High-intensity discharge (HID) lamps include mercury vapor, metal halide, and high-pressure sodium lamps. HID lamps consist of an arc tube enclosed in an outer glass bulb. The arc tube contains mercury vapor, metal halides, or sodium vapor and produces light through electric excitation of these elements. HID lamps have distinctive spectral power distributions that produce different color appearances. They require ballasts to regulate current and start reliably. Upon power interruption, HID lamps require time to restart due to high internal pressures that must decrease first.