High intensity discharge lighting includes both metal halide (MH) and high pressure sodium (HPS). HID lighting consists of three components: the ballast, the reflector or socket, and the bulb. Ballasts can be specific to the type of bulb (MH only or HPS only) or they can be switchable, or “smart” ballasts, which have the ability to operate either type of bulb. Most ballasts are specific to wattage; although some of the newer, digital ballasts can operate different wattage bulbs. The ballast connects to the power supply and acts as a power converter so the bulb receives the proper current.
The reflector, or socket, is the component that connects the ballast to the bulb and the bulb itself is the component that actually emits the light. After receiving the proper current from the ballast, the bulb converts that energy into light. HID lighting is still the most popular choice for horticulturists for a few reasons. First, this technology has been around for a long time and is used in other industries. This means there are many manufacturers which helps lower the cost. Initial cost is not the only reason horticulturists continue to choose HID lighting.
High wattage HID fixtures are very powerful and have the capability of penetrating a plant canopy along with covering a large area. For example, an indoor horticulturist who is using a 1000 watt HID light can adequately cover 25-50 square feet of garden space. Greenhouse growers looking for supplemental lighting can cover even more space with that same 1000 watt fixture. It is not uncommon for greenhouse gardeners to have a 1000 watt HID cover 100+ square feet for supplementary lighting purposes. Long story short, HID lighting has a low initial cost for a relatively large coverage area. This is the main reason HID lighting has remained so popular among horticulturists.