[Darksky]Re: shielding question

Jan Hollan
Fri, 22 Oct 2004 18:26:33 +0200 (CEST)

> We hear many times that "full cut off" (and full shield) shall apply to
> every luminaire which spread more than 1500 lumens ( about 100-150 watt
> incandescent).
> Well !  If we allow such light emission, why do we not also allow, for
> instance, to have an architectural , or an advertising post-top, having
> a high lumens HID (5000-20000), but with a cut-off system that allow in
> horizontal plane only like the unshielded 1500 lumens output one, and
> therefore reducing it to a ``faint source'' as glaring effect. Not 2,5%,
> not 10%, (like Cut-off, or Semi-cut-off) per total emission, but rather
> a certain low limit of emission (only to show the lampshade shape).
> More exactly to reduce the light horizontal emission to those of a 1500
> lumens unshielded ``faint source'', and , of course following the
> conditions about ``no exceed'' 2250 lumens cumulative upward flux in any
> 2 meter radius region.

Dear designers,

I appreciate your question, it deserves a thorough answer.

There was some reasoning about the issue in my commented proposal
http://www.astro.cz/darksky/cz_law/lp_en_ex.pdf; when I take the relevant
parts from its LaTeX source the comments read:


  ``The first of the emission limits, the value of 1500 lumens for sources
not subject to regulation, is very liberal. It enables the citizens to use
without restriction those light sources, which they use most usually.

  ``High pressure discharge sources produce light fluxes over 1500~lm
nominally, they are the main source of light pollution, and there is no
reason why their light should not be properly directed in all cases. The
more due to the fact that their installation is mostly done by
professionals, not by the citizens themselves.''

  ``The exemption of the `faint' sources from the obligation to shine just
downwards is provided for the ordinary citizens mainly. It should not be
interpreted in a sense that the pollution produced by such sources is
negligible. And as a recommendation to install further glaring lanterns,
globes etc. with strong compact fluorescent sources consuming over 15~W.
Such luminaires are unsuitable for lighting anything. To be pleasant to
look at themselves at night, they are to be much much fainter -- directly
visible 6W fluorescents may be nice in a strongly lit evening downtown,
but still obtrusive in a village. A limit for installations done by
lighting professionals should be not 1500~lm, but rather 150~lm only.

It is however a hint how to improve temporarily the most polluting
luminaires by changing a high-intensity discharge source for
a compact fluorescent one -- this can be done by an electrician,
cheaply and quickly, before the luminaire will be replaced by a new
one, which will direct the light flux of the source perfectly.''


In another words, that 1500 lm limit does not mean that such sources would
not be polluting. It just means, that in our time, their contribution to
the pollution is much much less than the contribution of stronger sources,
so exempting them from regulation does not compromise the goal of the
legislation appreciably. Having an even lower limit on a local level is

It has been shown in the papers of Pierantonio Cinzano (and in some texts
and computations by me) that the 0 cd/klm limit is a necessary one for the
majority of outdoor lighting. Even 1 cd/klm would be to much, for
directions from 90 to 120 degrees from nadir.

In most cases, this limit is fulfilled simply by allowing no light
over horizontal direction. The vast advantage is, that compliance with
the limit can be then verified by a mere look at the luminaire, in
daylight or at night.

But it can be fulfilled even in such a way, that some light penetrates the
top part of the luminaire. For a 2000 lm source, the luminous intensity
may approach 1 cd. This may be nice, even if it is a bit more than a
usual lampion (Chinese lantern) has, as the paper absorbs some light from
the candle inside (which has that 1 candela of course).

For a 10 klm source, the luminous intensity above horizontal directions
may approach 5 cd, which is surely enough to become conspicuous even in a
rather strongly lit environment (if luminaire would have some 0.3 m2
cross-section, its luminance could be whole 50 cd/m2). It could be too
glaring for people viewing a night landscape from a balcony. But of
course, an order of magnitude less glaring than an opal globe with a 1000
lm source inside.

> A luminaire's lampshade itself , but, is true, only with a very low
> lambertian glare surface (only to look the shape of lampshade in the
> dark(ness) may be an architectural fact. Isn't it ?

If much more than 20 cd/m2 is really needed, there is a way: using
sources below 1500 lm. I.e., separating the luminaires meant for
illumination (using HID sources) and those meant as a decoration.

I can imagine a rule that would limit the maximum luminance of any point
when the luminaire is viewed from above, depending on the environment
where the luminaire is installed, but I am sure that such a rule would be
far too complicated to be put into any law. The main obstacle is an easy
verification that the law is obeyed -- I cannot offer any. I can easily
verify the luminance from one observing point, by making and processing an
image by some CCD cameras (Fuji S5000 and Canon EOS D60 for the moment),
but not from all other observing points...

There is a difference from the rule I recommend for billboards, which have
a simple geometry.

I admit that having 200 cd as an upper limit for billboards may seem as
very liberal compared to the 0 cd/klm limit for luminaires.  Again, as in
the case of ``faint sources'', the liberal approach stems from the fact,
that vast majority of current pollution is produced by HID luminaires. The
rules for HID have to be much stricter also due to the fact, that such
luminaires remain in function for decades. If we are to ensure that
pollution will decline instead of rising as up to now, the rules have to
exclude unnecessary (and most polluting) light from the worst polluters.
Allowing anything like tens or even hundreds of candelas horizontally and
upward would inevitably mean no change to the current trend. No law would
be better than such a ineffective one.

> On the other hand, what do you think about the post-tops and the
> "cut-off"  details shown in the link
> http://luminaires-projects.netfirms.com/posttop_st/view_sections/pac_sections.htm
> Are those cut-off shields and luminaires in compliance with DarkSky
> conditions, as long as the horizontal light emission of lamp is stopped by
> the shields  ?

I'm sorry to have to say, they are in no way compliant to the laws
protecting the night environment. I am sure is not possible to reduce
horizontal and upward light to 0 cd/klm by internal baffles inside a
lantern. The only way is to put the light source into the opaque top of
the lantern.

For HID sources (incl. SOX) such luminaires are not recommendable for
outdoor use. On the other side, for fluorescent sources below 25 W, they
would be surely much better than globes or the most widespread kind of
Siteco ``Pilzleuchte''.

> An other question refers to the use of low voltage outdoor lighting in
> residential or home areas. Do you consider suitable.  Are there rules, low
> or restrictions for such fixtures ?

In Ketchum and Hailey (towns in Idaho), the limit for sources which are
allowed to be used in luminaires which are not fully shielded (= having
0cd/klm horizontally and above) is some 750 lm.

Again, this is a liberal limit and I believe that lighting designers,
unlike laymen, should use fully shielded luminaires for almost any outdoor
sources other then festoons. Candle, having 1 cd horizontally, is an
extreme which may be still pleasant to view at, in an artificially lit
outdoor environment (indoors, the environment may be lit enough by the
very candle). Lampion, with a 5 W incandescent inside, may become a
pleasant light in a city outdoor environment.  But not with a 25 W or

with best regards,
 j hollan