The following excerpts are from an engineer and amateur astronomer who has dealt extensively with light-pollution issues. They were written to address the concerns of a Houston-area amateur astronomer who has had difficulty in getting his residential neighbors to accept that cutoff lighting is a good thing; the Texas resident had paid for the cost of several Hubbell full-cutoff Skycaps which his town then installed on several streetlights near his home; the Hubbell Skycap has been documented in NELPAG Circulars, and there are links to information on the Skycap from the NELPAG main web page. The neighbors then demanded that the town remove the Skycaps, claiming that the streets were not being as well lit as before the Skycaps were installed. Warren Offutt's thoughtful and helpful reply is provided here as a public service, in the hopes that others may get some useful information from these comments:
The definitive source of information, as to increase in lighting levels
when a Skycap is installed, is found in the Hubbell technical literature.
They present measured data showing that the light level in the effective
area increases substantially (I seem to remember something like a
near-doubling of light level) over that which an unshielded NEMA head fixture
provides.
The most authoritative sources of general lighting specifications are
those published by IESNA (Illuminating Engineering Society of North America).
Their standards (and they are comprehensive and numerous) are adopted both
by ANSI and the IEEE and republished by those two professional engineering
societies.
The IESNA standards are, unfortunately, expensive. I laid out about
$200 of my own money for copies which are now in the hands of Code
Enforcement in the City of Alamogordo (NM). Since what you need is buried
in one or more of those standards, considering their expense, your best bet
is through a local university library, a local public library, or even
through your Company's library.
If your Homeowners' Association is meeting this week, and you have
less than 30 days to get support for your proposition, you have a very
difficult task facing you. Rather than trying to find engineering
information to support your case, which the illiterati probably would not
believe/understand/accept anyhow, you might consider seeking support from
the astronomy interests in your city, or in the nearby universities.
Convincing your mayor is probably not enough -- most politicos don't
care what they themselves believe, but rather are responsive to what they
think the voting public believes.
In your presentation, I would suggest you avoid coming on blustery or
aggressive; rather try to provide factual education. The various IDA and
NELPAG publications provide good source material, but you will have to cull
them to find the parts that are useful in your project.
In Alamogordo, whenever the issue of lighting comes up at a City
Commissioners meeting, we make sure the local astronomy club and others
interested in good lighting are there "in force"; even if most of them say
little but nod their heads appropriately, the town officials can see for
themselves where the interest lies.
To connect the issue to astronomy interests is to lose the battle.
Most people think of astronomers as semi-harmless kooks who stay up all
night, and maybe use their telescopes to spy on their neighbors. Tie your
battle to reduction of glare, which benefits all senior citizens and those
with sight impairment; to the potential savings of tax money by making
lighting more effective; to the elimination or at least reduction of "light
trespass" (I'm told that courts in several jurisdictions have ruled that
light trespass is just as onerous as excessive noise, noxious odors, etc..);
to enhanced security through improved visibility of trespassers; and things
of that sort. Maybe your city would like a court test of light trespass?
It does not take a rocket scientist to see that the effective radius
of illumination is about 3 to 4 times the height at which the fixture is
mounted: at that distance, the illumination level from an area fixture is
about 1/10 the level directly underneath the fixture -- and the human eye
does not provide good vision when the scene is illuminated with a contrast
light level of more than about 10:1. It is not the level of illumination
that makes for good visibility, it is a relatively uniform level combined
with a lack of direct glare.
The example I have used in talks here, which seems to strike a responsive
chord with audiences, is to ask the listener to recall the last time they
tried to drive in the direction of the sun at sunset (or sunrise). When the
sun is below an altitude of about 15 degrees (that is, about 1-1/2 fist
widths-at-arms-length above the horizon), it is very difficult or impossible
to see clearly. There is plenty of light, but visibiliy it impaired by the
direct glare.
The same is true with street lighting. That 15 degree angle corresponds
to a distance of about 3.7 times the height of the fixture.
But these are technical facts, and at this stage in your fight,
it might be too late for technicals.
I suspect the real objection you face is that a private citizen (namely,
you) caused the city to change the lighting in front of other people's houses
without their having been informed and having agreed. If that guess is
correct, the solution will probably have to come from politicing on your part
with the neighbors, one on one, outside the meeting, so you can get a couple
of vocal supporters on your side when it comes to a public discussion. A
couple of weeks is not much time to analyze the opposition and strategize
your campaign.
In any case, I think you could make a near demand for the city to
eliminate light trespass from the fixture in front of your house; I have seen
this accomodated by utilities on request by placing a piece of heavy kitchen
foil inside the NEMA diffuser, to block the light in a specific direction....
I wish there were better answers for you, but it is a difficult matter,
and your time is short. In any case, I think your solution, if there is one,
is to be found not in engineering data but in people-people relations with
an outspoken few in your homeowner's association, so you can get some voices
on your side at the next meeting, and maybe bolster that with supporting
remarks from elderly people and people with incipient sight impairment
as to the benefit of glare reduction.
If I had the engineering task of improving security lighting in a
subdivision with widely different pole spacings, here is what I would
recommend.
