Research matters - Illuminating Engineering Society

LIGHTING FOR QUALITY Is Cutoff Really In The Eye Of The Beholder?
Doug Paulin, LC
ONE OF THE MOST
misunderstood areas of lighting
performance is wrapped up in
words like “cutoff,” “glare control ”
and “shielding.” Nothing toasts my
cookies more than someone pontificating
on these subjects and
demonstrating that they really don’t
have a good handle on what cutoff
really is, what it does or how it’s
measured. I usually bite my tongue
during these times, and once in a
while I even learn something new
by hearing a layman express his
views on the subject.
The laymen I refer to here are
the “city fathers” who are trying to
deal with their lighting issues. Often,
they are speaking on full cutoff [or
“FCO” as it’s known in quasi-lighting
circles] and they don’t know
exactly what full cutoff is, but they
know what they like about it. And
what they like is that it tends not to
cause light trespass problems.These
laymen tend to generalize full cutoff
into any luminaire that acts in a similar
fashion. In other words, it has its
aperture parallel to the ground, and
it “runs out of light” not too far
away from the pole.
It was interesting to me when a
village planning commission chairman
went on and on about how
great some “lantern lights” are,
which were just installed near his
house. “How do you like those new
full cutoff lights . . .?” he says. I knew
these Colonial Lanterns (with cutoff
optics in the hood) were not truly full
cutoff, but since he liked them, why
argue the fine points? Besides, I get
enough strange looks when I talk
about lighting minutia on a daily
basis. But the impression this person
had was a good one, and it
made me think about what is really
important to ordinary people:
Controlling light to a given area, and
not worrying about the small stuff
(the light reflecting off the vertical
corners of the lantern). Indeed, this
Colonial Lantern was definitely not
full cutoff, because there was incidental
light emitted at 90 deg vertical,
but it was close enough to do
what the neighbors wanted.
Structural Elements
Play A Role
Let’s take this to the next logical
step. This incidental light was a
product of structural elements necessary
to the mechanical support of
the luminaire (the vertical corners
of the lantern were an integral part
of the luminaire, yet caused some
light to be emitted at 90 deg, thereby
knocking it out of full cutoff classification.*
Another logical step
takes us to any post-top luminaire
classified as full cutoff. What about
the “yoke” or “spider” attachment
that allows the luminaire to “hover”
over the pole? The answer to that
is the luminaire is tested without the
yoke. Then how different is a “shoebox”
mounted on a post-top yoke
from the Colonial Lantern from the
previous paragraph?
From here, it’s only a short jump
to poles. No, not North or South,
nor natives of Poland, but the poles
to which we fasten our lighting
apparati. Poles reflect light and
block light. Ergo, the “glint” that we
get from a brushed aluminum pole,
and the “pole shadow” that never
shows up on iso-footcandle diagrams.
Should we factor-in reflections
off of poles and pole-shadows?
Probably not, but as good little
lighting nerds, we need to know
that these things are present, and
stop thinking in absolute terms.
All of this builds a case for a little
tolerance...for the classification
called “IESNA Cutoff” which allows
a small amount of light at 90 deg and
above.** Yes,there is a small amount
of light above 90 deg if there is light
at 90 deg. Optics are just not that
precise because our light sources
do not come from a true, point
source. All light “beams” have a
thickness depending on the orientation
of the arc-tube. But this little
bit of “fuzz” above horizontal can do
some good things in certain environments.
In a commercial/ retail
area where the buildings are taller
than the luminaires, those buildings
get some light on them and don’t
form a “cave” of light and darkness.
Semi-Tough
Lastly, let’s dissect semi-cutoff
luminaires briefly. There are some
semi-cutoff luminaires that look like
they should be full cutoff but aren’t.
Luminaires that have flat-glass lenses
(parallel to the ground) and are
classified as semi-cutoff tell you that
they failed the full cutoff “test” at
80 deg vertical. They emit more
than 10 candelas per 1000 lamp
lumens at this angle. If they have
the “magical” flat glass lens, they
probably don’t miss it by much, and
they might miss it in only one direction.
I know of at least one “forward
throw” luminaire that only
misses the criteria in one direction
. . . “forward.” Duh. Is this bad? It
depends on the application, and the
. . . . . . . . . . . . . . . . . . . . . . June . . 2004 . . . LD+A . . . . 22. . www.iesna.org
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