The interior and exterior mirrors with automatic dimming recognize glaringly bright headlight beams behind the car and darken the mirror glass automatically. An electronic control unit varies the darkening effect quickly and continuously according to actual ambient light conditions.
This technology makes use of a transparent, electrically-conductive material that becomes increasingly impervious to light when an electric current is applied to it. A very thin layer of this material is sandwiched between the actual mirror reflector and the outer glass.
The mirror also contains two photoelectric cells. One of these faces forward and determines the ambient light intensity. The second is in the rear-facing surface of the mirror, and reacts when a bright light strikes it from behind the car. If this light is significantly brighter than the ambient light intensity, the photoelectric cell generates the necessary electrical signal, the electrically-conductive layer darkens and the image in the mirror no longer dazzles the driver.