To complicate things further i just read that for growing cannabis you should ideally be between 10000 and 15000 lumens per square foot.

Depends on what your growing.

cannabis requires less light then fruiting plants (tomatoes peppers ect) I'm not very knowable on the subject but if I was you I would start out with a smaller bulb (400 or so ) and if it looks like it needs more add a 2nd one.

LED's would be your most effective lumens/watt used and reduced heat output (sometimes a big issue) but they cost more. If you needed 1000w HID I would probably aim for 800w LED which a good quality one could cost $500-1000+.

To complicate things even more I'm going to throw wavelength into the mix rather then lumens, lux, or watts.

Watts is expressive of how much energy a bulb uses to produce light. It is possible to lower the amount of watts and still produce and equal amount of light. Example: An incandescent 60-watt bulb gives off 800 lumens of light. LED bulbs can deliver the same amount of light using as little as 10 watts. I would disregard choosing a grow light set up on wattage alone and wattage=/=light. However, all other things being equal, go for the lower wattage.

Lux is a factor of both illuminance over a given area and luminous emittance from a given area. Lux/unit area is going to be highly dependent on your specific set up. While the luminous emittance from a bulb may be 500lx, the illuminance of your grow area may be only 100lx based on the light being diffused throughout the area and absorbed/reflected. To find this out, you would need the gear for measuring it, but it is going to give you most accurate measurement of how much visible light (measured in lumens) your plants are actually receiving in a given area, but this brings us to the lumens factor of our growing set ups.

Lumens is a measurement of light visible to human eye. The problem is, plants don't have human eyes, and lumens are not photons, which are what the plant actually uses for photosynthesis. In this way, a plant "sees" light as a matter of wavelengths rather then lumens.

The energy of a photon is dependent on wavelength rather than lumens. A photon of red light has less energy then a photon of blue light. Thus, we can purchase a 2600 lumen blub for growing at 3500K (closer to the red spectrum) and we can purchase a 2600 lumen bulb at 6500K (in the blue spectrum) and while their lumens are equal, their photons are not, making one better then the other for growing plants.

I currently have two fixtures one has light bulbs at 2,600 lumens, 5,000K and the other has bulbs at 2,600 lumens and 6,500K. I had onions under both and the ones growing under the 5,000K lights were more yellow then then ones growing under the 6,500K lights. Anecdotal, but take it for what it's worth.

"you should ideally be between 10000 and 15000 lumens per square foot"

This goes back to the lumens/photons things. You can have 10,000 lumens/sqft, but your photon measurement (õmol/m2.s) which is what your are looking for when calculating photosynthesis, will be much different depending on the spectrum of your light.

This post was edited by ZachS on Fri, Mar 7, 14 at 15:16