High-intensity discharge (HID) lamps generate light through an electric arc between two electrodes in an arc tube, similar to fluorescents but with a shorter, hotter arc that produces more light. HID lamps require ballasts and take a few seconds to turn on as the ballast establishes the arc. They include high-pressure mercury vapor, metal halide, and high-pressure sodium lamps, with metal halide being the most efficacious in producing lumens per watt. HID lamps are commonly used for street lighting due to their high efficacy.