Originally Posted by [b
How many lumens your lights emit tells us how bright your lights are.
How much of that light is available to be used by your plants depends on a lot of factors, but chiefly how far they are away from it. For a bulb or a single tube the amount of usable light is roughly related to the square of the distance the plant is from it. So, compared to a plant one unit of distance from the light a plant twice as far away receives a quarter of the energy and one three times as far a ninth.
Wow, so you are growing your plants between 2x (rlhirst) and 3x (killerplantsguy) mine. I guess my light levels would possibly be similar in summer, I grow high light orchids and use little shading. I'm really interested now to measure this next year. The reddening I am seing now seems most likely just due to the plants suddenly being shocked by a sudden jump in light in the middle of the miserable British winter.
One slight word of warning - the light your plants get under lights is theoretically the lumens / area the light illuminates. However, reflectors do not evenly spread the light. My reflector (pulsar) has a very uneven light spread, light being relatively low right under the bulb, highest (3x the centre) in two stripes under the trailing edges of the reflector. I don't know if this is typical, or if I have just bought a bad design.
It is also rather difficult to judge what is an illuminated area, as human eyes are very poor at judging absolute light levels. What looks bright to you is not necessarily any good to a plant.
Only way to know for sure - get a light meter or even better (far more expensive) a PAR meter which measures the components of the light the actually matter to plants
I'm sorry if I seem ignorant, but could someone do this calculation for me? My lights are 45" above the plants, and hit a 30" by 60" area. I estimate lumen output of all lights to be... 14,000 + 4,000 + 4,000 + 8,800= 30,000 lumens without al the sunlight coming through my window. Can anyone calculate the foot candles for me please [img]http://www.**********.com/iB_html/non-cgi/emoticons/new/smile.gif[/img] .