What is the real definition of foot-candle?

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  • #1
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I've searched around the internet and thought it would be a simple thing find and perhaps it is, but the more I dig the worse it becomes. According to online dictionary, 1 foot-candle is the amount of light from a 1 lumen light source on 1 sft of surface area at a distance of 1 foot. Sounded reasonable. However another source indicates that 1 foot-candle is the amount of light on the surface of a sphere with radius of 1 foot with 1 lumen source at center. Also, sounds reasonable. Unless I'm mistaken these are totally different values. It comes across that there is a "rule of thumb foot-candle", "a lighting engineering foot-candle" and a "physics foot-candle". My question is there a reputable science organization with a documented and usable "foot-candle" unit definition?

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  • #2
russ_watters
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The definition is the first one. I suggest providing your source for the second, so we can look at it and try to see what's wrong. As for sources, I'm sure any into physics textbook should include the units. You could also search NIST for their use.
 
  • #3
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The following from wiki, along with other sites, "The unit foot-candle is defined as the amount of illumination the inside surface of a one-foot-radius sphere would be receiving if there were a uniform point source of one candela in the exact center of the sphere". The reason I thought it was this definition is because this is how radiant energy from the sun is calculated using Stephen-Boltzmann equation.
 
  • #4
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For what it's worth... Just read a paper from the University of Berkeley and they're using the wiki definition.
 
  • #5
russ_watters
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The following from wiki, along with other sites, "The unit foot-candle is defined as the amount of illumination the inside surface of a one-foot-radius sphere would be receiving if there were a uniform point source of one candela in the exact center of the sphere". The reason I thought it was this definition is because this is how radiant energy from the sun is calculated using Stephen-Boltzmann equation.
Hmm....I guess maybe I misunderstood; this is a unit of intensity, not power, which means it is independent of area. So either would be true.
 
  • #6
Meir Achuz
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"1 sft of surface area at a distance of 1 foot" implies to me that they mean a spherical surface, which is the only type where every point is at the same 1 foot distance. The two definitions are identical.
 
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  • #7
Meir Achuz
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Thank you. I didn't notice that the second derivation did not say 'on one square foot of the surface of a sphere'.
 

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