Foot candle and lumen/sqft are the same thing.

foot-can·dle (fʊt'kăn'dl)
n. (Abbr. fc or ft-c)

A unit of measure of the intensity of light falling on a surface, equal to one lumen per square foot and originally defined with reference to a standardized candle burning at one foot from a given surface.

The fall off of light is an inverste relationship [ f(x)=k/x ] where k is a constant the is related to the amount of light and x is the distance from the light.

This reminds me I really need to write my tutorial on this.

 

Ok, soooo uhmm, How do I figure out how many foot candles I can provide a certain plant?
Lets say I got two 40 watt bulbs, in a 29 gallon viv. And I got an orchid about 6 inches away from the lights. I threw away the package that says the lumens they put out.
Sorry if I am being slow on this subject, but I just cant make heads or tails of the function you gave me. I forgot my calculus.
Thanks

 

Footcandles (fc) = Total Lumens (lm) ÷ Area in Square Feet

Remember that its not just about LIGHT - when plants are involved, the wavelength is a crucial consideration. Plant chlorophyll absorbs light at wavelengths of 400 to 700 nm. All light is not the same.

 

i have a chart which gives the measurement for four 40 watt bulbs which are new. used bulbs give less light, so replace them every 6 months to one year. at a distance of 6 inches from the bulbs the ft.-c is 1000. at 12 inches it's 600 ft.c at 18 inches it's 420ft.c. if you only have two bulbs you can half these numbers to get an idea of the number, but it's not exact.

this link tells you how to use an old manual camera light meter then convert the f-stop to foot candles. http://en.wikipedia.org/wiki/Foot-candle

 

Thanks guys. I dont have that kind of camera so I cant do anything with the link, but its very interesting.

I got one 48" long 40 watt sunglo T-10. It emitts 4200K light spectrum, and 300 Lumen.
So I suppose I'm getting about 1000 Foot Candles at about 6 inches away from the lights.

Is that light spectrum good enough for plants? And can someone explain lumens? I'll look it up on wiki too. THanks.