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\begin{document}
\subsection*{Luminaires which do not shine upwards: efficiency and technologies
}
\addcontentsline{toc}{subsection}{Luminaires which do not shine upwards: efficiency and technologies}


The key for reducing the most harmful light emissions is using \emph{just
such luminaires, which are not visible as high-luminance objects when
viewed from above}. An elementary check they do not shine upwards:
their luminance should be lower than the luminance of the lit areas
below and around them. Directions close to horizontal ones are especially
critical, as they cause pollution propagating to distances of over one
hundred kilometres, and make the far-away luminaires visible as dominant
points in the night landscape whenever they are not below horizon.


In many regions no other luminaires can be installed outdoors (if they
use lamps producing more light than some limit, 700~lm to 3500~lm,
usually 1500~lm). The wording of corresponding legislation contains
usually a very small allowance for horizontal direction (or for
direction parallel to a very long slope): even such luminaire is
compliant, whose specific luminous intensity is less then half a candela
per thousand lumens of the luminous flux produced inside it. This
allowance is needed to obey the law completely in real world, where
random tilts of luminaires (along the road, usually) up to 0.5
degrees are common even when using a level to position the luminaires
ideally (still larger tilts are usual for luminaires hanged on cables).
To illustrate it: a luminaire with a 100W discharge lamp can appear no
brighter than four candle-flames together, when viewed from anywhere
above itself (in comparison, its maximum luminous intensity when
observed from below can reach ten thousand candelas). Properly installed
luminaires obeying this rule are often called \textbf{fully shielded}''
in the scientific literature (we shall use its short \textbf{FS}
occasionally). They offer plenty of space for architects willing to make
the luminaires decently conspicuous; in fact, all luminaires are allowed
(due to the above mentioned generous limit of $<$~0.49~cd/klm) to have
luminances ten times larger than those of the lit areas, sometimes even
hundred times larger! (Let's remember that common bad luminaires,
comprising over 95 per cent of current European stock, have luminances
exceeding those of the lit ground thousands of times, even dozens
of thousands of times).


The technology for achieving zero emissions horizontally and upwards is
very simple. The luminaire is to have an opaque body containing a
cavity with the lamp (or lamps), ending by a horizontal opening at the
bottom. All old-time luminaires applied at railways were of this type,
with no highly \href{http://en.wikipedia.org/wiki/Specular}{specular} optical
surfaces inside their cavity. Modern luminaires which should distribute
the emitted light optimally onto the target surface have to be equipped
by one or several mirrors inside, with a complicated geometry, to
distribute light in a sophisticated way (tuned up to the configuration of
terrain and to positions and properties of other luminaires).


Old FS luminaires used to have an open cavity, with no transparent or
translucent element (a lens, flat or meniscus-shaped) at or above
the bottom opening. Modern FS luminaires tend to include such an
element to prevent dust entering the cavity and reducing the
specularity or even \href{http://en.wikipedia.org/wiki/Reflectivity}{reflectivity} of the
mirrors. To keep the emissions at horizontal directions at zero, such
an element (called a refractor as well, or sometimes a diffuser, if it
is meant to randomise the directions of the outgoing rays) is to be
hidden inside the cavity as a whole. An easy daylight check is that the
luminaire can be laid on the table by its bottom opening and remains
stable, or that the refractor ceases to be visible from a side (and
therefor from all the upper hemisphere).


