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Light for indoor growing marijuana
Indoors, 2000 lumens per sq. ft. is about as low as you want to go indoors. If you get under this mark, plant growth will certainly not go as fast as possible, and internode/stem length will increase. Also, light distance to plants will be much more critical. Daily adjustments to the lamps will be necessary, meaning you get no vacations.
2500 lumens psf should be a good target, and 3000 is optimal if your going to inject or enrich CO2 levels (more on that later).
High Intensity Discharge lamps are the best solution for most indoor growers. HID lamps come in 3 basic flavors: High Pressure Sodium (HPS), Metal Halide (MH) and Mercury Vapor. Metal Halide is an improved spectrum, higher intensity Mercury Vapor design. HPS is a yellowish sort of light, maybe a bit pink or orange. Same as some street lamps.
HPS lamps can be used to grow a crop from start to finish. Tests show that the HPS crop will mature 1 week later than a similar crop under MH, but it will be a bigger yield, so it is better to wait the extra week.
The easiest HID to buy, and least expensive initially are the flourescent and mercury vapor lamps. MV will put out about 8000 lumens per 175 watts, and 150 watts of HPS puts out about 15k lumens, so HPS is almost twice as efficient. But the color spectrum from MV lamp output is not as good. HPS is high in reds, which works well for flowering, while the Metal Halide is rich in blues, needed for the best vegetative growth. Unfortunately, MV lamps provide the worst spectrum for plant growth, but are very inexpensive to purchase.They are not recommended, unless you find them free, and even then, the electricity/efficiency issues outweigh the initial costs saved.
400 watt HPS will output around 45k lumens. For every 500 watts of continuous use, you use about $20 a month in electricity, so it is evident that a lamp taking half the power to output the same lumens (or twice the lumens at the same power level) will pay for itself in a year or so, and from then on, continuous savings will be reaped. This is a simple initial cost vs. operating costs calculation, and does not take into account the faster growth and increased yield the HPS lamp will give you, due to more light being available. If this is factored into the calculation the HPS lamp will pay for itself with the first crop, when compared to MV or fluorescent lamps, since it is easily twice as efficient and grows flowers faster and bigger.