I still see so many threads started with very basic lighting questions, and I just don't think anyone has made it simple enough so I thought I would give it a try. Lesson 1 lets learn about light: Watts - Is simply the amount of power (radiant energy) that you light uses and can be used to size your electrical system (wires and breakers) and calculate your electrical bill. When people ask about the watt per square foot or per plant it drives me crazy. You can have a light that is 100W but loses much of it's energy as heat and non visible light (i.e. incandescent bulbs). Kelvin - This is the temperature or the color of the light produced. The Sun is about 6500K (it varies by season and time of day). Most growers recommend that you do cuttings and veg with high color (hot) (5500K to 8500K) and lower color for flowering (cool) (2000K to 3500K). Personally I like a cool light for veg and flowering. Wavelength in Nanometers - Some bulb manufactures use charts to show you the nano-meters of their bulb and where they peak compared to where photosynthesis occurs. There is a relationship between Kelvin and Wavelength in Wien's law if anyone is really board you can look it up. Most people agree that photosynthesis occurs in two major bands of light: Lumens - Now here is what you are looking for this is the measurement of visible light. You have to be careful when using this to buy a bulb, it is also expressed as candlelight and bulbs lumens usually vary from start up to after the bulb is running, and they vary depending on the distance that it is measured from the bulb and how old the bulb is. I generally like to to select bulbs that get at least 100 initial lumens per watt (this tells you it's an efficient bulb) and I like 5,000 lumens per square foot, or 30,000 lumens per plant (I like big plants). Most agricultural schools teach that photosynthesis doesn't even start until about 400 lumens per square foot and peaks about about 2000. Lux - This is your measurement of lumes per square meter. Distance - Now this is huge, if your were paying attention you noticed above that I said I like 5,000 lumens per square foot, but a plant can only absorb about 2,000. That is because the 5,000 is at the bulb (or usually measured about 6" from the bulb). There is an inverse square ratio that you need to consider with lighting, in lang-man's terms the further you get from the light source the weaker it gets and fast. My 5,000 lumens at the bulb is only 2,000 lumens at 11" away from the bulb, and less than 400 at 27" (remember what the minimum lumens were above for photosynthesis?). Light penetration - This is something I still do not fully understand, but certain lights can penetrate deeper into the canopy and produce photosynthesis then others. 1000W HID lights are by far the best followed by lower wattage HIDs then LEDs and florescents. I think it has something to do with not visible light (maybe UV) that pushes the unused photons past the leaves instead of being reflected? I hope this helps a little with understanding light. I hope I can continue this tomorrow, but I am out of time today.