High-intensity discharge headlights emit a brighter white or bluish light than halogen lights. HIDs are legal only on vehicles with original equipment manufacturer systems. After-market kits used to install HID systems on other vehicles are illegal in all 50 states.
Halogen headlights have a metal filament. When electrically charged, the filament heats up until it glows. HID lights have no filament. They are filled with xenon gas and equipped with a ballast, which converts a car’s normal DC voltage to higher AC current. This creates an arc that ignites the gas and makes it glow.
HIDs installed in headlight systems designed for halogen lights produce substantially different beam patterns. They are difficult to aim, and emit light that exceeds the intensity allowed under the U.S. Department of Transportation's Federal Motor Vehicle Safety Standards.
Under federal regulation FMVSS 108, replacement lights must conform to the specifications for the original equipment manufacturer's system they will be installed in. The National Highway Transportation Safety Administration has determined that HID conversion kits can never be designed to meet this standard. The NHTSA declared HID kits illegal in 2004.
