There are only a few types of HID lamps available to consumers. The three main types on the market are high-pressure sodium lights, metal halide lights and mercury vapor lights.
Both metal halide and mercury vapor lights are known for emitting very bright white light that has a slight bluish tint. High-pressure sodium lights emit more of a yellow, orange and reddish light. Light output is traditionally measured in lumens. Of all three types, high-pressure sodium lights have the highest lumen output per watt.
While they can be used indoors in warehouse-type settings, most HID lights are typically used in outdoor applications. Street lighting typically employs the use of some type of HID light. Due to their high efficacy and long life, high-pressure sodium lights are used for most street and roadway lighting. Metal halide and mercury vapor lights are often used in warehouses because they emit a whiter light. Because of the blue-white light they emit, metal halide and mercury vapor lights can be used to encourage plant growth. Because of the orange-yellow light they emit, high-pressure sodium lights are ideal for encouraging plants to flower.
Each type of HID light is rated to last for different amounts of time. A mercury vapor light has a typical life of around 18,000 hours. A 400-watt metal halide light lasts about 20,000 hours. Most high-pressure sodium lights, regardless of wattage, last around 24,000 hours.
All high-intensity discharge lights contain certain levels of mercury gas. For example, a 250-watt high-pressure sodium light typically contains 15 mg of mercury. A 250-watt metal halide light contains 38 mg of mercury, more than any type of HID light. Overexposure to mercury found in HID lights can be toxic. Because of their mercury content, all HID lights should be handled with care. Inhaling, ingesting or even improperly handling mercury from a broken HID light can result in tremors, insomnia, respiratory failure, even death.