Although fluorescent lamps were a common type of indoor growth lamps in the past, HID lamps are now the most popular. High intensity discharge lamps have high lumen/watt efficiency. There are several different types of HID lamps, including mercury vapor, metal halide, high pressure sodium, and conversion bulbs. Metal halide and HPS lamps produce some color spectrum somewhat similar to the sun and can be used to grow plants. Mercury vapor lamps are the first type of HIDs and are widely used in street lighting, but when it comes to indoor gardening, they have relatively poor spectrum for plant growth, so they are mostly replaced by other types of HIDs for growing plants.

All HID growth lamps require ballasts to operate, and each ballast has a specific wattage. Popular HID power includes 150W, 250W, 400W, 600W and 1000W. Among all sizes, the 600W HID lamp has the highest luminous efficiency, followed by 1000W, and 600W HPS has 7% more light than 1000W HPS.

Although all HID lamps use the same principle, different types of bulbs have different starting and voltage requirements, as well as different operating characteristics and physical shapes. Because such a bulb will not work properly unless a matching ballast is used, even if the bulb is installed. In addition to producing lower levels of light, mismatched bulbs and ballasts can stop working or even burn out immediately.
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