Lighting for individual sites
Several years ago, the IESNA developed
a new backlight-uplight-glare (BUG)
rating for outdoor lighting fixtures,
superseding the full- or semi-cutoff rating terminology that is deeply ingrained
within the lighting-design industry.
Each B, U and G factor has a rating
of 0 to 5. For example, a certain luminaire might be rated B1-U0-G2. Such a
grade would inform potential purchasers
that the fixture would have no uplight
to add to sky glow, but it would cause
a small amount of backlight, or light
trespass, and it would cause a moderate
amount of glare.
The MLO specifies the maximum
BUG rating a light fixture can have in
each of the five lighting zones. For example, that fixture rated B1-U0-G2 could
be used in LZ2, but not in the more
rural zones LZ0 or LZ1. Once a city has
a lighting law based on the MLO, then
just by looking at the numbers, lighting designers and city planners can see
180°
100°
90°
80°
60°
Backlight
30°
Back
100°
90°
80°
30°
60°
Forward
light
Front
Sternberg Lighting, U. S.A.
Backlight, uplight and glare zones from
a hypothetical luminaire. The B zones,
which are opposite from the area of
intended use, create light trespass. The
U zones create sky glow, especially the
lower UL zone, which most degrades
professional and amateur astronomy. The UH zone is mostly wasted
energy. The G rating for glare takes into
account the amount of lighting from the
FH, FVH, BH and BVH zones.
Municipalities can
customize the MLO
by overlaying “
lighting zones” (LZs) to
various neighborhoods and regions
within its borders.
whether proposed lighting fixtures would
be allowed in a given environmental zone.
The model ordinance also lays out
two methods for determining whether
a non-residential property complies
with the law. Both methods limit the
amount of light used on a property,
but do not control how the light will
be used to illuminate outside space.
The prescriptive method simply specifies the “total site lumen limit” for a
given area, based on its lighting zone
and hardscape area. The performance
method of compliance allows for more
flexibility in decorative lighting, but it
also requires the lighting designer to
demonstrate whether the lighting will
produce such off-site impacts as glare
and light trespass.
Mark S. Rea, director of the Lighting
Research Center at Rensselaer Polytech-
nic Institute (Troy, N. Y., U.S.A.), and
his colleagues have developed a method
called outdoor site-lighting performance
(OSP) for predicting and measuring all
three components of light pollution.
Using commercially available software,
a lighting designer draws a virtual box
around a proposed outdoor lighting
installation and then calculates how to
minimize the amount of light leaving
that box. “If you’re pumping a lot of
lumens into your box, a lot are going to
leave,” Rea said. “If you’re trying to pre-
vent sky glow, it’s about light level—it’s
not about optics.”
OSP provides the framework for a
quantitative, not qualitative, discussion
with a community about holding night-
time activities while minimizing light
pollution and energy waste. “We’re not
there to tell you whether you should or
you shouldn’t have high school football
on Friday nights,” Rea said. Turning off
the lights when they’re not being used
for sporting events is one of the most
important strategies for reducing a site’s
contribution to light pollution.
Prospects for the future
In the United States, light pollution
is probably more prominent in the
public eye than ever before. This spring,
Arizona Governor Jan Brewer vetoed
a proposed state law that would have
permitted bright LED billboards. Independent filmmaker Ian Cheney recently
released a documentary, The City Dark,
which describes all aspects of modern-day light pollution, from stargazing
to sea turtles; it has been making the
round of this year’s film festivals. IDA
is designating “dark sky” communities,
parks and reserves in North America and
Europe, and several entrepreneurs have
set up their own amateur-astronomy
resorts in the darkest corners of the
American Southwest.
Still, Parks bemoans the deeply
ingrained attitude that property owners in most places have more of a right
to shine their security lights into their
neighbors’ windows than to blast their
stereos at 3 a.m. “Public nuisance law is
the next frontier,” he said. t
Patricia Daukantas ( patd@nasw.org) is a freelance writer specializing in optics and photonics.
[ References and Resources ]
>> A. Barghini and B.A.S. de Medeiros.
Environ. Health Perspect. 118, 1503
(2010).
>> M.S. Rea et al. Phys. Today 63( 6), 8
(June 2010).
>> D.E. Blask. Sleep Med. Rev. 13, 257
(2009).
>> C. Luginbuhl et al. Phys. Today 62( 12),
32 (December 2009).
>> R.G. Stevens. Mutation Res.: Rev. in
Mutation Res. 682, 1 (2009).
ONLINE EXTRA: For an expanded
list of resources concerning light
pollution, see the online version of
this article at www.osa-opn.org.
