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.
--
Regards from Virginia Beach,

Earl

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.

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.
--
Regards from Virginia Beach,

Earl

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.

If you find your illuminance is too high, you can add reducing filters
between the light sources and the sign face.

If you're determined to do some calculations, the 0.25 fc at 25'
translates to ~150 Cd luminous intensity.  The luminous flux of 8600
lumens is only an industry average.  The actual flux will vary from tube
to tube and from ballast to ballast, but you can overestimate and say
that the flux will never exceed 9000 lumens per bulb and design around
that.  You're already aware of the light loss due to the sign
transmittance, but if your sign box has a white or other non-black
interior, you may get internal reflections that increase the amount of
light being emitted from the sign.  The relation between luminous
intensity (Cd) and luminous flux (lumens) is that 1 Cd is equal to 1
lumen per solid angle (lm/sr) for an isotropic source (a source that
puts and equal amount of light in all directions in a sphere).  There
are 4pi sr in a sphere.

Earl Kiosterud wrote:
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.

--
Douglas Cummins
Calcoast - ITL

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