Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible

growers who claim their plants flower under a 6-hour dark and 12 hour light photoperiod. This expedited, 18-hour photoperiod regimen is supposed to work, but I’m not sold on it. Growers say that their harvest is undiminished, and that they are getting 25 percent more marijuana in the same time. I have not visited their grow rooms to verify these claims. No electricity is saved by adopting this regimen.

You can put outdoor plants in a closet daily to induce flowering with 12 hours of uninterrupted darkness.
Intensity
    High intensity discharge lamps are bright–very, very bright. Growers who properly manage this intense brightness harvest more weed per watt. Intensity is the magnitude of light energy per unit of area. It is greatest near the bulb and diminishes rapidly as it moves away from the source.

The Inverse Square Law dictates light intensity in relation to distance.

Tie a string 12 to 36 inches long to the HID reflector. Use the string to measure the distance between the bulb and plant canopy.

This ‘Chronic’ cola received plenty of bright PAR watts. It developed super-thick, dense, heavy buds.
    For example, plants that are two feet from a lamp receive one-fourth the amount of light received by plants one foot away! An HID that emits 100,000 lumens produces a paltry 25,000 lumens two feet (60 cm) away. A 1000-watt HID that emits 100,000 initial lumens yields 11,111 lumens three feet (90 cm) away. Couple this meager sum with a poorly designed reflective hood, and beautiful buds suffer big time! The closer marijuana is to a light source, the more PAR watts it receives and the better it grows, as long as it is not so close that heat from the lamp burns foliage.

Leaves reach for the light. Strong, well-illuminated plants orient foliage to catch the maximum amount of light possible.

Loose buds like this form when they do not receive enough light.
The Inverse Square Law
    The relationship between light emitted from a point source (bulb) and distance are defined by the inverse square law. This law affirms that the intensity of light changes in inverse proportion to the square of the distance.
    I = L/D 2
    Intensity = light output/distance 2
    For example:
                   = light output/distance 2
    100,000 = 100,000/1
    25,000 = 100,000/4
    11,111 = 100,000/9
    6250 = 100,000/16
Link to the best light handbook on the Internet http://www.intl-light.com/handbook.html
    A 1000-watt standard metal halide emits from 80,000 to 110,000 initial lumens and 65,000 to 88,000 average (mean) lumens. One lumen is equal to the amount of light emitted by one candle that falls on one square foot of surface one foot away. Super halides emit 115,000 initial lumens and 92,000 mean lumens. A 1000-watt HP sodium emits 140,000 initial lumens, and a 600-watt HP sodium emits 90,000; watt for watt, that’s seven percent more lumens than the 1000-watt HPS. Lumens emitted are only part of the equation. Lumens received by the plant are much more important.
    Lumens received are measured in watts-per-square-foot or in foot-candles (fc). One foot-candle equals the amount of light that falls on one square foot of surface located one foot away from one candle.
    Watts-per-square-foot (or m 2 ) is easy to calculate, but is an erroneous way to determine usable light for a garden. It measures how many watts are available from a light source in an area. For example, a 400-watt incandescent bulb emits the same watts-per-square-foot as a 400-watt metal halide. Mounting height is not considered in watts per-square-foot; the lamp could be mounted at any height from four to eight feet. Nor does it consider PAR watts or efficiency of the bulb.

A 175-watt HID yields enough light to grow a 2 × 2-foot (60 × 60 cm) garden well. Notice how fast light intensity diminishes more than a foot from the bulb.
A 250-watt HID will illuminate up to a 3 × 3 foot (90 × 90 cm) area. Keep the bulb from 12 to 18 inches (30-45 cm) above plants.
A 400-watt HID delivers plenty of light to illuminate a 4× 4-foot (1.2 × 1.2 m) area well. Hang the lamp from 12 to 24 inches (30-60 cm) above the canopy of the garden.
A 600-watt HP delivers enough light to illuminate a 4 × 4-foot (1.2 × 1.2 cm) area well. Hang the lamp from 18 to 24 inches (30-60 cm) above plants.
A 1000-watt HID delivers enough light to illuminate a 6 × 6-foot (1.8 × 1.8 m) area well. Some reflective hoods are designed to throw light over a rectangular area. Large 1000-watt HIDs can burn