it's at any distance. lumens is a measurement of total light. as you move the lamp away, the light is spread out over a larger area so the intensity drops, but the total light (lumens) stays the same. intensity is measured in LUX and this figure is very roughly halfed for every foot of distance
Ah I understand, so you really cant figure out the intensity of a light untill you have it running with a light meter under it. How does foot candles play into this?
The correct answer to your question is 1 Foot, as 1 lumen = 1 Lux = 1 x candle power at a distance of 1 foot, total ling the amount of light in 1 square foot. (Google it) 400w Mh light = 38000 lumens 600w HPS = 98000 lumens or: Lumen (band), Russian alternative rock band Lumen Pierce, a fictional character played by Julia Stiles from the television series Dexter USS Lumen (AKA-30), a US Navy ship
sort of, there is probably a pattern somewhere so you may be able to work it out, I'm not sure a foot candle is an old imperial way of measuring intensity. most people use the metric LUX now. except Americans of course lol, they love imperial measurements 1 LUX = 0.092903 foot candle
If your using CFLs they lose a ton of light the farther away they are, from what I understand they are pretty much useless to a plant after 6-8".
the intensity diminishes at the same rate no matter what the source. but I agree after about 8" the intensity is very low. filling the canopy, keeping it even, and keeping the lamps within a few inches is very important with CFLs