Metal Halide bulbs, often abbreviated as MH, are a type of high intensity grow bulb designed specifically to emit a light in the blue / violet area of the light spectrum and are used in Metal Halide grow lights. The reason MH bulbs utilize this …part of the spectrum is to mimic the natural light that the Sun provides during the spring. As such they are often better used for the vegetative stage of growing and MH bulbs are not often advised to be used during flowering if an HPS or other grow light with a spectrum more suited to flowering or one designed for all stages of growth is available. MH bulbs need to be replaced roughly once per year depending on usage. When it comes time to replace your MH bulb shop HTG Supply for the best quality and prices available online!