Metal halide lighting is a type of High Intensity Discharge (HID) lighting. Other types of HID include mercury vapour, high pressure sodium, and low pressure sodium. All HID lamps work on the same principle: an electrical arc across a gas or vapour using the gas or vapour as a conductor, the same way incandescent lamps use a tungsten filament as its conductor. They were previously used primarily outdoors due to noise and colour rendering, but with these problems now mostly corrected, they are frequently found indoors in high-bay venues such as gymnasiums and warehouses. Safety issues can include high operating temperature, 'non-passive envelope failure' (a quaint term for a lamp with a vapour pressure between 70 and 90 pounds per square inch blowing), and high UV output.
