Metal Halide:
Used commonly  for plants in the vegetative stage of growth, these systems provide a full spectrum of light but do not have as much light penetration as HPS systems.

High Pressure Sodium (HPS):
These systems are used predominantly for flowering stages due to the bulbs arc that outputs mostly red/orange spectrum of light that increase flowering, these lights are usually brighter than Metal Halide which means the light travels deeper into the canopy.

Fluorescent Tube:
Used mainly for veg and during clone rooting these lighting systems provide a full spectrum and run very cool meaning they can be put much closer to the top of your garden and will not burn.

LED is a tech that is just now really getting going in the cannabis grow scene. The LED systems you would want for cannabis would be built of an array of 3-5 watt CREE diodes and would be white for veg and red and white for flower. LED systems cost a lot of money but run very cool and can be used even on a very large scale.

The internal electrodeless lamp or induction light is a gas discharge lamp in which the power required to generate light is transferred from outside the lamp envelope to the gas inside via an electric or magnetic field, in contrast with a typical gas discharge lamp that uses internal electrodes connected to the power supply by conductors that pass through the lamp envelope.

CFL (Compact Florescent) bulbs are popular for micro grows and vegging but there are still a few gardener’s out there that actually flower using these bulbs and build massive 1000+ watt arrays. CFL’s actually produce more usable light than HID lights and have the added benefit of being able to be positioned only a few inches away from the tops of your canopy, these lights are made up to 250 watt bulbs and can be both blue and red spectrum.
[CFL’s DO NOT have as much light penetration as HID lighting]

Green LED’s:
During lights off many cultivators choose to have green LED’s turn on which will not disrupt the grow cycle and allows growers to be able to work in their garden even after lights off! (This can also be used outdoors, you can even buy green LED head lamps for ease of use!)

Measuring Light:
All light is measured in foot-candles, lumens and lux.
Foot-candles and lux measure light visible to the human eye (which can see much less of the light spectrum than plants can absorb.) The human eye is most sensitive to light between 525-625 nanometers. The importance of the blue and red portion of the spectrum is diminished greatly when a light is measured in foot-candles, lux or lumens. A foot-candle is a unit of illumination equal to the intensity of one candle at a distance of one foot. The lux scale is similar to that of the foot-candle; one foot-candle is equal to 10.76 lux.

Light is also measured in spectrum with kelvin temperature which expresses the exact color a bulb emits. Bulbs with a kelvin temperature from 3000 to 6500 are best for cultivating cannabis. The PAR section explains that plants use specific portions of the spectrum- a complete range from blues to reds.

Light Meters:
Light meters measure light in foot-candles or lux, both in which are visible to the human eye (they don’t not measure plant usable light) This information is still valuable because it helps record the amount of light spread over your garden canopy. You can take the info gathered from a light meter and combine it with the PAR rating of your bulb. Always use the highest PAR-rated bulbs to grow cannabis!