First: The single most important factor is the elimination of direct glare
from each luminaire. This factor plays the dominant role in determining the
community's security benefit from street lighting. Elimination of direct
glare also results in the most pleasing illumination to the eye. Its sole
disadvantage is that some residents will say, "But I cannot see the light
itself, so how can it be bright enough?" It is a case of subjective opinion
being misleading.
(People who have never experienced good lighting sometimes think that
a bright light seen directly implies good security; however, numerous studies
show that this kind of lighting actually reduces security.
Schools in several parts of the USA and abroad, have reported decreases in
vandalism when lights were shut off to save money; patrolling police report
that it is much easier for police and neighbors to detect prowlers when the
prowler is forced to provide their own light, rather than being able to depend
on the glare-induced shadows for their unlawful purposes. It is unfortunate
that people's perception of security is at variance with measured results,
but the lighting designer must deal both with perception and with reality and
try to find a happy medium.
In fact, a study conducted by the New York City Police Department several
years ago determined that something over 95% of the street crimes were
committed on the 20% of the streets that had the "best" lighting ... something
to think about!)
Second: No matter what is done with luminaires and shields for different pole
spacings, no area illumination fixture can provide useful security
illumination farther than about 5 or 6 times the height of the fixture above
ground. This is an extreme limit which greatly exceeds good design limits.
Good design limits place the outer fringe of useful illumination at about
3 or 4 times the fixture height. However, where the pole locations have
been forced by other factors, it might be necessary to accept a much less
than ideal situation. (Spot lights and search lights can reach farther
under some conditions, but they are not for general security.)
An observer might be able to "see his own shadow" at greater distances,
but the contribution to community security is an illusion because of the
extreme ratio of bright to dim parts of the scene. Community security is
measured not by the perception of law abiding people, but by the reduction of
accidents, by reduction in vandalism and house breakins, by early detection
of fires and similar considerations. Breaches of security are not caused by
the actions of conservative law-abiding residents, but by the actions of
persons having different values.
Third: Determine for each individual fixture where that limit is: either
an _absolute_ maximum of 5 to 6 times the fixture height, or half-way between
adjacent poles, which ever is the lesser distance. Station a sitting observer
at that point. Then, with the observer's assistance, adjust the fixture
shield so the observer sitting at the limit point cannot see the direct
radiation (that is, the direct glare) from the fixture itself. (The observer
should be sitting to minimize the effect of the height of the observer on
the adjustment.)
The purpose is to eliminate the direct glare coming from the fixture,
because it is the direct glare that reduces the security value of street
lighting when the "throw" of the fixture is being stretched to, or beyond,
the limit.
Note that this requires shields different from prepackaged commercial
shields, which are designed for the 3 to 4 times light throw distance, rather
than a greater limit distance.
The disadvantage is the need to fabricate shields for your purpose; the
advantage is that you will be able to tailor the security benefit to your
exact situation, including different cutoff angles in different directions,
including elimination of light trespass into residents' windows, etc. A
four-sided shutter design, fabricated out of light-weight sheet aluminum
with overlapping corners, is inexpensive to fabricate and easy to adust.
Adjustment could be done during daylight hours, if you can arrange for the
lamp to be temporarily illuminated. However, I would recommend adjustment
after dark.
For reference, a cutoff at the 5 to 6 times distance corresponds to a
luminaire cutoff angle of about 10 degrees below the horizontal. If you
wish to test this glare angle, wait until sunset, and then look in the
direction of the sun when it is 10 degrees above the horizon. You can judge
how well road visibility can be maintained with the direct-glare source at
that height compared to other elevation angles, corresponding to other cutoff
angles.
I have worked on matters connected with good lighting for several years
here in Otero County. I have provided advice and consulting to our City's
code enforcement department, and I personally authored the good-lighting
ordinance for the Village of Cloudcroft. My credentials include a Doctorate
in Engineering: I can certify that the challenges involved in community
lighting do have good and practical solutions, although some of those
solutions might seem counter-intuitive at first.
Some of our local subdivision developers, having seen examples of good
community lighting, have approached us for recommendations for their new
developments, believing that the marketability of the properties is
enhanced by the beauty of glare-controlled lighting. Our local County
Planning and Zoning Board includes a one-page specification for community
lighting to be followed by new subdivisions. (I wrote that specification
in cooperation with the Zoning Board chairman; if your Homeowner's
Association would like a copy, I'd be happy to furnish it.)
The good solutions to community lighting provide real enhanced security
while at the same time reducing objectionable light trespass, reducing
wasteful electric costs, enhancing the community beauty and property values,
and reducing environment pollution. People of good faith working together
always find workable solutions, although it takes patience to achieve them.
People who become angry with their neighbors never find them.
From your description of the careful approach of your Homeowners'
Association, I think your community is fortunate to be searching for
reasoned solutions. If you don't lose patience with the process, you will
find solutions that will work to everyone's advantage --- and I believe the
result will enhance property values for all residents when time comes to
sell.
You may use any of the above with attribution if you think it will be
helpful to your community.
Regards,
Warren Offutt, D.E.
Charter Member, ODSA
offutt@galileo.apo.nmsu.edu
Cloudcroft, NM