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Bi-Xenon Headlights refers to HID (High Intensity Discharge) Xenon headlights that offer both low (dipped) and high (main) beam lighting from a dual headlight system (as opposed to a quad headlight system).
A bi-xenon system eliminates the need for a separate halogen high beam bulb/lens by either moving the xenon bulb within the lens or by moving a shield up or down in front of the bulb (depending on the reflector design) to allow light to escape from the lens in a different pattern.
A bi-xenon system typically features dynamic headlight leveling and on some vehicles direction adjustability.
The four lamp Bi - Xenon systems on some cars (BMW for example) use beam deflection on one pair of HID Xenon lamps for main and high beams. The second pair of lamps contain standard halogen bulbs for the headlamp flash function, they are not used in normal running. HID lamps take a few seconds to reach full output and their service life is seriously reduced by flashing on and off, for these reasons they are not currently used for any flash function.