There have been a lot of lighting questions lately so I thought I'd throw this out there.
Proper lighting may be the most important piece of the indoor garden. Photosynthesis and chlorophyll production occur throughout the spectrum with peaks in chlorophyll synthesis around 400nm in the violet wavelengths and photosynthesis around 700nm in the orange-red wavelengths. It's easy to see that any artificial light producing the visible spectrum can be used to grow plants. Only a few types of artificial lights will grow plants well
Fluorescents and high intensity discharge (HID) lights have been the only useful artificials until the recent past when LED and MPS (microwave powered sulfur plasma) became available. The price of LED & MPS is still prohibitive for the grower on a small budget. Currently they are about twice the price of a dual 1000W system/bulbs/reflector/fans. However, LED's use much less energy and have a much longer life than HID and flourescents so their savings is realized slowly. MPS systems use 1000W of energy; they only offer savings in the less frequent bulb changes. MPS bulbs only need to be changed every 7 yrs while MH & HPS should be changed yearly.
Incandescent, halogen, and mercury vapor lights will grow plants, just not well. They produce a lot of heat and little light so they are much less efficient than the others. Their spectrums are heavily weighted on the red side which induces stretching (longer internode spacing). Heat also induces stretching which combined with the redder spectrum produces a spindly plant sparsely populated with leaves.
Lights are often rated in color temperature - degrees Kelvin (K). Low numbers indicate redder light while high numbers indicate bluer light; red light is designated warm and blue light is designated cool.
2100K - HPS
3000K - warm white
4200K - cool white, MH
5000K - "full spectrum"
6500K - "sunlight" or "daylight"
>9000K - specialty MH and fluoros
Fluoros come in 2 types, standard tubes and compact fluorescents (CF or CFL). Tube fixtures house the ballasts that power the bulbs while CFL's can be screwed into any household socket. Conventional CFL's, that is. CFL's designed specifically for horticultural use often require a MOGUL socket. In both cases, the ballast is housed in the base of the bulb so they generate a bit more heat than tubes. The plants can't be quite as close. The biggest drawback with fluorescents is they only penetrate about 6". Blacklights produce uvA & uvB light and are useful as supplementals only. A better choice would be a 10,000K aquarium light. Fluoros designed specifically for horticultural use are available. Some employ targeted spectrum phosphors emitting a purple glow.
High Intensity Discharge (HID)
There are 3 varieties of HID lights useful for growing, HPS, MH, and MPS. HID lights require a ballast to provide the power to ignite them. Some have the ballast built into the fixture and others are available with a remote ballast. I recommend the remote ballast types for two reasons. First, the ballast is heavy and incorporating it into the fixture creates a very heavy fixture to hang over your delicate plants. Second, ballasts produce heat and being able to locate a remote ballast outside the growspace helps control heat. Electronic ballasts produce much less heat but are relatively expensive now. Horticultural lighting systems are available with shrouded cords. You simply hang the light and plug it in.
As the name implies, HID lights have a higher intensity and penetrate farther than fluoros.
50-70W penetrates about 1ft
150-250W penetrates about 1.5ft
400-600W penetrates about 3ft
1000W penetrates about 4ft
High Pressure Sodium (HPS)
HPS can be identified by the rosey glow it emits. Watt for watt, it produces more overall light than MH. HPS induces stretch (longer internode intervals) due to its redder light. It emits a spectrum useful for growing throughout the lifecycle although many growers use them only for flowering. Plain HPS bulbs work just fine, providing ample useable light for growth, but there are bulbs available with an enhanced blue spectrum to aid foliage growth and combat stretching. EYE Hortilux, Sunmaster, and Phillips Son Agro are 3 well known brands. Currently their cost is about 5 times that of a regular HPS bulb and worth it.
MH emits a more natural white light. Watt for watt they emit less light than HPS (36,000 lumens for a 400W MH vs 50,000 lumens for a 400W HPS). Plants grow very compactly under the bluer light of MH, therefore many growers use them for vegetative growing and switch to HPS for flowering. Plants do very well under MH throughout their lifecycle, but the lack of red light inhibits flower/fruit production so the plants yield somewhat less. Sunmaster and others now make MH in enhanced spectrums as well; cool MH (4000-6000K) for vegging and warm MH (2000-3000K) for flowering. Light systems designed for aquariums are available with MH in the 9000K range. MH also produces a *small* amount of uvB light which is theorized to increase resin production and therefore increase potency.
There are several websites to visit. LED Grow Lights.com
offers proof that they work and excellent information on growing applications, with specific information on plants with low light requirements and those with high light requirements.
How much light do I need?
Cannabis is categorized as a high light plant. There are minimum levels of illumination required to insure compact plants. I have read that gardens can be successfully lit by as little as 2500 lumens/ft² when vegging and 5000 lumens/ft² when flowering. I have never used anything less than 3000 lumens/ft² for vegetative plants and 7500 lumens/ft² for flowering plants (in HID terms: 35W/ft² for veg and 60W/ft² to flower). Intensity degrades exponentially as you get farther from the source so the lights have to be kept close to the plants. To the indoor gardener that means less penetration through dense foliage. Ventilation is the key to keeping the light close. Air-cooled, tempered glass shielded hoods go a long way toward controlling heat. A small fan blowing right at the bulb works well also.
CaliGrower's Light Distance Charts
for HID's are invaluable aids, displaying levels of illumination at specific distances as well as PAR energy. Photosynthetically Active Radiation (PAR) is how plants "see" light. There is a line on the chart indicating where various wattages provide equivalent PAR to the sun. As long as your light is that close, you will see lush, compact growth. Interestingly, Sunmaster is the only vendor I find that lists PAR wattages for their bulbs. This link
is a FAQ with a more technical discussion of PAR as well as conversion factors for various lights.
The angle of light is important as well. Rays of sunlight are almost parallel by the time they reach earth due to the distance between earth and sun. A 10ft plant is illuminated equally from top to bottom outdoors. That is not the case indoors where the plants are a few feet at most from the light. Hanging lights vertically without reflectors at varying heights between rows of plants insures illumination to the bottom of the plants, penetration to the center, and dense growth along their entire height.