morphius1, you're close...remember 1 foot-candle = 1 lumen/ft². that's the definition. (you are taking an intensity over an area). so when you measure 4300 foot-candles, that is the same as 4300 lu/ft². that's how much light you actually have illuminating your plants. the 54,180 lumens would actually be the equivalent intensity of the source.
choader, you're close, too. lumens is the measure of intensity at a point source (no area). lumen/ft² is the intensity of the light spread over an area. and right on about the area being illuminated, when you double the distance, you get only 1/4 the light...which is somewhat dependent on the shape of your reflector...
so back to the envirolites. if you could somehow get all the light out of one envirolite bulb and project it onto a 1 ft² surface, you would have 8900 lu/ft². THAT would be good - the sun illuminates the earth at about 10,000 lu/ft².
hope that helps
