HID lighting is employed for many applications. Metal halide and high-pressure sodium bulbs are used for industrial lighting, outdoor lighting, warehouse lighting and even greenhouse lighting. Fluorescent tubes operate at relatively low temperatures and can be grouped together in fixtures to provide similar light output as high-intensity discharge bulbs. LEDs can be engineered into clusters within a single bulb to offer generous light outputs. LEDs have great potential in promoting plant growth. Blue light encourages vegetative plant growth, while red light promotes flowering. Indoor garden enthusiasts can employ blue and red LEDs to provide the type of lighting that is used most efficiently by plants.
HID bulbs are designed to last for many hours before they need replacing. High-pressure sodium bulbs last between 12,000 and 25,000 hours. Metal halide bulbs are engineered to last between 10,000 and 15,000 hours. Fluorescent bulbs are very efficient and can last for up to 10,000 hours while specially designed high-output fluorescent bulbs can last for upward of 20,000 hours. LEDs can last up to 50,000 hours.
Lumens measure the light output of a light source. Four high-output fluorescent bulbs offer similar light output as a 400-watt HID bulb. Regular 42-watt fluorescent bulbs offer approximately 66 lumens per watt. Metal halide HID bulbs offer 58 lumens per watt and high-pressure sodium HID bulbs offer 87 lumens per watt. A single LED produces about 20 lumens per watt and the average single LED draws less than 1/10 of a watt of power.
All HID bulbs contain some level of mercury, which is a very toxic material, according to the Environmental Protection Agency. HID bulbs contain dramatically more mercury than most other bulbs. The average fluorescent bulb contains around 5 mg of the toxic substance. The average high-pressure sodium bulb is designed with around 15 mg of mercury. Around 38 mg of mercury can be found in the average 250-watt metal halide bulb, while 400-watt metal halide bulbs can contain upward of 81 mg. LEDs contain no mercury; there is no risk of mercury exposure from handling LEDs.