i dont know if anyone has mentioned this but think of this.... plants outdoor receive aprox. 15,000 lumens per square foot... i saw this.... and thought i would give you that fact as an idea of what goes on outside.... also a 150 watt HPS will be at about 15,000 lumens,,, so im not sure if that means you would have to have one 150 watt hps for every square foot of growing (next to impossible i would think) or not... does anyone have any comments on that?
Yes. It's a bit more complicated. Outdoor sun is nominally 100,000 lux, which is 100,000 lumens per square meter, and approximately 10,000 lumens/square foot.
The 15000 lumens for a 150W HPS needs to be divided by an area that accounts for the distance from the bulb to the plant. With no reflector whatsoever, this would be 4 * pi * d^2, where d is the distance to the plant.
So say that you have your light 18" from your canopy with no reflector. You get :
15,000 lumens / (4 * pi * (18 inches * .0254 m / inch)^2) = 5700 lumens / m^2 = 5700 lux
If you move it down to 12", you bump up to nearly 13,000 lux. Add a reflector and some mylar and you can easily get even better.
When comparing, however, you have to remember that the sun is not out at full intensity for as long as your HID is on, and you haven't clouds to worry about.