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New to me permitting issue - foot-candles

Scotchbrite

Scotchbrite

No comment
Gotta start with the street cred info first; I've been permitting signs for over 20 years now. Today I had a request I've never been asked to provide before:
Please provide information that shows that sign lighting will not create more than one-tenth (0.1) foot-candle impact on habitable residential uses in residentially zoned areas south of property.
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The sign in question is in a strip mall that requires individual lit letter signs, and this is the 3rd sign I've permitted for this specific unit.

Aside from being extremely disgruntled about the request just on the face of it, I'm completely at a loss for how I would even begin to legitimately prove a sign will not create more than 0.1 foot-candle at some distant location! I tried doing some Google searching but didn't have any luck finding something that even hinted at a method to do this. The only thing I can find is a meter for measuring the light output once a sign has been installed.

I'm curious if anyone here as dealt with this issue?
 
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BigNate

New Member
simple conversion and math - look up the foot candle rating at a specific distance from the source, calculate the distance to the "some distant location" and then realize that the light density on an imaginary sphere leaving the light source drops off as a function of the cube of the distance from the source... or put a light meter the known distance from the spec'd light source and actually measure the difference between readings with the light on and off (I assume there is some background light as you probable are not making a darkroom just to test....)
 
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BigNate

New Member
... there is surely a chart that has all the calculations for reduction of light density vs distance from source...... try a physics book.
 
Gino

Gino

Premium Subscriber
I've been turned down on several jobs for customers who wanted LED signs on their property. I have no idea what chart or calculations the city used, but they said the sign being proposed would hafta go back about 1,800 more feet. That would put the sign in a forest. The closest residential house/unit to where we specced the sign was over a block away..... and that was considered too bright. Being able to control the lumens better, has relaxed the rules somewhat, but I don't know how to spec that calculation out. Give it to an engineer.... they do everything else.
 
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netsol

Active Member
bignate
i believe it is the square of the distance

this sound like a rare opportunity to buy a new meter of some sort that i do not have now.

this is exciting!!! (and you were going to look on a chart...)
 
Notarealsignguy

Notarealsignguy

Arial - it's almost helvetica
yeah, ummmm, that's not good enough, we're gonna need an engineer to go ahead and put that in writing in order to proceed.
See if the light manufacturer has a spec that you can throw in there, I am pretty sure that I have seen some with charts. They made my electrician friend do decibel readings on a standby generator he installed, sounds similar.
 
Scotchbrite

Scotchbrite

No comment
BigNate said:
simple conversion and math - look up the foot candle rating at a specific distance from the source, calculate the distance to the "some distant location" and then realize that the light density on an imaginary sphere leaving the light source drops off as a function of the cube of the distance from the source... or put a light meter the known distance from the spec'd light source and actually measure the difference between readings with the light on and off (I assume there is some background light as you probable are not making a darkroom just to test....)
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"cube of the distance from the source" is the bit of information I couldn't find.

netsol said:
bignate
i believe it is the square of the distance

this sound like a rare opportunity to buy a new meter of some sort that i do not have now.

this is exciting!!! (and you were going to look on a chart...)
Click to expand...
Well I'm not going to build and install the sign without a permit and risk the sign not meeting the requirement.

How could I even use a meter without knowing if the light it was reading was coming from the lot lights and the Carl's Jr signs that are much closer to the residential property?

Sounds like now I'm going to need a lighting engineer along with the structural engineer. And people will wonder why signs are so expensive.
 
Gino

Gino

Premium Subscriber
They're all just gonna buy magic markers and draw on their own windows. Cripes, half the fonts look like they are already.
 
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BigNate

New Member
... the volume of a sphere increases as a function of the cube of the radius... if you are measuring a point of light, the energy would be reduced as a function of a cube of the distance... IF you are measuring an energy density over an area, the function would be as a square of the distance... so kinda depends on the final units they want measured. a light meter may be the easiest way for a lay-person to prove this... However, a good spec of light output and distance could be used - just show all work so they will have to hire their own engineer to either disprove or prove your figures - there is nothing special about an engineer, they just plug through the math that the rest of us cringe at - but the math is not magic.
 
Scotchbrite

Scotchbrite

No comment
Nits is what I usually see for message centers which I've been told is a ridiculous method of measuring light output on those.

weyandsign said:
Use this calculator. Put in the lumen output of one LED module inside the sign, and also the distance. Take a screen shot of it and send it in with the permit showing the lux is less than 0.01. https://www.omnicalculator.com/physics/lumen

If they don't accept that, tell them to meet you at the site at 11PM with their certified luminometer, so they can do some measurements.
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Putting the numbers in that calculator, the nearest residence in sight of the sign would be receiving 0.0003 foot-candles. Well under the 0.1 limit they give. Incidentally I read that a full moon is 0.01 foot-candles.
 
Gino

Gino

Premium Subscriber
What if I put another sign exactly the same on top of it ?? Will that double it ?? What if I stack 5 signs on top of each other all the same ??
 
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netsol

Active Member
Scotchbrite said:
"cube of the distance from the source" is the bit of information I couldn't find.


Well I'm not going to build and install the sign without a permit and risk the sign not meeting the requirement.

How could I even use a meter without knowing if the light it was reading was coming from the lot lights and the Carl's Jr signs that are much closer to the residential property?

Sounds like now I'm going to need a lighting engineer along with the structural engineer. And people will wonder why signs are so expensive.
Click to expand...
I would have used a "spotmeter" (GOSSEN used to make them, in the analog,/ needle moves back and forth days. ) I probably have one, buried in a cabinet somewhere. It has a red dot (like a sniper in a police movie) so you are sure what you are reading.) . It has a chart to convert to foot candles.

I am not saying this is how you should do it. Just that I HAVE SEEN IT DONE THAT WAY.
 
GAC05

GAC05

Quit buggin' me
Tell them you are going to paint everything within the sphere of radiance coming off the sign with this stuff:
1655344724356.png

With an 99.4% light absorption rate no light will be able to escape
 
H

Humble PM

Mostly tolerates architects
netsol said:
I would have used a "spotmeter" (GOSSEN used to make them, in the analog,/ needle moves back and forth days. ) I probably have one, buried in a cabinet somewhere. It has a red dot (like a sniper in a police movie) so you are sure what you are reading.) . It has a chart to convert to foot candles.
Click to expand...
Spot metres are for reflected light (the light falling on to the subject). You'd want an incident meter to measure the light coming out from the source (LED through sign face, measured at 1ft, at two foot, the intensity would be quartered etc). No fancey laser spotting, though you could mount a light pointer on the meter, for that Better Call Saul effect.
 
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