Specification :

High Intensity Discharge lighting is light from a plasma discharge rather than a filament. The system involves a ballast to start the light and certain gases to create the light. Overall the system uses less energy than halogen lighting, while producing 3 times more light.
How HID lights work?
High Intensity Discharge Lights do not have a filament. Instead light is created from an electrical discharge between two electrodes in a micro-environment of xenon gas and metal halide salts. The light is emitted by an electrically energized gas -- a plasma discharge-- formed and sustained between the two electrodes. The distinctive blue-white light of HID lamps stimulates the reflective paints in road markers and signs creating a safer driving environment. Additionally, the increased light output is designed to illuminate a wider area in front of the vehicle, improving visibility and safety, without disturbing the vision of oncoming drivers. HID lamps have over three times the lumens per watt of traditional halogen light sources and are more efficient at converting electrical energy into light. They produce at least 70% more light than traditional lamps, and use less power while producing less heat. This gives designers new freedom to explore the frontiers of front end design. By creating more compact headlights to fit smaller spaces, better aerodynamics are possible.