Re: fco and up light

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

(again a lengthy commentary, available separately as
    http://amper.ped.muni.cz/jenik/letters/public/msg00206.html
   and why_fs.* (with some markup) within
    http://amper.ped.muni.cz/light/shielding for later access)

Let's distinguish several cases when fully shielded luminaires perform
much better than the non-complying ones, i.e.,
those with non-zero (>=0.5 cd/klm) horizontal emissions.

1) you are above them
2) far from them
3) concerned with skyglow

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;
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).


ad 1) 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.


ad 2) 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's extreme example, see
http://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. 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 30x 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.


ad 3) 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 cloudless sky, however, 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.


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.

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.

Outer shades (e.g., a rectangular box 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.

jenik

PS.
 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.