A METAL HALIDE (MH) Bulb produces light by using an electric arc and a gaseous mixture of mercury and metal halides (metal compounds with bromine or iodine) . They are HID (High Intensity Discharge) lamps and will emit a light with blues and whites.
They have an average life span of between 6,000 and 15,000 hours and emit 75 - 100 lumens per watt of electricity.
WHY SHOULD I USE A METAL HALIDE BULB?
Metal Halide bulbs are especially suited to being used within the vegetative stage of a plants growth. This is because they emit a bluish / white light which is what the plant desires at this point of its life. MH bulbs essentially mimic the light of spring and early summer and if used properly the plants will be thick and bushy. They are ideal from stopping plants from stretching.
In order to run a MH bulb you will need a ballast. This prevents the current within the bulb from increase to a level where the bulb will actually destroy itself.