This promotes fast vegetative growth and compact, stocky plants with short internodal leaf spacing. Metal Halide bulbs create light by passing electricity through an clear inner arc tube that is enclosed in the vacuum of an outer clear glass tube. This inner arc tube contains mercury and other metals in iodide form. When electricity is applied to these metal iodides they give off very intense light and heat. The outer casing can be either clear or phosphorus coated.
Most gardeners prefer the clear bulb as it produces the brightest white light available.
The bulbs themselves need to be burned in a specific position. The universal bulbs can be burned in any position, but they are more efficient when burned vertically. The super bulbs cost slightly more, but they are more efficient, making them a good choice in the long run. The 100 w super bulbs do not last as long as any of the others and should be replaced every 6500 hours, which is about 12 months if you run the bulb 18 hours a day. HPS bulbs are the most efficient bulbs that are commonly used by the hobby gardener.