline{toc}{section}{Basic and additional shielding}

The above 3 reasons
should be enough for introducing a legal obligation to use no other
luminares than fully shielded ones for $>$1klm sources. Of course, this
elementary obligation does not ensure that the lighting will be excellent.
But it's simple, easy to verify and avoids the worst impacts of outdoor
lighting.

\emph{An additional obligation should exist: to shield any luminaire,
when some its beams are obtrusive} for any driver, pedestrian or
inhabitant and not needed at that particular place for safety. Such an
obligation would become a tool how to make lighting designers to care
about the proper downward-going light distribution, to spare future
costs to the luminaire owners.

\emph{Outer shades} (e.g., a rectangular boxe with its long axis along the road)
should be added in many cases at the very time of the luminaire
installation. The reason is that current too small and poorly-directing
fixtures let a lot of direct light from the burner going sideways, e.g. to
windows lying behind and below them. A strange daytime view should be no
hindrance to achieve a good benefit/harm ratio at night. After all, at
least Phillips has some fixtures with a slit for inserting an outer
vertical mirror (a coated, non-corroding aluminium sheet) to reflect
houseside-going light back to the street. Low-cost and good performing
solutions can be always found, if there is a good will. Outdoor lighting
can cease to be a curse of our epoch.

\end{document}
