Things to know about lighting
Color rating- Measured in Kelvin (K). The higher the number, the more bluish the light. 4000K-7000K is mostly on the blue side of the spectrum, while 3000K and under goes from a white spectrum, to a redder spectrum.
How much light is needed?
The minimum amount of light required by marijuana plants is around 3000 lumens per square foot. However, it's not 100% accurate, since although you may have a 10,000 lumen light, the amount of light that reaches the plant varies with the distance between the light and plants, and reflectivity of the grow box. The ideal amount is somewhere around 7000-10,000 lumens/sqft, and as long as the plants do not burn, as much light can be used as you want.
(*note, the sun produces about 10,000 lumens/sqft, on a sunny summer day).
Determining lumens for your grow area:
Determine the square footage of your area (example in a 4 foot by 4 foot area, there is 16 square feet)
If you have a 1000 watt High Pressure Sodium, that produces (approx.) 107,000 lumens.
Divide this by 16 (your square footage) 107,000 / 16 = 6687 lumens per square foot.
So just divide the total amount of Lumens, by the total amount of Sq ft, and thats your lumens per square foot.
How far away from my plants do the lights go?
The lights in your grow room should be as close as possible to the plants without burning them. There is no such thing as too much, unless there is sufficient heat to dry out and burn the leaves. A good rule is to put your hand under the light, if its too hot for your hand, chances are that the plants will be too, so move the light up until your hand feels more comfortable. For seedlings and clones, I keep them a little further away from the light, because they are very susceptible to burning and drying out, at these stages.
Efficiency is very important when choosing a type of light. The wattage is not the most important thing, as you can see below, different types of light produce different amounts of lumens per watt. A 300 watt incandescent will produce about 5100 lumens. While a 300 watt Metal Halide (just an example, they do not come in 300 watts), will produce 27,000 lumens. Obviously far more efficient for growing, while still using the same amount of electricity.
Approximate light production:
Incandescents: 17 lumens/watt
Mercury vapor: 45-50 lumens/watt
Fluorescents: 60-70 lumens/watt
Metal halide: 90 lumens/watt
High pressure sodium: 107 lumens/watt
