HID bulbs use different types of gas in the bulb. The main kinds used in growth include metal halide (MH), and high-pressure sodium (HPS). The main difference between the two bulbs is the type of light spectrum they produce, depending on the gas inside them. Metal Halides produce more pure white light than the HPS lamps, closer to daylight frequencies making it especially useful for growing plants. The MH bulb focuses on more of a blue spectrum and is used primarily for the vegetative stage of plant life.