Minimum amount of light for indoor marijuana grow room

What amount of light should I be using in my marijuana garden? Should it be concentrated or spread out evenly?

Ideally, you should use a minimum of 30 watts HPS input per square foot (psf). If possible, delivering 60 to 80 watts per square foot would increase both the growth rate and the quality of the produce. Plants grown under lower wattages tend to be weaker and thinner while buds grown under intense light mature much faster and are tighter, larger, and covered with resin glands. Increased yields are directly proportional to the amount of light input as long as the plants receive adequate amounts of water, nutrients, and CO2.