To reduce those emissions which do not go steep down enough,
manufacturers prefer using a flat glass as the refractor. It concerns
surely all emissions to angles higher than 75 degrees from \href{http://en.wikipedia.org/wiki/Nadir}{nadir} (direction just down to
the ground), these are always unwanted. Using flat glass is no
technology necessity however, many luminaire types are large enough to
comprise a
\href{http://en.wikipedia.org/wiki/Lens_\%28optics\%29#Lens_construction}{meniscus} within them (the fact that it is hardly ever used hints on
suitability of flat glass to diminish emissions over 75 degrees). In any
case, glass should be of a low-iron type to be non-absorbing for light
(nothing very common nowadays, with a possible exception of really educated
manufacturers). Generally, there is no obligation to use a lens at all
-- dust and dirt can be cleaned from suitably coated mirrors on the
occasion of lamp replacement. Frequent claims that bare mirrors (which
can be coated by a durable mineral layer) deteriorate quickly are
supported by no field research, in my knowledge. An alternative is to
change the mirrors instead of cleaning them at the site (cleaning them
in the workshop, or even recycling them). Glare protection by refractor
is absent in such a case, but luminous intensity in needed directions can
be maximised at the beginning of the luminaire life, maybe even for the
lifetime of the luminaire. Hopefully, luminous intensity in unneeded
directions can be kept low in the same time by proper cavity geometry and
non-corroding mirror surfaces. (Technically speaking for insiders, IP 65 has
not been proved as the winning possibility yet, or?)


Glass lenses are often considered as less transparent than plastic
lenses. This might apply for iron-rich (partly absorbing, cheapest)
glass and \textbf{new} plastic refractors. Proper comparison should be
made, however, after at least fifteen years of duty. Diminished
\href{http://en.wikipedia.org/wiki/Translucent}{translucency}
of plastic refractor is to be expected, and, especially,
diminished transparency. Luminous intensity maxima at angles around 67
degrees (desirable for installations with extreme spacing) can be
expected to go down also due to the deterioration of mirrors. The still
employed assumption that all deterioration of the luminaire can be
expressed by a single number (scaling down all the original luminous
intensities) is surely wrong. Especially for plastic refractors. No
field research is known to the author as this is concerned, even no
research by luminaire manufacturers. In spite of tens of millions of
suitable luminaires being available for that purpose in the EU...


Luminaires which do not shine upwards are THE option for the
best-quality lighting which is employed by some enlightened agencies
even without a legal enforcement. This includes Departments of Transport
(responsible for road lighting) in several states of the U.S.A. for
decades already. And many municipal lighting utilities. There is nothing
revolutionary about such luminaires. Why the good experience does not
spread out itself to the whole world quickly?... hidden interests of
some industries? Then legal enforcement is to help the good thing to
win!

\subsubsection*{Hundreds of luminaires, thousands of light distributions}
\addcontentsline{toc}{subsubsection}{Hundreds of luminaires, thousands of light distributions}

The offer of fully shielded luminaires is giant already. So large that 
it may be a problem to select an optimum product (and its optimum optics 
setting) for any particular lighting task. As a first aid, I've 
made thumbnail overviews for many manufacturers, and hypertext 
numeric overviews sorted e.g. on maximum luminous intensity at about 70 
or 65 degrees. See e.g. directories
\href{http://amper.ped.muni.cz/light/ies2/EasyLight-SaveTheSky/}{../ies2/EasyLight-SaveTheSky}
\href{http://amper.ped.muni.cz/light/ies2/acuity_brands_lighting/}{../ies2/acuity\_brands\_lighting}
\href{http://amper.ped.muni.cz/light/ies2/siteco}{../ies2/siteco/}.
(I'd gladly add another sets, when I'd get recent photometry files of all 
fully shielded products from any manufacturer.)


From the computed images it is evident that many fully shielded
luminaires can be used even for extreme spacings from 1:5 to 1:6 (or
even for 1:7), achieving very good uniformity of illuminance or
luminance of the street. 


