HID lamps are categorized in three groups :Mercury Vapor , Metal Halide, and Pressurized Sodium. For plant cultivation, Metal Halide (MH) or Pressurized Sodium (HPS and LPS) are the ones commonly used in grow rooms.Mercury Vapor - Mercury Vapor lamps are common, but they are not the most energy efficient for lighting plants or anything else. Mercury Vapor lamps have an average life of 24,000 hours. A black coating that deposits it's self on the inside of the bulb decrease light output after a significant amount of use.

Pressurized Sodium:
High and low pressure sodium lamps (HPS and LPS) are considered the most efficient lighting option for plant's in flowering phase. Sodium causes this type of lighting to be a yellow/orange color. A ten minute warm-up period is needed before you can turn on light after turning it off.

The life of the HPS and LPS are the same as the Mercury Vapor, however, the energy requirements (wattage) is about half. HPS lamps are usually chosen for outside or warehouse applications where color is not important, and where instant lighting is not required. When Pressurized Sodium lights are used in plant applications, they are commonly used only during the plant's flowering phase.

Pressurized Sodium spectrum is more suitable for flowering plants than Metal Halide or Fluorescent lamps. Plants grown under Pressurized Sodium lamps yields larger, denser fruits and flowers.

Metal Halide:
Unlike Pressurized Sodium, Metal Halide lamps have instant startup and emits a white color, however, they have about half the lifetime of both Mercury Vapor and Pressurized Sodium lamps. Metal Halide have a lower wattage than Pressurized Sodium. Metal Halide are usually better for applications where color of light is important. Metal Halide can also be used as an artificial light source for growing plants indoors.

Metal Halide lamps are usually used during a plant's growth phase because its spectrum provides good growth rate and smaller compact plants. Metal Halide Lamps provide high system efficiency (up to 100 lumens/watt) compared to Mercury Vapor, fluorescent and incandescent lights. Metal Halide have a "white" color which makes a good approximation for daylight. Metal Halide lamps last for 10,000 to 15,000 hours. Metal Halide lamps have a shorter lifetime than high pressure sodium lamps. Metal Halide lamps, however, have a shorter lifetime than high pressure sodium lamps.