Lumens to food-candles

[Earl Kiosterud]

I have a friend who's constructing an internally lighted sign. He needs to ensure that the
light output will not be greater than 0.25 foot candles measured at 25' per city code. The
sign is to be illuminated with four 96" standard high-output fluorescent tubes, 8600 lumens
each, color temperature, 4100°K. We know there will be light lost internally and through the
translucent sign faces (both sides of the sign). How can we determine if such a light
source, worst case, would exceed this foot-candle limit? Thanks.

 

[Earl Kiosterud]

I have a friend who's constructing an internally lighted sign. He needs to ensure that the
light output will not be greater than 0.25 foot candles measured at 25' per city code. The
sign is to be illuminated with four 96" standard high-output fluorescent tubes, 8600
lumens
each, color temperature, 4100°K. We know there will be light lost internally and through
the
translucent sign faces (both sides of the sign). How can we determine if such a light
source, worst case, would exceed this foot-candle limit? Thanks.

Let's assume your sign is double sided so light goes out both sides,
and that it diffuses the light, so the light goes in all directions.

34,400 lumens into 4PI steradians gives an average of 2737 candela.

At 25 feet, 2737 candela will produce about 1.4 footcandles, which is
over 5 times the allowable limit...

Dean

Thanks for your reply.

First of all, I don't know why my newsreader isn't putting the > characters in your reply.
I seems to only fail to do that with your reply.

This is all new to me. If we have a total of 34,400 lumens, then a 25 foot sphere, whose
surface area would be 4*PI*25^2, or 7854 sq feet, would be illuminated with those lumens.
So the lumens per square foot would be 34,400 / 7854, or 4.38 lumens/sq ft. Wouldn't that
be the same as 4.38 foot-candles? It's wrong, but I don't understand why.

 

[Ron Gibbs]

Rather than try to calculate it - because you will most likely get it
wrong no matter how diligent you are - build a mockup and measure the
illuminance at the required distance. A decent illuminance meter doesn't
cost too much. Make your measurements at night or in a dark room so that
ambient light doesn't artificially inflate your measurement.

As someone who uses raytracing software for a living, I agree this is good
advice. Before the days of non-sequential optical raytracing, I used to try
and calculate radiometric/photometric flux, but didn't expect to be accurate
within a factor af two or three. I would always build a mock-up if it was
critical.

Nowadays, good sofware can be quite accurate, depending on the time
spent on getting the model sufficiently detailed, which can be expensive in
time. Fluorescents are easy enough, but diffusing surfaces are difficult to
model accurately.

Ron Gibbs