\subsubsection*{Coated glass, the only proper lens for any luminaire}
\addcontentsline{toc}{subsubsection}{Coated glass, the only proper lens for any luminaire}

In the third millennium, cheap an extremely efficient one-layer
antireflection  coatings from porous SiO\ensuremath{_{2}} have been
developed. They are employed both for window glazing and solar
collectors already. Demands for luminaires are very similar to demands
for solar collectors: high transmittance especially at large incidence
angles. Transmission over 98 per cent up to 50 degrees angle of 
incidence and over 80 per cent at 70 degrees (!) are achieved this way 
(see \href{http://amper.ped.muni.cz/light/EuP/transmis1.25.png}{transmis1.25.png}). 
And such values should last for decades. Coated low-iron glass (called 
white glass as well) is clearly the best technology for luminaire 
lenses, maximising the useful light output and still reducing glare at 
large angles. No plastic bowls protruding down from luminaires can 
perform better. The old argument of not-enough-large transmission of 
flat glass at 65 or 70 degrees is no more applicable! The cost 
difference of common bad (greenish) float glass and the best glass with 
over 95\% transmissivity ist small enough compared with total price of any 
luminaire. It pays off due to better luminaire efficiency which is not 
limited by lens ageing. (Some luminaires in the directories mentioned 
above use lenses with antireflection coatings, for Siteco the coatings 
are mentioned in several photometric files)


Number of producers of such glazing is rising quickly, there seems to 
be more than ten of them already. For links to basic physics,
technologies and producers see \href{antir_bookm.htm}{antir\_bookm.htm}. 
Common technology is 
sol-gel dip coating and subsequent hardening (lenses  are to be shaped
prior to hardening).


\subsubsection*{Novel parameters of quality streetlighting}
\addcontentsline{toc}{subsubsection}{Novel parameters of quality streetlighting}


In the course of being a stakeholder of EuP streetlighting preparatory
study, I came to a novel idea how to help promotion of such
high-quality, low-pollution technology. Road lighting standards include
a quantity which describes glare in some way. Most often, it is called
\emph{threshold increment} (of contrast threshold, at which the
object gets noticed; \textbf{TI}). This is computed from the so-called veiling
luminance, a bright fog'' produced by dispersion of light inside eyes
(esp. old eyes). All light entering the eyes contributes to veiling
luminance, to unwanted light signal which brings no information about
environment and makes detection of real objects difficult. But bright lights
close to the observed target contribute more than lights which are
(angularly) far from them.


Even good lights (having 0 cd/klm at 90 degrees from nadir and less than
30 cd/klm at 80 degrees) may be a source of considerable veiling
luminance when they are close to the observer, shining steep down.
However, it is easy to get rid of such glare: simply by a sun visor (of
a car or of your cap), a peak. The only light which is not easy to get
rid of is that one which comes from near the observed object (the object
is typically a road before you, the obtrusive lights are headlamps of a
car going toward you -- or, the bad streetlights in distance). The
difference between hight-quality lights and bad ones would be much more
apparent if the threshold increment would be computed just for light
which is difficult to screen by a visor: for light which is
less than 15 degrees from the target.


This advantage of top-quality lights should be extended by considering
twice higher veiling luminance, computing it for 66 years old people
instead for young ones. (Technically: I use $L_v = 18\,
\Sigma (L / \theta^{2})$, computed for
$\theta < 0.1$ degrees in my \href{http://amper.ped.muni.cz/light/luminance/}{raw2lum}. 
Common TI is then
$65 \times L_{v} / L_{av}^{0.8}$.)


My recommendation: introduce 3 additional parameters into EasyLight,
Roadpollution and into any standards: TI\_66-15, TI\_66, TI-15; 15 stands
for angle, 66 for age.


(A remark for the EuP study: maxima TI\_66-15 should become an important
system-level requirement. Should be below 5 in all cases).


\subsubsection*{Conclusion}
\addcontentsline{toc}{subsubsection}{Conclusion}


Various parameters can help to distinguish bad and good lighting. One is
however of prime importance, becoming an 
\href{http://en.wikipedia.org/wiki/Occam\%27s_razor}{Occam's razor}
for outdoor lighting: achieving \textbf{zero specific emissions horizontally
and above}. It's the easiest property to verify and it helps more than any
other one. And it is really simple to achieve.


\bigskip\noindent\itshape
jh, Feb 2007
\end{document}
